Refactoring Java parsing (3.19.x) (#10667)

* Porting java cleanup

* Update changelog

* Extension patch

* Remove extra allocations

* Fixing merge issues

* More merge fixes

* Fix TextFormat parser

40 files changed, 1747 insertions, 945 deletions

diff --git a/CHANGES.txt b/CHANGES.txt
index 189706720..1724240df 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,5 +1,23 @@
-2022-09-13 version 19.5 (C++/Java/Python/PHP/Objective-C/C#/Ruby)
+2022-09-27 version 3.19.6 (C++/Java/Python/PHP/Objective-C/C#/Ruby)
+  Java
+  * Refactoring java full runtime to reuse sub-message builders and prepare to
+    migrate parsing logic from parse constructor to builder.
+  * Move proto wireformat parsing functionality from the private "parsing
+    constructor" to the Builder class.
+  * Change the Lite runtime to prefer merging from the wireformat into mutable
+    messages rather than building up a new immutable object before merging. This
+    way results in fewer allocations and copy operations.
+  * Make message-type extensions merge from wire-format instead of building up
+    instances and merging afterwards. This has much better performance.
+  * Fix TextFormat parser to build up recurring (but supposedly not repeated)
+    sub-messages directly from text rather than building a new sub-message and
+    merging the fully formed message into the existing field.
+
+
+2022-09-13 version 3.19.5 (C++/Java/Python/PHP/Objective-C/C#/Ruby)
 
+  C++
+  * Reduce memory consumption of MessageSet parsing
 
 2022-01-28 version 3.19.4 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
diff --git a/java/core/src/main/java/com/google/protobuf/AbstractMessage.java b/java/core/src/main/java/com/google/protobuf/AbstractMessage.java
index 1364fce41..ebf4318b9 100644
--- a/java/core/src/main/java/com/google/protobuf/AbstractMessage.java
+++ b/java/core/src/main/java/com/google/protobuf/AbstractMessage.java
@@ -426,27 +426,22 @@ public abstract class AbstractMessage
         throws IOException {
       boolean discardUnknown = input.shouldDiscardUnknownFields();
       final UnknownFieldSet.Builder unknownFields =
-          discardUnknown ? null : UnknownFieldSet.newBuilder(getUnknownFields());
-      while (true) {
-        final int tag = input.readTag();
-        if (tag == 0) {
-          break;
-        }
-
-        MessageReflection.BuilderAdapter builderAdapter =
-            new MessageReflection.BuilderAdapter(this);
-        if (!MessageReflection.mergeFieldFrom(
-            input, unknownFields, extensionRegistry, getDescriptorForType(), builderAdapter, tag)) {
-          // end group tag
-          break;
-        }
-      }
+          discardUnknown ? null : getUnknownFieldSetBuilder();
+      MessageReflection.mergeMessageFrom(this, unknownFields, input, extensionRegistry);
       if (unknownFields != null) {
-        setUnknownFields(unknownFields.build());
+        setUnknownFieldSetBuilder(unknownFields);
       }
       return (BuilderType) this;
     }
 
+    protected UnknownFieldSet.Builder getUnknownFieldSetBuilder() {
+      return UnknownFieldSet.newBuilder(getUnknownFields());
+    }
+
+    protected void setUnknownFieldSetBuilder(final UnknownFieldSet.Builder builder) {
+      setUnknownFields(builder.build());
+    }
+
     @Override
     public BuilderType mergeUnknownFields(final UnknownFieldSet unknownFields) {
       setUnknownFields(
diff --git a/java/core/src/main/java/com/google/protobuf/ArrayDecoders.java b/java/core/src/main/java/com/google/protobuf/ArrayDecoders.java
index 1217e112e..39b79278c 100644
--- a/java/core/src/main/java/com/google/protobuf/ArrayDecoders.java
+++ b/java/core/src/main/java/com/google/protobuf/ArrayDecoders.java
@@ -234,6 +234,29 @@ final class ArrayDecoders {
   @SuppressWarnings({"unchecked", "rawtypes"})
   static int decodeMessageField(
       Schema schema, byte[] data, int position, int limit, Registers registers) throws IOException {
+    Object msg = schema.newInstance();
+    int offset = mergeMessageField(msg, schema, data, position, limit, registers);
+    schema.makeImmutable(msg);
+    registers.object1 = msg;
+    return offset;
+  }
+
+  /** Decodes a group value. */
+  @SuppressWarnings({"unchecked", "rawtypes"})
+  static int decodeGroupField(
+      Schema schema, byte[] data, int position, int limit, int endGroup, Registers registers)
+      throws IOException {
+    Object msg = schema.newInstance();
+    int offset = mergeGroupField(msg, schema, data, position, limit, endGroup, registers);
+    schema.makeImmutable(msg);
+    registers.object1 = msg;
+    return offset;
+  }
+
+  @SuppressWarnings({"unchecked", "rawtypes"})
+  static int mergeMessageField(
+      Object msg, Schema schema, byte[] data, int position, int limit, Registers registers)
+      throws IOException {
     int length = data[position++];
     if (length < 0) {
       position = decodeVarint32(length, data, position, registers);
@@ -242,27 +265,28 @@ final class ArrayDecoders {
     if (length < 0 || length > limit - position) {
       throw InvalidProtocolBufferException.truncatedMessage();
     }
-    Object result = schema.newInstance();
-    schema.mergeFrom(result, data, position, position + length, registers);
-    schema.makeImmutable(result);
-    registers.object1 = result;
+    schema.mergeFrom(msg, data, position, position + length, registers);
+    registers.object1 = msg;
     return position + length;
   }
 
-  /** Decodes a group value. */
   @SuppressWarnings({"unchecked", "rawtypes"})
-  static int decodeGroupField(
-      Schema schema, byte[] data, int position, int limit, int endGroup, Registers registers)
+  static int mergeGroupField(
+      Object msg,
+      Schema schema,
+      byte[] data,
+      int position,
+      int limit,
+      int endGroup,
+      Registers registers)
       throws IOException {
     // A group field must has a MessageSchema (the only other subclass of Schema is MessageSetSchema
     // and it can't be used in group fields).
     final MessageSchema messageSchema = (MessageSchema) schema;
-    Object result = messageSchema.newInstance();
     // It's OK to directly use parseProto2Message since proto3 doesn't have group.
     final int endPosition =
-        messageSchema.parseProto2Message(result, data, position, limit, endGroup, registers);
-    messageSchema.makeImmutable(result);
-    registers.object1 = result;
+        messageSchema.parseProto2Message(msg, data, position, limit, endGroup, registers);
+    registers.object1 = msg;
     return endPosition;
   }
 
@@ -847,26 +871,19 @@ final class ArrayDecoders {
           break;
         }
         case ENUM:
-        {
-          IntArrayList list = new IntArrayList();
-          position = decodePackedVarint32List(data, position, list, registers);
-          UnknownFieldSetLite unknownFields = message.unknownFields;
-          if (unknownFields == UnknownFieldSetLite.getDefaultInstance()) {
-            unknownFields = null;
-          }
-          unknownFields =
-              SchemaUtil.filterUnknownEnumList(
-                  fieldNumber,
-                  list,
-                  extension.descriptor.getEnumType(),
-                  unknownFields,
-                  unknownFieldSchema);
-          if (unknownFields != null) {
-            message.unknownFields = unknownFields;
+          {
+            IntArrayList list = new IntArrayList();
+            position = decodePackedVarint32List(data, position, list, registers);
+            SchemaUtil.filterUnknownEnumList(
+                message,
+                fieldNumber,
+                list,
+                extension.descriptor.getEnumType(),
+                null,
+                unknownFieldSchema);
+            extensions.setField(extension.descriptor, list);
+            break;
           }
-          extensions.setField(extension.descriptor, list);
-          break;
-        }
         default:
           throw new IllegalStateException(
               "Type cannot be packed: " + extension.descriptor.getLiteType());
@@ -878,13 +895,8 @@ final class ArrayDecoders {
         position = decodeVarint32(data, position, registers);
         Object enumValue = extension.descriptor.getEnumType().findValueByNumber(registers.int1);
         if (enumValue == null) {
-          UnknownFieldSetLite unknownFields = ((GeneratedMessageLite) message).unknownFields;
-          if (unknownFields == UnknownFieldSetLite.getDefaultInstance()) {
-            unknownFields = UnknownFieldSetLite.newInstance();
-            ((GeneratedMessageLite) message).unknownFields = unknownFields;
-          }
           SchemaUtil.storeUnknownEnum(
-              fieldNumber, registers.int1, unknownFields, unknownFieldSchema);
+              message, fieldNumber, registers.int1, null, unknownFieldSchema);
           return position;
         }
         // Note, we store the integer value instead of the actual enum object in FieldSet.
@@ -941,20 +953,45 @@ final class ArrayDecoders {
             value = registers.object1;
             break;
           case GROUP:
-            final int endTag = (fieldNumber << 3) | WireFormat.WIRETYPE_END_GROUP;
-            position = decodeGroupField(
-                Protobuf.getInstance().schemaFor(extension.getMessageDefaultInstance().getClass()),
-                data, position, limit, endTag, registers);
-            value = registers.object1;
-            break;
-
+            {
+              final int endTag = (fieldNumber << 3) | WireFormat.WIRETYPE_END_GROUP;
+              final Schema fieldSchema =
+                  Protobuf.getInstance()
+                      .schemaFor(extension.getMessageDefaultInstance().getClass());
+              if (extension.isRepeated()) {
+                position = decodeGroupField(fieldSchema, data, position, limit, endTag, registers);
+                extensions.addRepeatedField(extension.descriptor, registers.object1);
+              } else {
+                Object oldValue = extensions.getField(extension.descriptor);
+                if (oldValue == null) {
+                  oldValue = fieldSchema.newInstance();
+                  extensions.setField(extension.descriptor, oldValue);
+                }
+                position =
+                    mergeGroupField(
+                        oldValue, fieldSchema, data, position, limit, endTag, registers);
+              }
+              return position;
+            }
           case MESSAGE:
-            position = decodeMessageField(
-                Protobuf.getInstance().schemaFor(extension.getMessageDefaultInstance().getClass()),
-                data, position, limit, registers);
-            value = registers.object1;
-            break;
-
+            {
+              final Schema fieldSchema =
+                  Protobuf.getInstance()
+                      .schemaFor(extension.getMessageDefaultInstance().getClass());
+              if (extension.isRepeated()) {
+                position = decodeMessageField(fieldSchema, data, position, limit, registers);
+                extensions.addRepeatedField(extension.descriptor, registers.object1);
+              } else {
+                Object oldValue = extensions.getField(extension.descriptor);
+                if (oldValue == null) {
+                  oldValue = fieldSchema.newInstance();
+                  extensions.setField(extension.descriptor, oldValue);
+                }
+                position =
+                    mergeMessageField(oldValue, fieldSchema, data, position, limit, registers);
+              }
+              return position;
+            }
           case ENUM:
             throw new IllegalStateException("Shouldn't reach here.");
         }
@@ -962,17 +999,6 @@ final class ArrayDecoders {
       if (extension.isRepeated()) {
         extensions.addRepeatedField(extension.descriptor, value);
       } else {
-        switch (extension.getLiteType()) {
-          case MESSAGE:
-          case GROUP:
-            Object oldValue = extensions.getField(extension.descriptor);
-            if (oldValue != null) {
-              value = Internal.mergeMessage(oldValue, value);
-            }
-            break;
-          default:
-            break;
-        }
         extensions.setField(extension.descriptor, value);
       }
     }
diff --git a/java/core/src/main/java/com/google/protobuf/BinaryReader.java b/java/core/src/main/java/com/google/protobuf/BinaryReader.java
index d64574c2a..3a0e04dfa 100644
--- a/java/core/src/main/java/com/google/protobuf/BinaryReader.java
+++ b/java/core/src/main/java/com/google/protobuf/BinaryReader.java
@@ -247,6 +247,15 @@ abstract class BinaryReader implements Reader {
 
     private <T> T readMessage(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
         throws IOException {
+      T newInstance = schema.newInstance();
+      mergeMessageField(newInstance, schema, extensionRegistry);
+      schema.makeImmutable(newInstance);
+      return newInstance;
+    }
+
+    @Override
+    public <T> void mergeMessageField(
+        T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
       int size = readVarint32();
       requireBytes(size);
 
@@ -256,15 +265,10 @@ abstract class BinaryReader implements Reader {
       limit = newLimit;
 
       try {
-        // Allocate and read the message.
-        T message = schema.newInstance();
-        schema.mergeFrom(message, this, extensionRegistry);
-        schema.makeImmutable(message);
-
+        schema.mergeFrom(target, this, extensionRegistry);
         if (pos != newLimit) {
           throw InvalidProtocolBufferException.parseFailure();
         }
-        return message;
       } finally {
         // Restore the limit.
         limit = prevLimit;
@@ -287,19 +291,23 @@ abstract class BinaryReader implements Reader {
 
     private <T> T readGroup(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
         throws IOException {
+      T newInstance = schema.newInstance();
+      mergeGroupField(newInstance, schema, extensionRegistry);
+      schema.makeImmutable(newInstance);
+      return newInstance;
+    }
+
+    @Override
+    public <T> void mergeGroupField(
+        T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
       int prevEndGroupTag = endGroupTag;
       endGroupTag = WireFormat.makeTag(WireFormat.getTagFieldNumber(tag), WIRETYPE_END_GROUP);
 
       try {
-        // Allocate and read the message.
-        T message = schema.newInstance();
-        schema.mergeFrom(message, this, extensionRegistry);
-        schema.makeImmutable(message);
-
+        schema.mergeFrom(target, this, extensionRegistry);
         if (tag != endGroupTag) {
           throw InvalidProtocolBufferException.parseFailure();
         }
-        return message;
       } finally {
         // Restore the old end group tag.
         endGroupTag = prevEndGroupTag;
diff --git a/java/core/src/main/java/com/google/protobuf/CodedInputStreamReader.java b/java/core/src/main/java/com/google/protobuf/CodedInputStreamReader.java
index 7658f629d..1d992d75d 100644
--- a/java/core/src/main/java/com/google/protobuf/CodedInputStreamReader.java
+++ b/java/core/src/main/java/com/google/protobuf/CodedInputStreamReader.java
@@ -197,9 +197,15 @@ final class CodedInputStreamReader implements Reader {
     return readGroup(schema, extensionRegistry);
   }
 
-  // Should have the same semantics of CodedInputStream#readMessage()
-  private <T> T readMessage(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
-      throws IOException {
+  @Override
+  public <T> void mergeMessageField(
+      T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
+    requireWireType(WIRETYPE_LENGTH_DELIMITED);
+    mergeMessageFieldInternal(target, schema, extensionRegistry);
+  }
+
+  private <T> void mergeMessageFieldInternal(
+      T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
     int size = input.readUInt32();
     if (input.recursionDepth >= input.recursionLimit) {
       throw InvalidProtocolBufferException.recursionLimitExceeded();
@@ -207,39 +213,54 @@ final class CodedInputStreamReader implements Reader {
 
     // Push the new limit.
     final int prevLimit = input.pushLimit(size);
-    // Allocate and read the message.
-    T message = schema.newInstance();
     ++input.recursionDepth;
-    schema.mergeFrom(message, this, extensionRegistry);
-    schema.makeImmutable(message);
+    schema.mergeFrom(target, this, extensionRegistry);
     input.checkLastTagWas(0);
     --input.recursionDepth;
     // Restore the previous limit.
     input.popLimit(prevLimit);
-    return message;
   }
 
-  private <T> T readGroup(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
+  // Should have the same semantics of CodedInputStream#readMessage()
+  private <T> T readMessage(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
       throws IOException {
+    T newInstance = schema.newInstance();
+    mergeMessageFieldInternal(newInstance, schema, extensionRegistry);
+    schema.makeImmutable(newInstance);
+    return newInstance;
+  }
+
+  @Override
+  public <T> void mergeGroupField(
+      T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
+    requireWireType(WIRETYPE_START_GROUP);
+    mergeGroupFieldInternal(target, schema, extensionRegistry);
+  }
+
+  private <T> void mergeGroupFieldInternal(
+      T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry) throws IOException {
     int prevEndGroupTag = endGroupTag;
     endGroupTag = WireFormat.makeTag(WireFormat.getTagFieldNumber(tag), WIRETYPE_END_GROUP);
 
     try {
-      // Allocate and read the message.
-      T message = schema.newInstance();
-      schema.mergeFrom(message, this, extensionRegistry);
-      schema.makeImmutable(message);
-
+      schema.mergeFrom(target, this, extensionRegistry);
       if (tag != endGroupTag) {
         throw InvalidProtocolBufferException.parseFailure();
       }
-      return message;
     } finally {
       // Restore the old end group tag.
       endGroupTag = prevEndGroupTag;
     }
   }
 
+  private <T> T readGroup(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
+      throws IOException {
+    T newInstance = schema.newInstance();
+    mergeGroupFieldInternal(newInstance, schema, extensionRegistry);
+    schema.makeImmutable(newInstance);
+    return newInstance;
+  }
+
   @Override
   public ByteString readBytes() throws IOException {
     requireWireType(WIRETYPE_LENGTH_DELIMITED);
diff --git a/java/core/src/main/java/com/google/protobuf/DescriptorMessageInfoFactory.java b/java/core/src/main/java/com/google/protobuf/DescriptorMessageInfoFactory.java
index 797513659..21ded52d9 100644
--- a/java/core/src/main/java/com/google/protobuf/DescriptorMessageInfoFactory.java
+++ b/java/core/src/main/java/com/google/protobuf/DescriptorMessageInfoFactory.java
@@ -402,8 +402,8 @@ final class DescriptorMessageInfoFactory implements MessageInfoFactory {
     boolean enforceUtf8 = true;
     for (int i = 0; i < fieldDescriptors.size(); ++i) {
       FieldDescriptor fd = fieldDescriptors.get(i);
-      if (fd.getContainingOneof() != null) {
-        // Build a oneof member field.
+      if (fd.getContainingOneof() != null && !fd.getContainingOneof().isSynthetic()) {
+        // Build a oneof member field. But only if it is a real oneof, not a proto3 optional
         builder.withField(buildOneofMember(messageType, fd, oneofState, enforceUtf8, null));
         continue;
       }
diff --git a/java/core/src/main/java/com/google/protobuf/DynamicMessage.java b/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
index 8beebba24..51e6b0c27 100644
--- a/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
+++ b/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
@@ -421,7 +421,10 @@ public final class DynamicMessage extends AbstractMessage {
       fields.makeImmutable();
       DynamicMessage result =
           new DynamicMessage(
-              type, fields, java.util.Arrays.copyOf(oneofCases, oneofCases.length), unknownFields);
+              type,
+              fields,
+              java.util.Arrays.copyOf(oneofCases, oneofCases.length),
+              unknownFields);
       return result;
     }
 
diff --git a/java/core/src/main/java/com/google/protobuf/ExtensionSchema.java b/java/core/src/main/java/com/google/protobuf/ExtensionSchema.java
index 2eae22d26..bd391a2c1 100644
--- a/java/core/src/main/java/com/google/protobuf/ExtensionSchema.java
+++ b/java/core/src/main/java/com/google/protobuf/ExtensionSchema.java
@@ -59,6 +59,7 @@ abstract class ExtensionSchema<T extends FieldSet.FieldDescriptorLite<T>> {
    *     or UnknownFieldSetLite in lite runtime.
    */
   abstract <UT, UB> UB parseExtension(
+      Object containerMessage,
       Reader reader,
       Object extension,
       ExtensionRegistryLite extensionRegistry,
diff --git a/java/core/src/main/java/com/google/protobuf/ExtensionSchemaFull.java b/java/core/src/main/java/com/google/protobuf/ExtensionSchemaFull.java
index 90558518b..9376e8780 100644
--- a/java/core/src/main/java/com/google/protobuf/ExtensionSchemaFull.java
+++ b/java/core/src/main/java/com/google/protobuf/ExtensionSchemaFull.java
@@ -85,6 +85,7 @@ final class ExtensionSchemaFull extends ExtensionSchema<FieldDescriptor> {
 
   @Override
   <UT, UB> UB parseExtension(
+      Object containerMessage,
       Reader reader,
       Object extensionObject,
       ExtensionRegistryLite extensionRegistry,
@@ -202,7 +203,7 @@ final class ExtensionSchemaFull extends ExtensionSchema<FieldDescriptor> {
               } else {
                 unknownFields =
                     SchemaUtil.storeUnknownEnum(
-                        fieldNumber, number, unknownFields, unknownFieldSchema);
+                        containerMessage, fieldNumber, number, unknownFields, unknownFieldSchema);
               }
             }
             value = enumList;
@@ -221,7 +222,7 @@ final class ExtensionSchemaFull extends ExtensionSchema<FieldDescriptor> {
         Object enumValue = extension.descriptor.getEnumType().findValueByNumber(number);
         if (enumValue == null) {
           return SchemaUtil.storeUnknownEnum(
-              fieldNumber, number, unknownFields, unknownFieldSchema);
+              containerMessage, fieldNumber, number, unknownFields, unknownFieldSchema);
         }
         value = enumValue;
       } else {
diff --git a/java/core/src/main/java/com/google/protobuf/ExtensionSchemaLite.java b/java/core/src/main/java/com/google/protobuf/ExtensionSchemaLite.java
index 437cca2d9..7e20ed2ff 100644
--- a/java/core/src/main/java/com/google/protobuf/ExtensionSchemaLite.java
+++ b/java/core/src/main/java/com/google/protobuf/ExtensionSchemaLite.java
@@ -32,7 +32,6 @@ package com.google.protobuf;
 
 import com.google.protobuf.GeneratedMessageLite.ExtensionDescriptor;
 import java.io.IOException;
-import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
@@ -67,6 +66,7 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
 
   @Override
   <UT, UB> UB parseExtension(
+      Object containerMessage,
       Reader reader,
       Object extensionObject,
       ExtensionRegistryLite extensionRegistry,
@@ -178,6 +178,7 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
             reader.readEnumList(list);
             unknownFields =
                 SchemaUtil.filterUnknownEnumList(
+                    containerMessage,
                     fieldNumber,
                     list,
                     extension.descriptor.getEnumType(),
@@ -199,7 +200,7 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
         Object enumValue = extension.descriptor.getEnumType().findValueByNumber(number);
         if (enumValue == null) {
           return SchemaUtil.storeUnknownEnum(
-              fieldNumber, number, unknownFields, unknownFieldSchema);
+              containerMessage, fieldNumber, number, unknownFields, unknownFieldSchema);
         }
         // Note, we store the integer value instead of the actual enum object in FieldSet.
         // This is also different from full-runtime where we store EnumValueDescriptor.
@@ -253,12 +254,46 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
             value = reader.readString();
             break;
           case GROUP:
+            // Special case handling for non-repeated sub-messages: merge in-place rather than
+            // building up new sub-messages and merging those, which is too slow.
+            // TODO(b/249368670): clean this up
+            if (!extension.isRepeated()) {
+              Object oldValue = extensions.getField(extension.descriptor);
+              if (oldValue instanceof GeneratedMessageLite) {
+                Schema extSchema = Protobuf.getInstance().schemaFor(oldValue);
+                if (!((GeneratedMessageLite<?, ?>) oldValue).isMutable()) {
+                  Object newValue = extSchema.newInstance();
+                  extSchema.mergeFrom(newValue, oldValue);
+                  extensions.setField(extension.descriptor, newValue);
+                  oldValue = newValue;
+                }
+                reader.mergeGroupField(oldValue, extSchema, extensionRegistry);
+                return unknownFields;
+              }
+            }
             value =
                 reader.readGroup(
                     extension.getMessageDefaultInstance().getClass(), extensionRegistry);
             break;
 
           case MESSAGE:
+            // Special case handling for non-repeated sub-messages: merge in-place rather than
+            // building up new sub-messages and merging those, which is too slow.
+            // TODO(b/249368670): clean this up
+            if (!extension.isRepeated()) {
+              Object oldValue = extensions.getField(extension.descriptor);
+              if (oldValue instanceof GeneratedMessageLite) {
+                Schema extSchema = Protobuf.getInstance().schemaFor(oldValue);
+                if (!((GeneratedMessageLite<?, ?>) oldValue).isMutable()) {
+                  Object newValue = extSchema.newInstance();
+                  extSchema.mergeFrom(newValue, oldValue);
+                  extensions.setField(extension.descriptor, newValue);
+                  oldValue = newValue;
+                }
+                reader.mergeMessageField(oldValue, extSchema, extensionRegistry);
+                return unknownFields;
+              }
+            }
             value =
                 reader.readMessage(
                     extension.getMessageDefaultInstance().getClass(), extensionRegistry);
@@ -274,6 +309,7 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
         switch (extension.getLiteType()) {
           case MESSAGE:
           case GROUP:
+            // TODO(b/249368670): this shouldn't be reachable, clean this up
             Object oldValue = extensions.getField(extension.descriptor);
             if (oldValue != null) {
               value = Internal.mergeMessage(oldValue, value);
@@ -527,15 +563,13 @@ final class ExtensionSchemaLite extends ExtensionSchema<ExtensionDescriptor> {
       throws IOException {
     GeneratedMessageLite.GeneratedExtension<?, ?> extension =
         (GeneratedMessageLite.GeneratedExtension<?, ?>) extensionObject;
-    Object value = extension.getMessageDefaultInstance().newBuilderForType().buildPartial();
 
-    Reader reader = BinaryReader.newInstance(ByteBuffer.wrap(data.toByteArray()), true);
+    MessageLite.Builder builder = extension.getMessageDefaultInstance().newBuilderForType();
 
-    Protobuf.getInstance().mergeFrom(value, reader, extensionRegistry);
-    extensions.setField(extension.descriptor, value);
+    final CodedInputStream input = data.newCodedInput();
 
-    if (reader.getFieldNumber() != Reader.READ_DONE) {
-      throw InvalidProtocolBufferException.invalidEndTag();
-    }
+    builder.mergeFrom(input, extensionRegistry);
+    extensions.setField(extension.descriptor, builder.buildPartial());
+    input.checkLastTagWas(0);
   }
 }
diff --git a/java/core/src/main/java/com/google/protobuf/FieldSet.java b/java/core/src/main/java/com/google/protobuf/FieldSet.java
index 8d1136036..5deef4166 100644
--- a/java/core/src/main/java/com/google/protobuf/FieldSet.java
+++ b/java/core/src/main/java/com/google/protobuf/FieldSet.java
@@ -39,6 +39,7 @@ import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 
 /**
  * A class which represents an arbitrary set of fields of some message type. This is used to
@@ -123,6 +124,12 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
     if (isImmutable) {
       return;
     }
+    for (int i = 0; i < fields.getNumArrayEntries(); ++i) {
+      Entry<T, Object> entry = fields.getArrayEntryAt(i);
+      if (entry.getValue() instanceof GeneratedMessageLite) {
+        ((GeneratedMessageLite<?, ?>) entry.getValue()).makeImmutable();
+      }
+    }
     fields.makeImmutable();
     isImmutable = true;
   }
@@ -936,8 +943,27 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
       this.isMutable = true;
     }
 
-    /** Creates the FieldSet */
+    /**
+     * Creates the FieldSet
+     *
+     * @throws UninitializedMessageException if a message field is missing required fields.
+     */
     public FieldSet<T> build() {
+      return buildImpl(false);
+    }
+
+    /** Creates the FieldSet but does not validate that all required fields are present. */
+    public FieldSet<T> buildPartial() {
+      return buildImpl(true);
+    }
+
+    /**
+     * Creates the FieldSet.
+     *
+     * @param partial controls whether to do a build() or buildPartial() when converting submessage
+     *     builders to messages.
+     */
+    private FieldSet<T> buildImpl(boolean partial) {
       if (fields.isEmpty()) {
         return FieldSet.emptySet();
       }
@@ -946,7 +972,7 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
       if (hasNestedBuilders) {
         // Make a copy of the fields map with all Builders replaced by Message.
         fieldsForBuild = cloneAllFieldsMap(fields, /* copyList */ false);
-        replaceBuilders(fieldsForBuild);
+        replaceBuilders(fieldsForBuild, partial);
       }
       FieldSet<T> fieldSet = new FieldSet<>(fieldsForBuild);
       fieldSet.hasLazyField = hasLazyField;
@@ -954,22 +980,22 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
     }
 
     private static <T extends FieldDescriptorLite<T>> void replaceBuilders(
-        SmallSortedMap<T, Object> fieldMap) {
+        SmallSortedMap<T, Object> fieldMap, boolean partial) {
       for (int i = 0; i < fieldMap.getNumArrayEntries(); i++) {
-        replaceBuilders(fieldMap.getArrayEntryAt(i));
+        replaceBuilders(fieldMap.getArrayEntryAt(i), partial);
       }
       for (Map.Entry<T, Object> entry : fieldMap.getOverflowEntries()) {
-        replaceBuilders(entry);
+        replaceBuilders(entry, partial);
       }
     }
 
     private static <T extends FieldDescriptorLite<T>> void replaceBuilders(
-        Map.Entry<T, Object> entry) {
-      entry.setValue(replaceBuilders(entry.getKey(), entry.getValue()));
+        Map.Entry<T, Object> entry, boolean partial) {
+      entry.setValue(replaceBuilders(entry.getKey(), entry.getValue(), partial));
     }
 
     private static <T extends FieldDescriptorLite<T>> Object replaceBuilders(
-        T descriptor, Object value) {
+        T descriptor, Object value, boolean partial) {
       if (value == null) {
         return value;
       }
@@ -984,7 +1010,7 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
           List<Object> list = (List<Object>) value;
           for (int i = 0; i < list.size(); i++) {
             Object oldElement = list.get(i);
-            Object newElement = replaceBuilder(oldElement);
+            Object newElement = replaceBuilder(oldElement, partial);
             if (newElement != oldElement) {
               // If the list contains a Message.Builder, then make a copy of that list and then
               // modify the Message.Builder into a Message and return the new list. This way, the
@@ -998,14 +1024,21 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
           }
           return list;
         } else {
-          return replaceBuilder(value);
+          return replaceBuilder(value, partial);
         }
       }
       return value;
     }
 
-    private static Object replaceBuilder(Object value) {
-      return (value instanceof MessageLite.Builder) ? ((MessageLite.Builder) value).build() : value;
+    private static Object replaceBuilder(Object value, boolean partial) {
+      if (!(value instanceof MessageLite.Builder)) {
+        return value;
+      }
+      MessageLite.Builder builder = (MessageLite.Builder) value;
+      if (partial) {
+        return builder.buildPartial();
+      }
+      return builder.build();
     }
 
     /** Returns a new Builder using the fields from {@code fieldSet}. */
@@ -1024,7 +1057,7 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
         if (fields.isImmutable()) {
           result.makeImmutable();
         } else {
-          replaceBuilders(result);
+          replaceBuilders(result, true);
         }
         return result;
       }
@@ -1047,7 +1080,7 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
      */
     public Object getField(final T descriptor) {
       Object value = getFieldAllowBuilders(descriptor);
-      return replaceBuilders(descriptor, value);
+      return replaceBuilders(descriptor, value, true);
     }
 
     /** Same as {@link #getField(F)}, but allow a {@link MessageLite.Builder} to be returned. */
@@ -1133,7 +1166,7 @@ final class FieldSet<T extends FieldSet.FieldDescriptorLite<T>> {
         ensureIsMutable();
       }
       Object value = getRepeatedFieldAllowBuilders(descriptor, index);
-      return replaceBuilder(value);
+      return replaceBuilder(value, true);
     }
 
     /**
diff --git a/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java b/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
index 7db8f32ee..808fde8e7 100644
--- a/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
+++ b/java/core/src/main/java/com/google/protobuf/GeneratedMessageLite.java
@@ -62,11 +62,50 @@ public abstract class GeneratedMessageLite<
         BuilderType extends GeneratedMessageLite.Builder<MessageType, BuilderType>>
     extends AbstractMessageLite<MessageType, BuilderType> {
 
+  /* For use by lite runtime only */
+  static final int UNINITIALIZED_SERIALIZED_SIZE = 0x7FFFFFFF;
+  private static final int MUTABLE_FLAG_MASK = 0x80000000;
+  private static final int MEMOIZED_SERIALIZED_SIZE_MASK = 0x7FFFFFFF;
+
+  /**
+   * We use the high bit of memoizedSerializedSize as the explicit mutability flag. It didn't make
+   * sense to have negative sizes anyway. Messages start as mutable.
+   *
+   * <p>Adding a standalone boolean would have added 8 bytes to every message instance.
+   *
+   * <p>We also reserve 0x7FFFFFFF as the "uninitialized" value.
+   */
+  private int memoizedSerializedSize = MUTABLE_FLAG_MASK | UNINITIALIZED_SERIALIZED_SIZE;
+
+  /* For use by the runtime only */
+  static final int UNINITIALIZED_HASH_CODE = 0;
+
   /** For use by generated code only. Lazily initialized to reduce allocations. */
   protected UnknownFieldSetLite unknownFields = UnknownFieldSetLite.getDefaultInstance();
 
-  /** For use by generated code only. */
-  protected int memoizedSerializedSize = -1;
+  boolean isMutable() {
+    return (memoizedSerializedSize & MUTABLE_FLAG_MASK) != 0;
+  }
+
+  void markImmutable() {
+    memoizedSerializedSize &= ~MUTABLE_FLAG_MASK;
+  }
+
+  int getMemoizedHashCode() {
+    return memoizedHashCode;
+  }
+
+  void setMemoizedHashCode(int value) {
+    memoizedHashCode = value;
+  }
+
+  void clearMemoizedHashCode() {
+    memoizedHashCode = UNINITIALIZED_HASH_CODE;
+  }
+
+  boolean hashCodeIsNotMemoized() {
+    return UNINITIALIZED_HASH_CODE == getMemoizedHashCode();
+  }
 
   @Override
   @SuppressWarnings("unchecked") // Guaranteed by runtime.
@@ -86,6 +125,10 @@ public abstract class GeneratedMessageLite<
     return (BuilderType) dynamicMethod(MethodToInvoke.NEW_BUILDER);
   }
 
+  MessageType newMutableInstance() {
+    return (MessageType) dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
+  }
+
   /**
    * A reflective toString function. This is primarily intended as a developer aid, while keeping
    * binary size down. The first line of the {@code toString()} representation includes a commented
@@ -106,11 +149,19 @@ public abstract class GeneratedMessageLite<
   @SuppressWarnings("unchecked") // Guaranteed by runtime
   @Override
   public int hashCode() {
-    if (memoizedHashCode != 0) {
-      return memoizedHashCode;
+    if (isMutable()) {
+      return computeHashCode();
     }
-    memoizedHashCode = Protobuf.getInstance().schemaFor(this).hashCode(this);
-    return memoizedHashCode;
+
+    if (hashCodeIsNotMemoized()) {
+      setMemoizedHashCode(computeHashCode());
+    }
+
+    return getMemoizedHashCode();
+  }
+
+  int computeHashCode() {
+    return Protobuf.getInstance().schemaFor(this).hashCode(this);
   }
 
   @SuppressWarnings("unchecked") // Guaranteed by isInstance + runtime
@@ -173,6 +224,7 @@ public abstract class GeneratedMessageLite<
   /** Called by subclasses to complete parsing. For use by generated code only. */
   protected void makeImmutable() {
     Protobuf.getInstance().schemaFor(this).makeImmutable(this);
+    markImmutable();
   }
 
   protected final <
@@ -198,8 +250,7 @@ public abstract class GeneratedMessageLite<
   @SuppressWarnings("unchecked")
   public final BuilderType toBuilder() {
     BuilderType builder = (BuilderType) dynamicMethod(MethodToInvoke.NEW_BUILDER);
-    builder.mergeFrom((MessageType) this);
-    return builder;
+    return builder.mergeFrom((MessageType) this);
   }
 
   /**
@@ -256,14 +307,22 @@ public abstract class GeneratedMessageLite<
     return dynamicMethod(method, null, null);
   }
 
+  void clearMemoizedSerializedSize() {
+    setMemoizedSerializedSize(UNINITIALIZED_SERIALIZED_SIZE);
+  }
+
   @Override
   int getMemoizedSerializedSize() {
-    return memoizedSerializedSize;
+    return memoizedSerializedSize & MEMOIZED_SERIALIZED_SIZE_MASK;
   }
 
   @Override
   void setMemoizedSerializedSize(int size) {
-    memoizedSerializedSize = size;
+    if (size < 0) {
+      throw new IllegalStateException("serialized size must be non-negative, was " + size);
+    }
+    memoizedSerializedSize =
+        (memoizedSerializedSize & MUTABLE_FLAG_MASK) | (size & MEMOIZED_SERIALIZED_SIZE_MASK);
   }
 
   @Override
@@ -274,11 +333,41 @@ public abstract class GeneratedMessageLite<
   }
 
   @Override
+  int getSerializedSize(Schema schema) {
+    if (isMutable()) {
+      // The serialized size should never be memoized for mutable instances.
+      int size = computeSerializedSize(schema);
+      if (size < 0) {
+        throw new IllegalStateException("serialized size must be non-negative, was " + size);
+      }
+      return size;
+    }
+
+    // If memoizedSerializedSize has already been set, return it.
+    if (getMemoizedSerializedSize() != UNINITIALIZED_SERIALIZED_SIZE) {
+      return getMemoizedSerializedSize();
+    }
+
+    // Need to compute and memoize the serialized size.
+    int size = computeSerializedSize(schema);
+    setMemoizedSerializedSize(size);
+    return size;
+  }
+
+  @Override
   public int getSerializedSize() {
-    if (memoizedSerializedSize == -1) {
-      memoizedSerializedSize = Protobuf.getInstance().schemaFor(this).getSerializedSize(this);
+    // Calling this with 'null' to delay schema lookup in case the serializedSize is already
+    // memoized.
+    return getSerializedSize(null);
+  }
+
+  private int computeSerializedSize(Schema<?> nullableSchema) {
+    if (nullableSchema == null) {
+      return Protobuf.getInstance().schemaFor(this).getSerializedSize(this);
+    } else {
+      return ((Schema<GeneratedMessageLite<MessageType, BuilderType>>) nullableSchema)
+          .getSerializedSize(this);
     }
-    return memoizedSerializedSize;
   }
 
   /** Constructs a {@link MessageInfo} for this message type. */
@@ -318,6 +407,7 @@ public abstract class GeneratedMessageLite<
   protected static <T extends GeneratedMessageLite<?, ?>> void registerDefaultInstance(
       Class<T> clazz, T defaultInstance) {
     defaultInstanceMap.put(clazz, defaultInstance);
+    defaultInstance.makeImmutable();
   }
 
   protected static Object newMessageInfo(
@@ -342,13 +432,19 @@ public abstract class GeneratedMessageLite<
 
     private final MessageType defaultInstance;
     protected MessageType instance;
-    protected boolean isBuilt;
 
     protected Builder(MessageType defaultInstance) {
       this.defaultInstance = defaultInstance;
-      this.instance =
-          (MessageType) defaultInstance.dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
-      isBuilt = false;
+      if (defaultInstance.isMutable()) {
+        throw new IllegalArgumentException("Default instance must be immutable.");
+      }
+      // this.instance should be set to defaultInstance but some tests rely on newBuilder().build()
+      // creating unique instances.
+      this.instance = newMutableInstance();
+    }
+
+    private MessageType newMutableInstance() {
+      return defaultInstance.newMutableInstance();
     }
 
     /**
@@ -356,15 +452,13 @@ public abstract class GeneratedMessageLite<
      * state before the write happens to preserve immutability guarantees.
      */
     protected final void copyOnWrite() {
-      if (isBuilt) {
+      if (!instance.isMutable()) {
         copyOnWriteInternal();
-        isBuilt = false;
       }
     }
 
     protected void copyOnWriteInternal() {
-      MessageType newInstance =
-          (MessageType) instance.dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
+      MessageType newInstance = newMutableInstance();
       mergeFromInstance(newInstance, instance);
       instance = newInstance;
     }
@@ -376,27 +470,28 @@ public abstract class GeneratedMessageLite<
 
     @Override
     public final BuilderType clear() {
-      // No need to copy on write since we're dropping the instance anyways.
-      instance = (MessageType) instance.dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
+      // No need to copy on write since we're dropping the instance anyway.
+      if (defaultInstance.isMutable()) {
+        throw new IllegalArgumentException("Default instance must be immutable.");
+      }
+      instance = newMutableInstance(); // should be defaultInstance;
       return (BuilderType) this;
     }
 
     @Override
     public BuilderType clone() {
       BuilderType builder = (BuilderType) getDefaultInstanceForType().newBuilderForType();
-      builder.mergeFrom(buildPartial());
+      builder.instance = buildPartial();
       return builder;
     }
 
     @Override
     public MessageType buildPartial() {
-      if (isBuilt) {
+      if (!instance.isMutable()) {
         return instance;
       }
 
       instance.makeImmutable();
-
-      isBuilt = true;
       return instance;
     }
 
@@ -416,12 +511,15 @@ public abstract class GeneratedMessageLite<
 
     /** All subclasses implement this. */
     public BuilderType mergeFrom(MessageType message) {
+      if (getDefaultInstanceForType().equals(message)) {
+        return (BuilderType) this;
+      }
       copyOnWrite();
       mergeFromInstance(instance, message);
       return (BuilderType) this;
     }
 
-    private void mergeFromInstance(MessageType dest, MessageType src) {
+    private static <MessageType> void mergeFromInstance(MessageType dest, MessageType src) {
       Protobuf.getInstance().schemaFor(dest).mergeFrom(dest, src);
     }
 
@@ -932,7 +1030,9 @@ public abstract class GeneratedMessageLite<
     @Override
     protected void copyOnWriteInternal() {
       super.copyOnWriteInternal();
-      instance.extensions = instance.extensions.clone();
+      if (instance.extensions != FieldSet.emptySet()) {
+        instance.extensions = instance.extensions.clone();
+      }
     }
 
     private FieldSet<ExtensionDescriptor> ensureExtensionsAreMutable() {
@@ -946,7 +1046,7 @@ public abstract class GeneratedMessageLite<
 
     @Override
     public final MessageType buildPartial() {
-      if (isBuilt) {
+      if (!instance.isMutable()) {
         return instance;
       }
 
@@ -1530,7 +1630,7 @@ public abstract class GeneratedMessageLite<
       T instance, CodedInputStream input, ExtensionRegistryLite extensionRegistry)
       throws InvalidProtocolBufferException {
     @SuppressWarnings("unchecked") // Guaranteed by protoc
-    T result = (T) instance.dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
+    T result = instance.newMutableInstance();
     try {
       // TODO(yilunchong): Try to make input with type CodedInpuStream.ArrayDecoder use
       // fast path.
@@ -1561,15 +1661,12 @@ public abstract class GeneratedMessageLite<
       T instance, byte[] input, int offset, int length, ExtensionRegistryLite extensionRegistry)
       throws InvalidProtocolBufferException {
     @SuppressWarnings("unchecked") // Guaranteed by protoc
-    T result = (T) instance.dynamicMethod(MethodToInvoke.NEW_MUTABLE_INSTANCE);
+    T result = instance.newMutableInstance();
     try {
       Schema<T> schema = Protobuf.getInstance().schemaFor(result);
       schema.mergeFrom(
           result, input, offset, offset + length, new ArrayDecoders.Registers(extensionRegistry));
       schema.makeImmutable(result);
-      if (result.memoizedHashCode != 0) {
-        throw new RuntimeException();
-      }
     } catch (InvalidProtocolBufferException e) {
       if (e.getThrownFromInputStream()) {
         e = new InvalidProtocolBufferException(e);
diff --git a/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java b/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java
index 46427b3dd..01d03bade 100644
--- a/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java
+++ b/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java
@@ -133,6 +133,10 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
     return internalGetFieldAccessorTable().descriptor;
   }
 
+  // TODO(b/248143958): This method should be removed. It enables parsing directly into an
+  // "immutable" message. Have to leave it for now to support old gencode.
+  // @deprecated use newBuilder().mergeFrom() instead
+  @Deprecated
   protected void mergeFromAndMakeImmutableInternal(
       CodedInputStream input, ExtensionRegistryLite extensionRegistry)
       throws InvalidProtocolBufferException {
@@ -299,13 +303,14 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
 
   @Override
   public UnknownFieldSet getUnknownFields() {
-    throw new UnsupportedOperationException(
-        "This is supposed to be overridden by subclasses.");
+    return unknownFields;
   }
 
   /**
    * Called by subclasses to parse an unknown field.
    *
+   * <p>TODO(b/248153893) remove this method
+   *
    * @return {@code true} unless the tag is an end-group tag.
    */
   protected boolean parseUnknownField(
@@ -323,6 +328,8 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
   /**
    * Delegates to parseUnknownField. This method is obsolete, but we must retain it for
    * compatibility with older generated code.
+   *
+   * <p>TODO(b/248153893) remove this method
    */
   protected boolean parseUnknownFieldProto3(
       CodedInputStream input,
@@ -547,8 +554,18 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
     // to dispatch dirty invalidations. See GeneratedMessageV3.BuilderListener.
     private boolean isClean;
 
-    private UnknownFieldSet unknownFields =
-        UnknownFieldSet.getDefaultInstance();
+    /**
+     * This field holds either an {@link UnknownFieldSet} or {@link UnknownFieldSet.Builder}.
+     *
+     * <p>We use an object because it should only be one or the other of those things at a time and
+     * Object is the only common base. This also saves space.
+     *
+     * <p>Conversions are lazy: if {@link #setUnknownFields} is called, this will contain {@link
+     * UnknownFieldSet}. If unknown fields are merged into this builder, the current {@link
+     * UnknownFieldSet} will be converted to a {@link UnknownFieldSet.Builder} and left that way
+     * until either {@link #setUnknownFields} or {@link #buildPartial} or {@link #build} is called.
+     */
+    private Object unknownFieldsOrBuilder = UnknownFieldSet.getDefaultInstance();
 
     protected Builder() {
       this(null);
@@ -604,7 +621,7 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
      */
     @Override
     public BuilderType clear() {
-      unknownFields = UnknownFieldSet.getDefaultInstance();
+      unknownFieldsOrBuilder = UnknownFieldSet.getDefaultInstance();
       onChanged();
       return (BuilderType) this;
     }
@@ -757,7 +774,7 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
     }
 
     private BuilderType setUnknownFieldsInternal(final UnknownFieldSet unknownFields) {
-      this.unknownFields = unknownFields;
+      unknownFieldsOrBuilder = unknownFields;
       onChanged();
       return (BuilderType) this;
     }
@@ -776,12 +793,20 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
     }
 
     @Override
-    public BuilderType mergeUnknownFields(
-        final UnknownFieldSet unknownFields) {
-      return setUnknownFields(
-        UnknownFieldSet.newBuilder(this.unknownFields)
-                       .mergeFrom(unknownFields)
-                       .build());
+    public BuilderType mergeUnknownFields(final UnknownFieldSet unknownFields) {
+      if (UnknownFieldSet.getDefaultInstance().equals(unknownFields)) {
+        return (BuilderType) this;
+      }
+
+      if (UnknownFieldSet.getDefaultInstance().equals(unknownFieldsOrBuilder)) {
+        unknownFieldsOrBuilder = unknownFields;
+        onChanged();
+        return (BuilderType) this;
+      }
+
+      getUnknownFieldSetBuilder().mergeFrom(unknownFields);
+      onChanged();
+      return (BuilderType) this;
     }
 
 
@@ -817,7 +842,50 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
 
     @Override
     public final UnknownFieldSet getUnknownFields() {
-      return unknownFields;
+      if (unknownFieldsOrBuilder instanceof UnknownFieldSet) {
+        return (UnknownFieldSet) unknownFieldsOrBuilder;
+      } else {
+        return ((UnknownFieldSet.Builder) unknownFieldsOrBuilder).buildPartial();
+      }
+    }
+
+    /**
+     * Called by generated subclasses to parse an unknown field.
+     *
+     * @return {@code true} unless the tag is an end-group tag.
+     */
+    protected boolean parseUnknownField(
+        CodedInputStream input, ExtensionRegistryLite extensionRegistry, int tag)
+        throws IOException {
+      if (input.shouldDiscardUnknownFields()) {
+        return input.skipField(tag);
+      }
+      return getUnknownFieldSetBuilder().mergeFieldFrom(tag, input);
+    }
+
+    /** Called by generated subclasses to add to the unknown field set. */
+    protected final void mergeUnknownLengthDelimitedField(int number, ByteString bytes) {
+      getUnknownFieldSetBuilder().mergeLengthDelimitedField(number, bytes);
+    }
+
+    /** Called by generated subclasses to add to the unknown field set. */
+    protected final void mergeUnknownVarintField(int number, int value) {
+      getUnknownFieldSetBuilder().mergeVarintField(number, value);
+    }
+
+    @Override
+    protected UnknownFieldSet.Builder getUnknownFieldSetBuilder() {
+      if (unknownFieldsOrBuilder instanceof UnknownFieldSet) {
+        unknownFieldsOrBuilder = ((UnknownFieldSet) unknownFieldsOrBuilder).toBuilder();
+      }
+      onChanged();
+      return (UnknownFieldSet.Builder) unknownFieldsOrBuilder;
+    }
+
+    @Override
+    protected void setUnknownFieldSetBuilder(UnknownFieldSet.Builder builder) {
+      unknownFieldsOrBuilder = builder;
+      onChanged();
     }
 
     /**
@@ -1609,7 +1677,7 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
     private FieldSet<FieldDescriptor> buildExtensions() {
       return extensions == null
           ? (FieldSet<FieldDescriptor>) FieldSet.emptySet()
-          : extensions.build();
+          : extensions.buildPartial();
     }
 
     @Override
@@ -1815,6 +1883,20 @@ public abstract class GeneratedMessageV3 extends AbstractMessage
       }
     }
 
+    @Override
+    protected boolean parseUnknownField(
+        CodedInputStream input, ExtensionRegistryLite extensionRegistry, int tag)
+        throws IOException {
+      ensureExtensionsIsMutable();
+      return MessageReflection.mergeFieldFrom(
+          input,
+          input.shouldDiscardUnknownFields() ? null : getUnknownFieldSetBuilder(),
+          extensionRegistry,
+          getDescriptorForType(),
+          new MessageReflection.ExtensionBuilderAdapter(extensions),
+          tag);
+    }
+
     private void verifyContainingType(final FieldDescriptor field) {
       if (field.getContainingType() != getDescriptorForType()) {
         throw new IllegalArgumentException(
diff --git a/java/core/src/main/java/com/google/protobuf/MessageLiteToString.java b/java/core/src/main/java/com/google/protobuf/MessageLiteToString.java
index 9ad681679..c822dc7ac 100644
--- a/java/core/src/main/java/com/google/protobuf/MessageLiteToString.java
+++ b/java/core/src/main/java/com/google/protobuf/MessageLiteToString.java
@@ -32,12 +32,15 @@ package com.google.protobuf;
 
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
+import java.util.Arrays;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Set;
-import java.util.TreeSet;
+import java.util.TreeMap;
 
 /** Helps generate {@link String} representations of {@link MessageLite} protos. */
 final class MessageLiteToString {
@@ -46,6 +49,11 @@ final class MessageLiteToString {
   private static final String BUILDER_LIST_SUFFIX = "OrBuilderList";
   private static final String MAP_SUFFIX = "Map";
   private static final String BYTES_SUFFIX = "Bytes";
+  private static final char[] INDENT_BUFFER = new char[80];
+
+  static {
+    Arrays.fill(INDENT_BUFFER, ' ');
+  }
 
   /**
    * Returns a {@link String} representation of the {@link MessageLite} object. The first line of
@@ -73,37 +81,51 @@ final class MessageLiteToString {
     // Build a map of method name to method. We're looking for methods like getFoo(), hasFoo(),
     // getFooList() and getFooMap() which might be useful for building an object's string
     // representation.
-    Map<String, Method> nameToNoArgMethod = new HashMap<String, Method>();
-    Map<String, Method> nameToMethod = new HashMap<String, Method>();
-    Set<String> getters = new TreeSet<String>();
+    Set<String> setters = new HashSet<>();
+    Map<String, Method> hazzers = new HashMap<>();
+    Map<String, Method> getters = new TreeMap<>();
     for (Method method : messageLite.getClass().getDeclaredMethods()) {
-      nameToMethod.put(method.getName(), method);
-      if (method.getParameterTypes().length == 0) {
-        nameToNoArgMethod.put(method.getName(), method);
+      if (Modifier.isStatic(method.getModifiers())) {
+        continue;
+      }
+      if (method.getName().length() < 3) {
+        continue;
+      }
 
-        if (method.getName().startsWith("get")) {
-          getters.add(method.getName());
-        }
+      if (method.getName().startsWith("set")) {
+        setters.add(method.getName());
+        continue;
+      }
+
+      if (!Modifier.isPublic(method.getModifiers())) {
+        continue;
+      }
+
+      if (method.getParameterTypes().length != 0) {
+        continue;
+      }
+
+      if (method.getName().startsWith("has")) {
+        hazzers.put(method.getName(), method);
+      } else if (method.getName().startsWith("get")) {
+        getters.put(method.getName(), method);
       }
     }
 
-    for (String getter : getters) {
-      String suffix = getter.startsWith("get") ? getter.substring(3) : getter;
+    for (Entry<String, Method> getter : getters.entrySet()) {
+      String suffix = getter.getKey().substring(3);
       if (suffix.endsWith(LIST_SUFFIX)
           && !suffix.endsWith(BUILDER_LIST_SUFFIX)
           // Sometimes people have fields named 'list' that aren't repeated.
           && !suffix.equals(LIST_SUFFIX)) {
-        String camelCase =
-            suffix.substring(0, 1).toLowerCase()
-                + suffix.substring(1, suffix.length() - LIST_SUFFIX.length());
         // Try to reflectively get the value and toString() the field as if it were repeated. This
         // only works if the method names have not been proguarded out or renamed.
-        Method listMethod = nameToNoArgMethod.get(getter);
+        Method listMethod = getter.getValue();
         if (listMethod != null && listMethod.getReturnType().equals(List.class)) {
           printField(
               buffer,
               indent,
-              camelCaseToSnakeCase(camelCase),
+              suffix.substring(0, suffix.length() - LIST_SUFFIX.length()),
               GeneratedMessageLite.invokeOrDie(listMethod, messageLite));
           continue;
         }
@@ -111,12 +133,9 @@ final class MessageLiteToString {
       if (suffix.endsWith(MAP_SUFFIX)
           // Sometimes people have fields named 'map' that aren't maps.
           && !suffix.equals(MAP_SUFFIX)) {
-        String camelCase =
-            suffix.substring(0, 1).toLowerCase()
-                + suffix.substring(1, suffix.length() - MAP_SUFFIX.length());
         // Try to reflectively get the value and toString() the field as if it were a map. This only
         // works if the method names have not been proguarded out or renamed.
-        Method mapMethod = nameToNoArgMethod.get(getter);
+        Method mapMethod = getter.getValue();
         if (mapMethod != null
             && mapMethod.getReturnType().equals(Map.class)
             // Skip the deprecated getter method with no prefix "Map" when the field name ends with
@@ -127,29 +146,25 @@ final class MessageLiteToString {
           printField(
               buffer,
               indent,
-              camelCaseToSnakeCase(camelCase),
+              suffix.substring(0, suffix.length() - MAP_SUFFIX.length()),
               GeneratedMessageLite.invokeOrDie(mapMethod, messageLite));
           continue;
         }
       }
 
-      Method setter = nameToMethod.get("set" + suffix);
-      if (setter == null) {
+      if (!setters.contains("set" + suffix)) {
         continue;
       }
       if (suffix.endsWith(BYTES_SUFFIX)
-          && nameToNoArgMethod.containsKey(
-              "get" + suffix.substring(0, suffix.length() - "Bytes".length()))) {
+          && getters.containsKey("get" + suffix.substring(0, suffix.length() - "Bytes".length()))) {
         // Heuristic to skip bytes based accessors for string fields.
         continue;
       }
 
-      String camelCase = suffix.substring(0, 1).toLowerCase() + suffix.substring(1);
-
       // Try to reflectively get the value and toString() the field as if it were optional. This
       // only works if the method names have not been proguarded out or renamed.
-      Method getMethod = nameToNoArgMethod.get("get" + suffix);
-      Method hasMethod = nameToNoArgMethod.get("has" + suffix);
+      Method getMethod = getter.getValue();
+      Method hasMethod = hazzers.get("has" + suffix);
       // TODO(dweis): Fix proto3 semantics.
       if (getMethod != null) {
         Object value = GeneratedMessageLite.invokeOrDie(getMethod, messageLite);
@@ -159,7 +174,7 @@ final class MessageLiteToString {
                 : (Boolean) GeneratedMessageLite.invokeOrDie(hasMethod, messageLite);
         // TODO(dweis): This doesn't stop printing oneof case twice: value and enum style.
         if (hasValue) {
-          printField(buffer, indent, camelCaseToSnakeCase(camelCase), value);
+          printField(buffer, indent, suffix, value);
         }
         continue;
       }
@@ -215,10 +230,10 @@ final class MessageLiteToString {
    *
    * @param buffer the buffer to write to
    * @param indent the number of spaces the proto should be indented by
-   * @param name the field name (in lower underscore case)
+   * @param name the field name (in PascalCase)
    * @param object the object value of the field
    */
-  static final void printField(StringBuilder buffer, int indent, String name, Object object) {
+  static void printField(StringBuilder buffer, int indent, String name, Object object) {
     if (object instanceof List<?>) {
       List<?> list = (List<?>) object;
       for (Object entry : list) {
@@ -235,10 +250,8 @@ final class MessageLiteToString {
     }
 
     buffer.append('\n');
-    for (int i = 0; i < indent; i++) {
-      buffer.append(' ');
-    }
-    buffer.append(name);
+    indent(indent, buffer);
+    buffer.append(pascalCaseToSnakeCase(name));
 
     if (object instanceof String) {
       buffer.append(": \"").append(TextFormatEscaper.escapeText((String) object)).append('"');
@@ -248,9 +261,7 @@ final class MessageLiteToString {
       buffer.append(" {");
       reflectivePrintWithIndent((GeneratedMessageLite<?, ?>) object, buffer, indent + 2);
       buffer.append("\n");
-      for (int i = 0; i < indent; i++) {
-        buffer.append(' ');
-      }
+      indent(indent, buffer);
       buffer.append("}");
     } else if (object instanceof Map.Entry<?, ?>) {
       buffer.append(" {");
@@ -258,19 +269,33 @@ final class MessageLiteToString {
       printField(buffer, indent + 2, "key", entry.getKey());
       printField(buffer, indent + 2, "value", entry.getValue());
       buffer.append("\n");
-      for (int i = 0; i < indent; i++) {
-        buffer.append(' ');
-      }
+      indent(indent, buffer);
       buffer.append("}");
     } else {
       buffer.append(": ").append(object.toString());
     }
   }
 
-  private static final String camelCaseToSnakeCase(String camelCase) {
+  private static void indent(int indent, StringBuilder buffer) {
+    while (indent > 0) {
+      int partialIndent = indent;
+      if (partialIndent > INDENT_BUFFER.length) {
+        partialIndent = INDENT_BUFFER.length;
+      }
+      buffer.append(INDENT_BUFFER, 0, partialIndent);
+      indent -= partialIndent;
+    }
+  }
+
+  private static String pascalCaseToSnakeCase(String pascalCase) {
+    if (pascalCase.isEmpty()) {
+      return pascalCase;
+    }
+
     StringBuilder builder = new StringBuilder();
-    for (int i = 0; i < camelCase.length(); i++) {
-      char ch = camelCase.charAt(i);
+    builder.append(Character.toLowerCase(pascalCase.charAt(0)));
+    for (int i = 1; i < pascalCase.length(); i++) {
+      char ch = pascalCase.charAt(i);
       if (Character.isUpperCase(ch)) {
         builder.append("_");
       }
diff --git a/java/core/src/main/java/com/google/protobuf/MessageReflection.java b/java/core/src/main/java/com/google/protobuf/MessageReflection.java
index 6741e1cb9..0028adc7c 100644
--- a/java/core/src/main/java/com/google/protobuf/MessageReflection.java
+++ b/java/core/src/main/java/com/google/protobuf/MessageReflection.java
@@ -30,6 +30,7 @@
 
 package com.google.protobuf;
 
+import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor;
 import java.io.IOException;
 import java.util.ArrayList;
@@ -323,6 +324,34 @@ class MessageReflection {
         Message defaultInstance)
         throws IOException;
 
+    /**
+     * Read the given group field from the wire, merging with the existing field if it is already
+     * present.
+     *
+     * <p>For extensions, defaultInstance must be specified. For regular fields, defaultInstance can
+     * be null.
+     */
+    void mergeGroup(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException;
+
+    /**
+     * Read the given message field from the wire, merging with the existing field if it is already
+     * present.
+     *
+     * <p>For extensions, defaultInstance must be specified. For regular fields, defaultInstance can
+     * be null.
+     */
+    void mergeMessage(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException;
+
     /** Returns the UTF8 validation level for the field. */
     WireFormat.Utf8Validation getUtf8Validation(Descriptors.FieldDescriptor descriptor);
 
@@ -349,6 +378,7 @@ class MessageReflection {
   static class BuilderAdapter implements MergeTarget {
 
     private final Message.Builder builder;
+    private boolean hasNestedBuilders = true;
 
     @Override
     public Descriptors.Descriptor getDescriptorForType() {
@@ -364,6 +394,17 @@ class MessageReflection {
       return builder.getField(field);
     }
 
+    private Message.Builder getFieldBuilder(Descriptors.FieldDescriptor field) {
+      if (hasNestedBuilders) {
+        try {
+          return builder.getFieldBuilder(field);
+        } catch (UnsupportedOperationException e) {
+          hasNestedBuilders = false;
+        }
+      }
+      return null;
+    }
+
     @Override
     public boolean hasField(Descriptors.FieldDescriptor field) {
       return builder.hasField(field);
@@ -371,6 +412,12 @@ class MessageReflection {
 
     @Override
     public MergeTarget setField(Descriptors.FieldDescriptor field, Object value) {
+      if (!field.isRepeated() && value instanceof MessageLite.Builder) {
+        if (value != getFieldBuilder(field)) {
+          builder.setField(field, ((MessageLite.Builder) value).buildPartial());
+        }
+        return this;
+      }
       builder.setField(field, value);
       return this;
     }
@@ -384,12 +431,18 @@ class MessageReflection {
     @Override
     public MergeTarget setRepeatedField(
         Descriptors.FieldDescriptor field, int index, Object value) {
+      if (value instanceof MessageLite.Builder) {
+        value = ((MessageLite.Builder) value).buildPartial();
+      }
       builder.setRepeatedField(field, index, value);
       return this;
     }
 
     @Override
     public MergeTarget addRepeatedField(Descriptors.FieldDescriptor field, Object value) {
+      if (value instanceof MessageLite.Builder) {
+        value = ((MessageLite.Builder) value).buildPartial();
+      }
       builder.addRepeatedField(field, value);
       return this;
     }
@@ -500,14 +553,87 @@ class MessageReflection {
     }
 
     @Override
+    public void mergeGroup(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        Message.Builder subBuilder;
+        if (hasField(field)) {
+          subBuilder = getFieldBuilder(field);
+          if (subBuilder != null) {
+            input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+            return;
+          } else {
+            subBuilder = newMessageFieldInstance(field, defaultInstance);
+            subBuilder.mergeFrom((Message) getField(field));
+          }
+        } else {
+          subBuilder = newMessageFieldInstance(field, defaultInstance);
+        }
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder.buildPartial());
+      } else {
+        Message.Builder subBuilder = newMessageFieldInstance(field, defaultInstance);
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    @Override
+    public void mergeMessage(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        Descriptors.FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        Message.Builder subBuilder;
+        if (hasField(field)) {
+          subBuilder = getFieldBuilder(field);
+          if (subBuilder != null) {
+            input.readMessage(subBuilder, extensionRegistry);
+            return;
+          } else {
+            subBuilder = newMessageFieldInstance(field, defaultInstance);
+            subBuilder.mergeFrom((Message) getField(field));
+          }
+        } else {
+          subBuilder = newMessageFieldInstance(field, defaultInstance);
+        }
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder.buildPartial());
+      } else {
+        Message.Builder subBuilder = newMessageFieldInstance(field, defaultInstance);
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    private Message.Builder newMessageFieldInstance(
+        FieldDescriptor field, Message defaultInstance) {
+      // When default instance is not null. The field is an extension field.
+      if (defaultInstance != null) {
+        return defaultInstance.newBuilderForType();
+      } else {
+        return builder.newBuilderForField(field);
+      }
+    }
+
+    @Override
     public MergeTarget newMergeTargetForField(
         Descriptors.FieldDescriptor field, Message defaultInstance) {
       Message.Builder subBuilder;
-      if (defaultInstance != null) {
-        subBuilder = defaultInstance.newBuilderForType();
-      } else {
-        subBuilder = builder.newBuilderForField(field);
+      if (!field.isRepeated() && hasField(field)) {
+        subBuilder = getFieldBuilder(field);
+        if (subBuilder != null) {
+          return new BuilderAdapter(subBuilder);
+        }
       }
+
+      subBuilder = newMessageFieldInstance(field, defaultInstance);
       if (!field.isRepeated()) {
         Message originalMessage = (Message) getField(field);
         if (originalMessage != null) {
@@ -543,7 +669,7 @@ class MessageReflection {
 
     @Override
     public Object finish() {
-      return builder.buildPartial();
+      return builder;
     }
   }
 
@@ -666,6 +792,281 @@ class MessageReflection {
     }
 
     @Override
+    public void mergeGroup(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        if (hasField(field)) {
+          MessageLite.Builder current = ((MessageLite) getField(field)).toBuilder();
+          input.readGroup(field.getNumber(), current, extensionRegistry);
+          Object unused = setField(field, current.buildPartial());
+          return;
+        }
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder.buildPartial());
+      } else {
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    @Override
+    public void mergeMessage(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        if (hasField(field)) {
+          MessageLite.Builder current = ((MessageLite) getField(field)).toBuilder();
+          input.readMessage(current, extensionRegistry);
+          Object unused = setField(field, current.buildPartial());
+          return;
+        }
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder.buildPartial());
+      } else {
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    @Override
+    public Object parseMessageFromBytes(
+        ByteString bytes,
+        ExtensionRegistryLite registry,
+        Descriptors.FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      Message.Builder subBuilder = defaultInstance.newBuilderForType();
+      if (!field.isRepeated()) {
+        Message originalMessage = (Message) getField(field);
+        if (originalMessage != null) {
+          subBuilder.mergeFrom(originalMessage);
+        }
+      }
+      subBuilder.mergeFrom(bytes, registry);
+      return subBuilder.buildPartial();
+    }
+
+    @Override
+    public MergeTarget newMergeTargetForField(
+        Descriptors.FieldDescriptor descriptor, Message defaultInstance) {
+      throw new UnsupportedOperationException("newMergeTargetForField() called on FieldSet object");
+    }
+
+    @Override
+    public MergeTarget newEmptyTargetForField(
+        Descriptors.FieldDescriptor descriptor, Message defaultInstance) {
+      throw new UnsupportedOperationException("newEmptyTargetForField() called on FieldSet object");
+    }
+
+    @Override
+    public WireFormat.Utf8Validation getUtf8Validation(Descriptors.FieldDescriptor descriptor) {
+      if (descriptor.needsUtf8Check()) {
+        return WireFormat.Utf8Validation.STRICT;
+      }
+      // TODO(b/248145492): support lazy strings for ExtesnsionSet.
+      return WireFormat.Utf8Validation.LOOSE;
+    }
+
+    @Override
+    public Object finish() {
+      throw new UnsupportedOperationException("finish() called on FieldSet object");
+    }
+  }
+
+  static class ExtensionBuilderAdapter implements MergeTarget {
+
+    private final FieldSet.Builder<Descriptors.FieldDescriptor> extensions;
+
+    ExtensionBuilderAdapter(FieldSet.Builder<Descriptors.FieldDescriptor> extensions) {
+      this.extensions = extensions;
+    }
+
+    @Override
+    public Descriptors.Descriptor getDescriptorForType() {
+      throw new UnsupportedOperationException("getDescriptorForType() called on FieldSet object");
+    }
+
+    @Override
+    public Object getField(Descriptors.FieldDescriptor field) {
+      return extensions.getField(field);
+    }
+
+    @Override
+    public boolean hasField(Descriptors.FieldDescriptor field) {
+      return extensions.hasField(field);
+    }
+
+    @Override
+    @CanIgnoreReturnValue
+    public MergeTarget setField(Descriptors.FieldDescriptor field, Object value) {
+      extensions.setField(field, value);
+      return this;
+    }
+
+    @Override
+    @CanIgnoreReturnValue
+    public MergeTarget clearField(Descriptors.FieldDescriptor field) {
+      extensions.clearField(field);
+      return this;
+    }
+
+    @Override
+    @CanIgnoreReturnValue
+    public MergeTarget setRepeatedField(
+        Descriptors.FieldDescriptor field, int index, Object value) {
+      extensions.setRepeatedField(field, index, value);
+      return this;
+    }
+
+    @Override
+    @CanIgnoreReturnValue
+    public MergeTarget addRepeatedField(Descriptors.FieldDescriptor field, Object value) {
+      extensions.addRepeatedField(field, value);
+      return this;
+    }
+
+    @Override
+    public boolean hasOneof(Descriptors.OneofDescriptor oneof) {
+      return false;
+    }
+
+    @Override
+    @CanIgnoreReturnValue
+    public MergeTarget clearOneof(Descriptors.OneofDescriptor oneof) {
+      // Nothing to clear.
+      return this;
+    }
+
+    @Override
+    public Descriptors.FieldDescriptor getOneofFieldDescriptor(Descriptors.OneofDescriptor oneof) {
+      return null;
+    }
+
+    @Override
+    public ContainerType getContainerType() {
+      return ContainerType.EXTENSION_SET;
+    }
+
+    @Override
+    public ExtensionRegistry.ExtensionInfo findExtensionByName(
+        ExtensionRegistry registry, String name) {
+      return registry.findImmutableExtensionByName(name);
+    }
+
+    @Override
+    public ExtensionRegistry.ExtensionInfo findExtensionByNumber(
+        ExtensionRegistry registry, Descriptors.Descriptor containingType, int fieldNumber) {
+      return registry.findImmutableExtensionByNumber(containingType, fieldNumber);
+    }
+
+    @Override
+    public Object parseGroup(
+        CodedInputStream input,
+        ExtensionRegistryLite registry,
+        Descriptors.FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      Message.Builder subBuilder = defaultInstance.newBuilderForType();
+      if (!field.isRepeated()) {
+        Message originalMessage = (Message) getField(field);
+        if (originalMessage != null) {
+          subBuilder.mergeFrom(originalMessage);
+        }
+      }
+      input.readGroup(field.getNumber(), subBuilder, registry);
+      return subBuilder.buildPartial();
+    }
+
+    @Override
+    public Object parseMessage(
+        CodedInputStream input,
+        ExtensionRegistryLite registry,
+        Descriptors.FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      Message.Builder subBuilder = defaultInstance.newBuilderForType();
+      if (!field.isRepeated()) {
+        Message originalMessage = (Message) getField(field);
+        if (originalMessage != null) {
+          subBuilder.mergeFrom(originalMessage);
+        }
+      }
+      input.readMessage(subBuilder, registry);
+      return subBuilder.buildPartial();
+    }
+
+    @Override
+    public void mergeGroup(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        if (hasField(field)) {
+          Object fieldOrBuilder = extensions.getFieldAllowBuilders(field);
+          MessageLite.Builder subBuilder;
+          if (fieldOrBuilder instanceof MessageLite.Builder) {
+            subBuilder = (MessageLite.Builder) fieldOrBuilder;
+          } else {
+            subBuilder = ((MessageLite) fieldOrBuilder).toBuilder();
+            extensions.setField(field, subBuilder);
+          }
+          input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+          return;
+        }
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder);
+      } else {
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readGroup(field.getNumber(), subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    @Override
+    public void mergeMessage(
+        CodedInputStream input,
+        ExtensionRegistryLite extensionRegistry,
+        FieldDescriptor field,
+        Message defaultInstance)
+        throws IOException {
+      if (!field.isRepeated()) {
+        if (hasField(field)) {
+          Object fieldOrBuilder = extensions.getFieldAllowBuilders(field);
+          MessageLite.Builder subBuilder;
+          if (fieldOrBuilder instanceof MessageLite.Builder) {
+            subBuilder = (MessageLite.Builder) fieldOrBuilder;
+          } else {
+            subBuilder = ((MessageLite) fieldOrBuilder).toBuilder();
+            extensions.setField(field, subBuilder);
+          }
+          input.readMessage(subBuilder, extensionRegistry);
+          return;
+        }
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = setField(field, subBuilder);
+      } else {
+        Message.Builder subBuilder = defaultInstance.newBuilderForType();
+        input.readMessage(subBuilder, extensionRegistry);
+        Object unused = addRepeatedField(field, subBuilder.buildPartial());
+      }
+    }
+
+    @Override
     public Object parseMessageFromBytes(
         ByteString bytes,
         ExtensionRegistryLite registry,
@@ -700,7 +1101,7 @@ class MessageReflection {
       if (descriptor.needsUtf8Check()) {
         return WireFormat.Utf8Validation.STRICT;
       }
-      // TODO(liujisi): support lazy strings for ExtesnsionSet.
+      // TODO(b/248145492): support lazy strings for ExtesnsionSet.
       return WireFormat.Utf8Validation.LOOSE;
     }
 
@@ -829,13 +1230,13 @@ class MessageReflection {
       switch (field.getType()) {
         case GROUP:
           {
-            value = target.parseGroup(input, extensionRegistry, field, defaultInstance);
-            break;
+            target.mergeGroup(input, extensionRegistry, field, defaultInstance);
+            return true;
           }
         case MESSAGE:
           {
-            value = target.parseMessage(input, extensionRegistry, field, defaultInstance);
-            break;
+            target.mergeMessage(input, extensionRegistry, field, defaultInstance);
+            return true;
           }
         case ENUM:
           final int rawValue = input.readEnum();
@@ -870,6 +1271,29 @@ class MessageReflection {
     return true;
   }
 
+  /** Read a message from the given input stream into the provided target and UnknownFieldSet. */
+  static void mergeMessageFrom(
+      Message.Builder target,
+      UnknownFieldSet.Builder unknownFields,
+      CodedInputStream input,
+      ExtensionRegistryLite extensionRegistry)
+      throws IOException {
+    BuilderAdapter builderAdapter = new BuilderAdapter(target);
+    Descriptor descriptorForType = target.getDescriptorForType();
+    while (true) {
+      final int tag = input.readTag();
+      if (tag == 0) {
+        break;
+      }
+
+      if (!mergeFieldFrom(
+          input, unknownFields, extensionRegistry, descriptorForType, builderAdapter, tag)) {
+        // end group tag
+        break;
+      }
+    }
+  }
+
   /** Called by {@code #mergeFieldFrom()} to parse a MessageSet extension into MergeTarget. */
   private static void mergeMessageSetExtensionFromCodedStream(
       CodedInputStream input,
diff --git a/java/core/src/main/java/com/google/protobuf/MessageSchema.java b/java/core/src/main/java/com/google/protobuf/MessageSchema.java
index 4170f4fbe..76220cc47 100644
--- a/java/core/src/main/java/com/google/protobuf/MessageSchema.java
+++ b/java/core/src/main/java/com/google/protobuf/MessageSchema.java
@@ -42,7 +42,6 @@ import static com.google.protobuf.ArrayDecoders.decodeFixed64;
 import static com.google.protobuf.ArrayDecoders.decodeFixed64List;
 import static com.google.protobuf.ArrayDecoders.decodeFloat;
 import static com.google.protobuf.ArrayDecoders.decodeFloatList;
-import static com.google.protobuf.ArrayDecoders.decodeGroupField;
 import static com.google.protobuf.ArrayDecoders.decodeGroupList;
 import static com.google.protobuf.ArrayDecoders.decodeMessageField;
 import static com.google.protobuf.ArrayDecoders.decodeMessageList;
@@ -66,6 +65,8 @@ import static com.google.protobuf.ArrayDecoders.decodeVarint32;
 import static com.google.protobuf.ArrayDecoders.decodeVarint32List;
 import static com.google.protobuf.ArrayDecoders.decodeVarint64;
 import static com.google.protobuf.ArrayDecoders.decodeVarint64List;
+import static com.google.protobuf.ArrayDecoders.mergeGroupField;
+import static com.google.protobuf.ArrayDecoders.mergeMessageField;
 import static com.google.protobuf.ArrayDecoders.skipField;
 
 import com.google.protobuf.ArrayDecoders.Registers;
@@ -1176,6 +1177,7 @@ final class MessageSchema<T> implements Schema<T> {
 
   @Override
   public void mergeFrom(T message, T other) {
+    checkMutable(message);
     if (other == null) {
       throw new NullPointerException();
     }
@@ -1374,47 +1376,83 @@ final class MessageSchema<T> implements Schema<T> {
     }
   }
 
-  private void mergeMessage(T message, T other, int pos) {
+  private void mergeMessage(T targetParent, T sourceParent, int pos) {
+    if (!isFieldPresent(sourceParent, pos)) {
+      return;
+    }
+
     final int typeAndOffset = typeAndOffsetAt(pos);
     final long offset = offset(typeAndOffset);
 
-    if (!isFieldPresent(other, pos)) {
+    final Object source = UNSAFE.getObject(sourceParent, offset);
+    if (source == null) {
+      throw new IllegalStateException(
+          "Source subfield " + numberAt(pos) + " is present but null: " + sourceParent);
+    }
+
+    final Schema fieldSchema = getMessageFieldSchema(pos);
+    if (!isFieldPresent(targetParent, pos)) {
+      if (!isMutable(source)) {
+        // Can safely share source if it is immutable
+        UNSAFE.putObject(targetParent, offset, source);
+      } else {
+        // Make a safetey copy of source
+        final Object copyOfSource = fieldSchema.newInstance();
+        fieldSchema.mergeFrom(copyOfSource, source);
+        UNSAFE.putObject(targetParent, offset, copyOfSource);
+      }
+      setFieldPresent(targetParent, pos);
       return;
     }
 
-    Object mine = UnsafeUtil.getObject(message, offset);
-    Object theirs = UnsafeUtil.getObject(other, offset);
-    if (mine != null && theirs != null) {
-      Object merged = Internal.mergeMessage(mine, theirs);
-      UnsafeUtil.putObject(message, offset, merged);
-      setFieldPresent(message, pos);
-    } else if (theirs != null) {
-      UnsafeUtil.putObject(message, offset, theirs);
-      setFieldPresent(message, pos);
+    // Sub-message is present, merge from source
+    Object target = UNSAFE.getObject(targetParent, offset);
+    if (!isMutable(target)) {
+      Object newInstance = fieldSchema.newInstance();
+      fieldSchema.mergeFrom(newInstance, target);
+      UNSAFE.putObject(targetParent, offset, newInstance);
+      target = newInstance;
     }
+    fieldSchema.mergeFrom(target, source);
   }
 
-  private void mergeOneofMessage(T message, T other, int pos) {
-    int typeAndOffset = typeAndOffsetAt(pos);
+  private void mergeOneofMessage(T targetParent, T sourceParent, int pos) {
     int number = numberAt(pos);
-    long offset = offset(typeAndOffset);
-
-    if (!isOneofPresent(other, number, pos)) {
+    if (!isOneofPresent(sourceParent, number, pos)) {
       return;
     }
-    Object mine = null;
-    if (isOneofPresent(message, number, pos)) {
-      mine = UnsafeUtil.getObject(message, offset);
+
+    long offset = offset(typeAndOffsetAt(pos));
+    final Object source = UNSAFE.getObject(sourceParent, offset);
+    if (source == null) {
+      throw new IllegalStateException(
+          "Source subfield " + numberAt(pos) + " is present but null: " + sourceParent);
     }
-    Object theirs = UnsafeUtil.getObject(other, offset);
-    if (mine != null && theirs != null) {
-      Object merged = Internal.mergeMessage(mine, theirs);
-      UnsafeUtil.putObject(message, offset, merged);
-      setOneofPresent(message, number, pos);
-    } else if (theirs != null) {
-      UnsafeUtil.putObject(message, offset, theirs);
-      setOneofPresent(message, number, pos);
+
+    final Schema fieldSchema = getMessageFieldSchema(pos);
+    if (!isOneofPresent(targetParent, number, pos)) {
+      if (!isMutable(source)) {
+        // Can safely share source if it is immutable
+        UNSAFE.putObject(targetParent, offset, source);
+      } else {
+        // Make a safety copy of theirs
+        final Object copyOfSource = fieldSchema.newInstance();
+        fieldSchema.mergeFrom(copyOfSource, source);
+        UNSAFE.putObject(targetParent, offset, copyOfSource);
+      }
+      setOneofPresent(targetParent, number, pos);
+      return;
     }
+
+    // Sub-message is present, merge from source
+    Object target = UNSAFE.getObject(targetParent, offset);
+    if (!isMutable(target)) {
+      Object newInstance = fieldSchema.newInstance();
+      fieldSchema.mergeFrom(newInstance, target);
+      UNSAFE.putObject(targetParent, offset, newInstance);
+      target = newInstance;
+    }
+    fieldSchema.mergeFrom(target, source);
   }
 
   @Override
@@ -3853,6 +3891,7 @@ final class MessageSchema<T> implements Schema<T> {
     if (extensionRegistry == null) {
       throw new NullPointerException();
     }
+    checkMutable(message);
     mergeFromHelper(unknownFieldSchema, extensionSchema, message, reader, extensionRegistry);
   }
 
@@ -3889,6 +3928,7 @@ final class MessageSchema<T> implements Schema<T> {
             }
             unknownFields =
                 extensionSchema.parseExtension(
+                    message,
                     reader,
                     extension,
                     extensionRegistry,
@@ -3955,21 +3995,10 @@ final class MessageSchema<T> implements Schema<T> {
               break;
             case 9:
               { // MESSAGE:
-                if (isFieldPresent(message, pos)) {
-                  Object mergedResult =
-                      Internal.mergeMessage(
-                          UnsafeUtil.getObject(message, offset(typeAndOffset)),
-                          reader.readMessageBySchemaWithCheck(
-                              (Schema<T>) getMessageFieldSchema(pos), extensionRegistry));
-                  UnsafeUtil.putObject(message, offset(typeAndOffset), mergedResult);
-                } else {
-                  UnsafeUtil.putObject(
-                      message,
-                      offset(typeAndOffset),
-                      reader.readMessageBySchemaWithCheck(
-                          (Schema<T>) getMessageFieldSchema(pos), extensionRegistry));
-                  setFieldPresent(message, pos);
-                }
+                final MessageLite current = (MessageLite) mutableMessageFieldForMerge(message, pos);
+                reader.mergeMessageField(
+                    current, (Schema<MessageLite>) getMessageFieldSchema(pos), extensionRegistry);
+                storeMessageField(message, pos, current);
                 break;
               }
             case 10: // BYTES:
@@ -3990,7 +4019,7 @@ final class MessageSchema<T> implements Schema<T> {
                 } else {
                   unknownFields =
                       SchemaUtil.storeUnknownEnum(
-                          number, enumValue, unknownFields, unknownFieldSchema);
+                          message, number, enumValue, unknownFields, unknownFieldSchema);
                 }
                 break;
               }
@@ -4012,21 +4041,10 @@ final class MessageSchema<T> implements Schema<T> {
               break;
             case 17:
               { // GROUP:
-                if (isFieldPresent(message, pos)) {
-                  Object mergedResult =
-                      Internal.mergeMessage(
-                          UnsafeUtil.getObject(message, offset(typeAndOffset)),
-                          reader.readGroupBySchemaWithCheck(
-                              (Schema<T>) getMessageFieldSchema(pos), extensionRegistry));
-                  UnsafeUtil.putObject(message, offset(typeAndOffset), mergedResult);
-                } else {
-                  UnsafeUtil.putObject(
-                      message,
-                      offset(typeAndOffset),
-                      reader.readGroupBySchemaWithCheck(
-                          (Schema<T>) getMessageFieldSchema(pos), extensionRegistry));
-                  setFieldPresent(message, pos);
-                }
+                final MessageLite current = (MessageLite) mutableMessageFieldForMerge(message, pos);
+                reader.mergeGroupField(
+                    current, (Schema<MessageLite>) getMessageFieldSchema(pos), extensionRegistry);
+                storeMessageField(message, pos, current);
                 break;
               }
             case 18: // DOUBLE_LIST:
@@ -4089,6 +4107,7 @@ final class MessageSchema<T> implements Schema<T> {
                 reader.readEnumList(enumList);
                 unknownFields =
                     SchemaUtil.filterUnknownEnumList(
+                        message,
                         number,
                         enumList,
                         getEnumFieldVerifier(pos),
@@ -4155,6 +4174,7 @@ final class MessageSchema<T> implements Schema<T> {
                 reader.readEnumList(enumList);
                 unknownFields =
                     SchemaUtil.filterUnknownEnumList(
+                        message,
                         number,
                         enumList,
                         getEnumFieldVerifier(pos),
@@ -4235,24 +4255,15 @@ final class MessageSchema<T> implements Schema<T> {
               readString(message, typeAndOffset, reader);
               setOneofPresent(message, number, pos);
               break;
-            case 60: // ONEOF_MESSAGE:
-              if (isOneofPresent(message, number, pos)) {
-                Object mergedResult =
-                    Internal.mergeMessage(
-                        UnsafeUtil.getObject(message, offset(typeAndOffset)),
-                        reader.readMessageBySchemaWithCheck(
-                            getMessageFieldSchema(pos), extensionRegistry));
-                UnsafeUtil.putObject(message, offset(typeAndOffset), mergedResult);
-              } else {
-                UnsafeUtil.putObject(
-                    message,
-                    offset(typeAndOffset),
-                    reader.readMessageBySchemaWithCheck(
-                        getMessageFieldSchema(pos), extensionRegistry));
-                setFieldPresent(message, pos);
+            case 60:
+              { // ONEOF_MESSAGE:
+                final MessageLite current =
+                    (MessageLite) mutableOneofMessageFieldForMerge(message, number, pos);
+                reader.mergeMessageField(
+                    current, (Schema<MessageLite>) getMessageFieldSchema(pos), extensionRegistry);
+                storeOneofMessageField(message, number, pos, current);
+                break;
               }
-              setOneofPresent(message, number, pos);
-              break;
             case 61: // ONEOF_BYTES:
               UnsafeUtil.putObject(message, offset(typeAndOffset), reader.readBytes());
               setOneofPresent(message, number, pos);
@@ -4272,7 +4283,7 @@ final class MessageSchema<T> implements Schema<T> {
                 } else {
                   unknownFields =
                       SchemaUtil.storeUnknownEnum(
-                          number, enumValue, unknownFields, unknownFieldSchema);
+                          message, number, enumValue, unknownFields, unknownFieldSchema);
                 }
                 break;
               }
@@ -4296,17 +4307,19 @@ final class MessageSchema<T> implements Schema<T> {
                   message, offset(typeAndOffset), Long.valueOf(reader.readSInt64()));
               setOneofPresent(message, number, pos);
               break;
-            case 68: // ONEOF_GROUP:
-              UnsafeUtil.putObject(
-                  message,
-                  offset(typeAndOffset),
-                  reader.readGroupBySchemaWithCheck(getMessageFieldSchema(pos), extensionRegistry));
-              setOneofPresent(message, number, pos);
-              break;
+            case 68:
+              { // ONEOF_GROUP:
+                final MessageLite current =
+                    (MessageLite) mutableOneofMessageFieldForMerge(message, number, pos);
+                reader.mergeGroupField(
+                    current, (Schema<MessageLite>) getMessageFieldSchema(pos), extensionRegistry);
+                storeOneofMessageField(message, number, pos, current);
+                break;
+              }
             default:
               // Assume we've landed on an empty entry. Treat it as an unknown field.
               if (unknownFields == null) {
-                unknownFields = unknownFieldSchema.newBuilder();
+                unknownFields = unknownFieldSchema.getBuilderFromMessage(message);
               }
               if (!unknownFieldSchema.mergeOneFieldFrom(unknownFields, reader)) {
                 return;
@@ -4333,7 +4346,8 @@ final class MessageSchema<T> implements Schema<T> {
     } finally {
       for (int i = checkInitializedCount; i < repeatedFieldOffsetStart; i++) {
         unknownFields =
-            filterMapUnknownEnumValues(message, intArray[i], unknownFields, unknownFieldSchema);
+            filterMapUnknownEnumValues(
+                message, intArray[i], unknownFields, unknownFieldSchema, message);
       }
       if (unknownFields != null) {
         unknownFieldSchema.setBuilderToMessage(message, unknownFields);
@@ -4343,6 +4357,8 @@ final class MessageSchema<T> implements Schema<T> {
 
   @SuppressWarnings("ReferenceEquality")
   static UnknownFieldSetLite getMutableUnknownFields(Object message) {
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
     UnknownFieldSetLite unknownFields = ((GeneratedMessageLite) message).unknownFields;
     if (unknownFields == UnknownFieldSetLite.getDefaultInstance()) {
       unknownFields = UnknownFieldSetLite.newInstance();
@@ -4603,24 +4619,13 @@ final class MessageSchema<T> implements Schema<T> {
         } else {
           break;
         }
-        UnknownFieldSetLite unknownFields = ((GeneratedMessageLite) message).unknownFields;
-        if (unknownFields == UnknownFieldSetLite.getDefaultInstance()) {
-          // filterUnknownEnumList() expects the unknownFields parameter to be mutable or null.
-          // Since we don't know yet whether there exist unknown enum values, we'd better pass
-          // null to it instead of allocating a mutable instance. This is also needed to be
-          // consistent with the behavior of generated parser/builder.
-          unknownFields = null;
-        }
-        unknownFields =
-            SchemaUtil.filterUnknownEnumList(
-                number,
-                (ProtobufList<Integer>) list,
-                getEnumFieldVerifier(bufferPosition),
-                unknownFields,
-                (UnknownFieldSchema<UnknownFieldSetLite, UnknownFieldSetLite>) unknownFieldSchema);
-        if (unknownFields != null) {
-          ((GeneratedMessageLite) message).unknownFields = unknownFields;
-        }
+        SchemaUtil.filterUnknownEnumList(
+            message,
+            number,
+            (ProtobufList<Integer>) list,
+            getEnumFieldVerifier(bufferPosition),
+            null,
+            (UnknownFieldSchema<UnknownFieldSetLite, UnknownFieldSetLite>) unknownFieldSchema);
         break;
       case 33: // SINT32_LIST:
       case 47: // SINT32_LIST_PACKED:
@@ -4774,20 +4779,11 @@ final class MessageSchema<T> implements Schema<T> {
         break;
       case 60: // ONEOF_MESSAGE:
         if (wireType == WireFormat.WIRETYPE_LENGTH_DELIMITED) {
+          final Object current = mutableOneofMessageFieldForMerge(message, number, bufferPosition);
           position =
-              decodeMessageField(
-                  getMessageFieldSchema(bufferPosition), data, position, limit, registers);
-          final Object oldValue =
-              unsafe.getInt(message, oneofCaseOffset) == number
-                  ? unsafe.getObject(message, fieldOffset)
-                  : null;
-          if (oldValue == null) {
-            unsafe.putObject(message, fieldOffset, registers.object1);
-          } else {
-            unsafe.putObject(
-                message, fieldOffset, Internal.mergeMessage(oldValue, registers.object1));
-          }
-          unsafe.putInt(message, oneofCaseOffset, number);
+              mergeMessageField(
+                  current, getMessageFieldSchema(bufferPosition), data, position, limit, registers);
+          storeOneofMessageField(message, number, bufferPosition, current);
         }
         break;
       case 61: // ONEOF_BYTES:
@@ -4827,21 +4823,18 @@ final class MessageSchema<T> implements Schema<T> {
         break;
       case 68: // ONEOF_GROUP:
         if (wireType == WireFormat.WIRETYPE_START_GROUP) {
+          final Object current = mutableOneofMessageFieldForMerge(message, number, bufferPosition);
           final int endTag = (tag & ~0x7) | WireFormat.WIRETYPE_END_GROUP;
           position =
-              decodeGroupField(
-                  getMessageFieldSchema(bufferPosition), data, position, limit, endTag, registers);
-          final Object oldValue =
-              unsafe.getInt(message, oneofCaseOffset) == number
-                  ? unsafe.getObject(message, fieldOffset)
-                  : null;
-          if (oldValue == null) {
-            unsafe.putObject(message, fieldOffset, registers.object1);
-          } else {
-            unsafe.putObject(
-                message, fieldOffset, Internal.mergeMessage(oldValue, registers.object1));
-          }
-          unsafe.putInt(message, oneofCaseOffset, number);
+              mergeGroupField(
+                  current,
+                  getMessageFieldSchema(bufferPosition),
+                  data,
+                  position,
+                  limit,
+                  endTag,
+                  registers);
+          storeOneofMessageField(message, number, bufferPosition, current);
         }
         break;
       default:
@@ -4878,6 +4871,7 @@ final class MessageSchema<T> implements Schema<T> {
   int parseProto2Message(
       T message, byte[] data, int position, int limit, int endGroup, Registers registers)
       throws IOException {
+    checkMutable(message);
     final sun.misc.Unsafe unsafe = UNSAFE;
     int currentPresenceFieldOffset = NO_PRESENCE_SENTINEL;
     int currentPresenceField = 0;
@@ -4994,18 +4988,11 @@ final class MessageSchema<T> implements Schema<T> {
               break;
             case 9: // MESSAGE
               if (wireType == WireFormat.WIRETYPE_LENGTH_DELIMITED) {
+                final Object current = mutableMessageFieldForMerge(message, pos);
                 position =
-                    decodeMessageField(
-                        getMessageFieldSchema(pos), data, position, limit, registers);
-                if ((currentPresenceField & presenceMask) == 0) {
-                  unsafe.putObject(message, fieldOffset, registers.object1);
-                } else {
-                  unsafe.putObject(
-                      message,
-                      fieldOffset,
-                      Internal.mergeMessage(
-                          unsafe.getObject(message, fieldOffset), registers.object1));
-                }
+                    mergeMessageField(
+                        current, getMessageFieldSchema(pos), data, position, limit, registers);
+                storeMessageField(message, pos, current);
                 currentPresenceField |= presenceMask;
                 continue;
               }
@@ -5054,20 +5041,18 @@ final class MessageSchema<T> implements Schema<T> {
               break;
             case 17: // GROUP
               if (wireType == WireFormat.WIRETYPE_START_GROUP) {
+                final Object current = mutableMessageFieldForMerge(message, pos);
                 final int endTag = (number << 3) | WireFormat.WIRETYPE_END_GROUP;
                 position =
-                    decodeGroupField(
-                        getMessageFieldSchema(pos), data, position, limit, endTag, registers);
-                if ((currentPresenceField & presenceMask) == 0) {
-                  unsafe.putObject(message, fieldOffset, registers.object1);
-                } else {
-                  unsafe.putObject(
-                      message,
-                      fieldOffset,
-                      Internal.mergeMessage(
-                          unsafe.getObject(message, fieldOffset), registers.object1));
-                }
-
+                    mergeGroupField(
+                        current,
+                        getMessageFieldSchema(pos),
+                        data,
+                        position,
+                        limit,
+                        endTag,
+                        registers);
+                storeMessageField(message, pos, current);
                 currentPresenceField |= presenceMask;
                 continue;
               }
@@ -5165,7 +5150,8 @@ final class MessageSchema<T> implements Schema<T> {
               message,
               intArray[i],
               unknownFields,
-              (UnknownFieldSchema<UnknownFieldSetLite, UnknownFieldSetLite>) unknownFieldSchema);
+              (UnknownFieldSchema<UnknownFieldSetLite, UnknownFieldSetLite>) unknownFieldSchema,
+              message);
     }
     if (unknownFields != null) {
       ((UnknownFieldSchema<UnknownFieldSetLite, UnknownFieldSetLite>) unknownFieldSchema)
@@ -5183,9 +5169,65 @@ final class MessageSchema<T> implements Schema<T> {
     return position;
   }
 
+  private Object mutableMessageFieldForMerge(T message, int pos) {
+    final Schema fieldSchema = getMessageFieldSchema(pos);
+    final long offset = offset(typeAndOffsetAt(pos));
+
+    // Field not present, create a new one
+    if (!isFieldPresent(message, pos)) {
+      return fieldSchema.newInstance();
+    }
+
+    // Field present, if mutable, ready to merge
+    final Object current = UNSAFE.getObject(message, offset);
+    if (isMutable(current)) {
+      return current;
+    }
+
+    // Field present but immutable, make a new mutable copy
+    final Object newMessage = fieldSchema.newInstance();
+    if (current != null) {
+      fieldSchema.mergeFrom(newMessage, current);
+    }
+    return newMessage;
+  }
+
+  private void storeMessageField(T message, int pos, Object field) {
+    UNSAFE.putObject(message, offset(typeAndOffsetAt(pos)), field);
+    setFieldPresent(message, pos);
+  }
+
+  private Object mutableOneofMessageFieldForMerge(T message, int fieldNumber, int pos) {
+    final Schema fieldSchema = getMessageFieldSchema(pos);
+
+    // Field not present, create it and mark it present
+    if (!isOneofPresent(message, fieldNumber, pos)) {
+      return fieldSchema.newInstance();
+    }
+
+    // Field present, if mutable, ready to merge
+    final Object current = UNSAFE.getObject(message, offset(typeAndOffsetAt(pos)));
+    if (isMutable(current)) {
+      return current;
+    }
+
+    // Field present but immutable, make a new mutable copy
+    final Object newMessage = fieldSchema.newInstance();
+    if (current != null) {
+      fieldSchema.mergeFrom(newMessage, current);
+    }
+    return newMessage;
+  }
+
+  private void storeOneofMessageField(T message, int fieldNumber, int pos, Object field) {
+    UNSAFE.putObject(message, offset(typeAndOffsetAt(pos)), field);
+    setOneofPresent(message, fieldNumber, pos);
+  }
+
   /** Parses a proto3 message and returns the limit if parsing is successful. */
   private int parseProto3Message(
       T message, byte[] data, int position, int limit, Registers registers) throws IOException {
+    checkMutable(message);
     final sun.misc.Unsafe unsafe = UNSAFE;
     int currentPresenceFieldOffset = NO_PRESENCE_SENTINEL;
     int currentPresenceField = 0;
@@ -5307,16 +5349,11 @@ final class MessageSchema<T> implements Schema<T> {
               break;
             case 9: // MESSAGE:
               if (wireType == WireFormat.WIRETYPE_LENGTH_DELIMITED) {
+                final Object current = mutableMessageFieldForMerge(message, pos);
                 position =
-                    decodeMessageField(
-                        getMessageFieldSchema(pos), data, position, limit, registers);
-                final Object oldValue = unsafe.getObject(message, fieldOffset);
-                if (oldValue == null) {
-                  unsafe.putObject(message, fieldOffset, registers.object1);
-                } else {
-                  unsafe.putObject(
-                      message, fieldOffset, Internal.mergeMessage(oldValue, registers.object1));
-                }
+                    mergeMessageField(
+                        current, getMessageFieldSchema(pos), data, position, limit, registers);
+                storeMessageField(message, pos, current);
                 currentPresenceField |= presenceMask;
                 continue;
               }
@@ -5447,18 +5484,73 @@ final class MessageSchema<T> implements Schema<T> {
 
   @Override
   public void makeImmutable(T message) {
-    // Make all repeated/map fields immutable.
-    for (int i = checkInitializedCount; i < repeatedFieldOffsetStart; i++) {
-      long offset = offset(typeAndOffsetAt(intArray[i]));
-      Object mapField = UnsafeUtil.getObject(message, offset);
-      if (mapField == null) {
-        continue;
-      }
-      UnsafeUtil.putObject(message, offset, mapFieldSchema.toImmutable(mapField));
+    if (!isMutable(message)) {
+      return;
+    }
+
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
+    if (message instanceof GeneratedMessageLite) {
+      GeneratedMessageLite<?, ?> generatedMessage = ((GeneratedMessageLite<?, ?>) message);
+      generatedMessage.clearMemoizedSerializedSize();
+      generatedMessage.clearMemoizedHashCode();
+      generatedMessage.markImmutable();
     }
-    final int length = intArray.length;
-    for (int i = repeatedFieldOffsetStart; i < length; i++) {
-      listFieldSchema.makeImmutableListAt(message, intArray[i]);
+
+    final int bufferLength = buffer.length;
+    for (int pos = 0; pos < bufferLength; pos += INTS_PER_FIELD) {
+      final int typeAndOffset = typeAndOffsetAt(pos);
+      final long offset = offset(typeAndOffset);
+      switch (type(typeAndOffset)) {
+        case 17: // GROUP
+        case 9: // MESSAGE
+          if (isFieldPresent(message, pos)) {
+            getMessageFieldSchema(pos).makeImmutable(UNSAFE.getObject(message, offset));
+          }
+          break;
+        case 18: // DOUBLE_LIST:
+        case 19: // FLOAT_LIST:
+        case 20: // INT64_LIST:
+        case 21: // UINT64_LIST:
+        case 22: // INT32_LIST:
+        case 23: // FIXED64_LIST:
+        case 24: // FIXED32_LIST:
+        case 25: // BOOL_LIST:
+        case 26: // STRING_LIST:
+        case 27: // MESSAGE_LIST:
+        case 28: // BYTES_LIST:
+        case 29: // UINT32_LIST:
+        case 30: // ENUM_LIST:
+        case 31: // SFIXED32_LIST:
+        case 32: // SFIXED64_LIST:
+        case 33: // SINT32_LIST:
+        case 34: // SINT64_LIST:
+        case 35: // DOUBLE_LIST_PACKED:
+        case 36: // FLOAT_LIST_PACKED:
+        case 37: // INT64_LIST_PACKED:
+        case 38: // UINT64_LIST_PACKED:
+        case 39: // INT32_LIST_PACKED:
+        case 40: // FIXED64_LIST_PACKED:
+        case 41: // FIXED32_LIST_PACKED:
+        case 42: // BOOL_LIST_PACKED:
+        case 43: // UINT32_LIST_PACKED:
+        case 44: // ENUM_LIST_PACKED:
+        case 45: // SFIXED32_LIST_PACKED:
+        case 46: // SFIXED64_LIST_PACKED:
+        case 47: // SINT32_LIST_PACKED:
+        case 48: // SINT64_LIST_PACKED:
+        case 49: // GROUP_LIST:
+          listFieldSchema.makeImmutableListAt(message, offset);
+          break;
+        case 50: // MAP:
+          {
+            Object mapField = UNSAFE.getObject(message, offset);
+            if (mapField != null) {
+              UNSAFE.putObject(message, offset, mapFieldSchema.toImmutable(mapField));
+            }
+          }
+          break;
+      }
     }
     unknownFieldSchema.makeImmutable(message);
     if (hasExtensions) {
@@ -5495,8 +5587,12 @@ final class MessageSchema<T> implements Schema<T> {
         extensionRegistry);
   }
 
-  private final <UT, UB> UB filterMapUnknownEnumValues(
-      Object message, int pos, UB unknownFields, UnknownFieldSchema<UT, UB> unknownFieldSchema) {
+  private <UT, UB> UB filterMapUnknownEnumValues(
+      Object message,
+      int pos,
+      UB unknownFields,
+      UnknownFieldSchema<UT, UB> unknownFieldSchema,
+      Object containerMessage) {
     int fieldNumber = numberAt(pos);
     long offset = offset(typeAndOffsetAt(pos));
     Object mapField = UnsafeUtil.getObject(message, offset);
@@ -5511,25 +5607,32 @@ final class MessageSchema<T> implements Schema<T> {
     // Filter unknown enum values.
     unknownFields =
         filterUnknownEnumMap(
-            pos, fieldNumber, mapData, enumVerifier, unknownFields, unknownFieldSchema);
+            pos,
+            fieldNumber,
+            mapData,
+            enumVerifier,
+            unknownFields,
+            unknownFieldSchema,
+            containerMessage);
     return unknownFields;
   }
 
   @SuppressWarnings("unchecked")
-  private final <K, V, UT, UB> UB filterUnknownEnumMap(
+  private <K, V, UT, UB> UB filterUnknownEnumMap(
       int pos,
       int number,
       Map<K, V> mapData,
       EnumVerifier enumVerifier,
       UB unknownFields,
-      UnknownFieldSchema<UT, UB> unknownFieldSchema) {
+      UnknownFieldSchema<UT, UB> unknownFieldSchema,
+      Object containerMessage) {
     Metadata<K, V> metadata =
         (Metadata<K, V>) mapFieldSchema.forMapMetadata(getMapFieldDefaultEntry(pos));
     for (Iterator<Map.Entry<K, V>> it = mapData.entrySet().iterator(); it.hasNext(); ) {
       Map.Entry<K, V> entry = it.next();
       if (!enumVerifier.isInRange((Integer) entry.getValue())) {
         if (unknownFields == null) {
-          unknownFields = unknownFieldSchema.newBuilder();
+          unknownFields = unknownFieldSchema.getBuilderFromMessage(containerMessage);
         }
         int entrySize =
             MapEntryLite.computeSerializedSize(metadata, entry.getKey(), entry.getValue());
@@ -5746,6 +5849,28 @@ final class MessageSchema<T> implements Schema<T> {
     return value & OFFSET_MASK;
   }
 
+  private static boolean isMutable(Object message) {
+    if (message == null) {
+      return false;
+    }
+
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
+    if (message instanceof GeneratedMessageLite<?, ?>) {
+      return ((GeneratedMessageLite<?, ?>) message).isMutable();
+    }
+
+    // For other types, we'll assume this is true because that's what was
+    // happening before we started checking.
+    return true;
+  }
+
+  private static void checkMutable(Object message) {
+    if (!isMutable(message)) {
+      throw new IllegalArgumentException("Mutating immutable message: " + message);
+    }
+  }
+
   private static <T> double doubleAt(T message, long offset) {
     return UnsafeUtil.getDouble(message, offset);
   }
diff --git a/java/core/src/main/java/com/google/protobuf/MessageSetSchema.java b/java/core/src/main/java/com/google/protobuf/MessageSetSchema.java
index 187dc8b8a..eae93b912 100644
--- a/java/core/src/main/java/com/google/protobuf/MessageSetSchema.java
+++ b/java/core/src/main/java/com/google/protobuf/MessageSetSchema.java
@@ -61,7 +61,13 @@ final class MessageSetSchema<T> implements Schema<T> {
   @SuppressWarnings("unchecked")
   @Override
   public T newInstance() {
-    return (T) defaultInstance.newBuilderForType().buildPartial();
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
+    if (defaultInstance instanceof GeneratedMessageLite) {
+      return (T) ((GeneratedMessageLite<?, ?>) defaultInstance).newMutableInstance();
+    } else {
+      return (T) defaultInstance.newBuilderForType().buildPartial();
+    }
   }
 
   @Override
@@ -132,6 +138,8 @@ final class MessageSetSchema<T> implements Schema<T> {
   public void mergeFrom(
       T message, byte[] data, int position, int limit, ArrayDecoders.Registers registers)
       throws IOException {
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
     UnknownFieldSetLite unknownFields = ((GeneratedMessageLite) message).unknownFields;
     if (unknownFields == UnknownFieldSetLite.getDefaultInstance()) {
       unknownFields = UnknownFieldSetLite.newInstance();
@@ -180,9 +188,12 @@ final class MessageSetSchema<T> implements Schema<T> {
             if (wireType == WireFormat.WIRETYPE_VARINT) {
               position = ArrayDecoders.decodeVarint32(data, position, registers);
               typeId = registers.int1;
+              // TODO(b/248560713) decide if we're keeping support for Full in schema classes and
+              // handle this better.
               extension =
-                  (GeneratedMessageLite.GeneratedExtension<?, ?>) extensionSchema
-                      .findExtensionByNumber(registers.extensionRegistry, defaultInstance, typeId);
+                  (GeneratedMessageLite.GeneratedExtension<?, ?>)
+                      extensionSchema.findExtensionByNumber(
+                          registers.extensionRegistry, defaultInstance, typeId);
               continue;
             }
             break;
diff --git a/java/core/src/main/java/com/google/protobuf/NewInstanceSchemaLite.java b/java/core/src/main/java/com/google/protobuf/NewInstanceSchemaLite.java
index 9b9226676..00cfe3b6c 100644
--- a/java/core/src/main/java/com/google/protobuf/NewInstanceSchemaLite.java
+++ b/java/core/src/main/java/com/google/protobuf/NewInstanceSchemaLite.java
@@ -33,7 +33,8 @@ package com.google.protobuf;
 final class NewInstanceSchemaLite implements NewInstanceSchema {
   @Override
   public Object newInstance(Object defaultInstance) {
-    return ((GeneratedMessageLite) defaultInstance)
-        .dynamicMethod(GeneratedMessageLite.MethodToInvoke.NEW_MUTABLE_INSTANCE);
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
+    return ((GeneratedMessageLite<?, ?>) defaultInstance).newMutableInstance();
   }
 }
diff --git a/java/core/src/main/java/com/google/protobuf/Reader.java b/java/core/src/main/java/com/google/protobuf/Reader.java
index 705096f2d..b99ee43d4 100644
--- a/java/core/src/main/java/com/google/protobuf/Reader.java
+++ b/java/core/src/main/java/com/google/protobuf/Reader.java
@@ -158,6 +158,14 @@ interface Reader {
   <T> T readGroupBySchemaWithCheck(Schema<T> schema, ExtensionRegistryLite extensionRegistry)
       throws IOException;
 
+  /** Read a message field from the wire format and merge the results into the given target. */
+  <T> void mergeMessageField(T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry)
+      throws IOException;
+
+  /** Read a group field from the wire format and merge the results into the given target. */
+  <T> void mergeGroupField(T target, Schema<T> schema, ExtensionRegistryLite extensionRegistry)
+      throws IOException;
+
   /**
    * Reads and returns the next field of type {@code BYTES} and advances the reader to the next
    * field.
diff --git a/java/core/src/main/java/com/google/protobuf/SchemaUtil.java b/java/core/src/main/java/com/google/protobuf/SchemaUtil.java
index 1d5e6ba69..7c236dba0 100644
--- a/java/core/src/main/java/com/google/protobuf/SchemaUtil.java
+++ b/java/core/src/main/java/com/google/protobuf/SchemaUtil.java
@@ -59,6 +59,8 @@ final class SchemaUtil {
    * GeneratedMessageLite}.
    */
   public static void requireGeneratedMessage(Class<?> messageType) {
+    // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle this
+    // better.
     if (!GeneratedMessageLite.class.isAssignableFrom(messageType)
         && GENERATED_MESSAGE_CLASS != null
         && !GENERATED_MESSAGE_CLASS.isAssignableFrom(messageType)) {
@@ -808,6 +810,8 @@ final class SchemaUtil {
 
   private static Class<?> getGeneratedMessageClass() {
     try {
+      // TODO(b/248560713) decide if we're keeping support for Full in schema classes and handle
+      // this better.
       return Class.forName("com.google.protobuf.GeneratedMessageV3");
     } catch (Throwable e) {
       return null;
@@ -900,7 +904,9 @@ final class SchemaUtil {
   }
 
   /** Filters unrecognized enum values in a list. */
+  @CanIgnoreReturnValue
   static <UT, UB> UB filterUnknownEnumList(
+      Object containerMessage,
       int number,
       List<Integer> enumList,
       EnumLiteMap<?> enumMap,
@@ -921,7 +927,9 @@ final class SchemaUtil {
           }
           ++writePos;
         } else {
-          unknownFields = storeUnknownEnum(number, enumValue, unknownFields, unknownFieldSchema);
+          unknownFields =
+              storeUnknownEnum(
+                  containerMessage, number, enumValue, unknownFields, unknownFieldSchema);
         }
       }
       if (writePos != size) {
@@ -931,7 +939,9 @@ final class SchemaUtil {
       for (Iterator<Integer> it = enumList.iterator(); it.hasNext(); ) {
         int enumValue = it.next();
         if (enumMap.findValueByNumber(enumValue) == null) {
-          unknownFields = storeUnknownEnum(number, enumValue, unknownFields, unknownFieldSchema);
+          unknownFields =
+              storeUnknownEnum(
+                  containerMessage, number, enumValue, unknownFields, unknownFieldSchema);
           it.remove();
         }
       }
@@ -940,7 +950,9 @@ final class SchemaUtil {
   }
 
   /** Filters unrecognized enum values in a list. */
+  @CanIgnoreReturnValue
   static <UT, UB> UB filterUnknownEnumList(
+      Object containerMessage,
       int number,
       List<Integer> enumList,
       EnumVerifier enumVerifier,
@@ -961,7 +973,9 @@ final class SchemaUtil {
           }
           ++writePos;
         } else {
-          unknownFields = storeUnknownEnum(number, enumValue, unknownFields, unknownFieldSchema);
+          unknownFields =
+              storeUnknownEnum(
+                  containerMessage, number, enumValue, unknownFields, unknownFieldSchema);
         }
       }
       if (writePos != size) {
@@ -971,7 +985,9 @@ final class SchemaUtil {
       for (Iterator<Integer> it = enumList.iterator(); it.hasNext(); ) {
         int enumValue = it.next();
         if (!enumVerifier.isInRange(enumValue)) {
-          unknownFields = storeUnknownEnum(number, enumValue, unknownFields, unknownFieldSchema);
+          unknownFields =
+              storeUnknownEnum(
+                  containerMessage, number, enumValue, unknownFields, unknownFieldSchema);
           it.remove();
         }
       }
@@ -981,9 +997,13 @@ final class SchemaUtil {
 
   /** Stores an unrecognized enum value as an unknown value. */
   static <UT, UB> UB storeUnknownEnum(
-      int number, int enumValue, UB unknownFields, UnknownFieldSchema<UT, UB> unknownFieldSchema) {
+      Object containerMessage,
+      int number,
+      int enumValue,
+      UB unknownFields,
+      UnknownFieldSchema<UT, UB> unknownFieldSchema) {
     if (unknownFields == null) {
-      unknownFields = unknownFieldSchema.newBuilder();
+      unknownFields = unknownFieldSchema.getBuilderFromMessage(containerMessage);
     }
     unknownFieldSchema.addVarint(unknownFields, number, enumValue);
     return unknownFields;
diff --git a/java/core/src/main/java/com/google/protobuf/TextFormat.java b/java/core/src/main/java/com/google/protobuf/TextFormat.java
index 21f1ff09d..b35052493 100644
--- a/java/core/src/main/java/com/google/protobuf/TextFormat.java
+++ b/java/core/src/main/java/com/google/protobuf/TextFormat.java
@@ -592,7 +592,7 @@ public final class TextFormat {
 
         case MESSAGE:
         case GROUP:
-          print((Message) value, generator);
+          print((MessageOrBuilder) value, generator);
           break;
       }
     }
diff --git a/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLite.java b/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLite.java
index b2cb7be4b..85a773a6c 100644
--- a/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLite.java
+++ b/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLite.java
@@ -388,7 +388,7 @@ public final class UnknownFieldSetLite {
   // Package private for unsafe experimental runtime.
   void storeField(int tag, Object value) {
     checkMutable();
-    ensureCapacity();
+    ensureCapacity(count + 1);
 
     tags[count] = tag;
     objects[count] = value;
@@ -396,13 +396,23 @@ public final class UnknownFieldSetLite {
   }
 
   /** Ensures that our arrays are long enough to store more metadata. */
-  private void ensureCapacity() {
-    if (count == tags.length) {
-      int increment = count < (MIN_CAPACITY / 2) ? MIN_CAPACITY : count >> 1;
-      int newLength = count + increment;
+  private void ensureCapacity(int minCapacity) {
+    if (minCapacity > this.tags.length) {
+      // Increase by at least 50%
+      int newCapacity = count + count / 2;
+
+      // Or new capacity if higher
+      if (newCapacity < minCapacity) {
+        newCapacity = minCapacity;
+      }
+
+      // And never less than MIN_CAPACITY
+      if (newCapacity < MIN_CAPACITY) {
+        newCapacity = MIN_CAPACITY;
+      }
 
-      tags = Arrays.copyOf(tags, newLength);
-      objects = Arrays.copyOf(objects, newLength);
+      this.tags = Arrays.copyOf(this.tags, newCapacity);
+      this.objects = Arrays.copyOf(this.objects, newCapacity);
     }
   }
 
@@ -487,4 +497,19 @@ public final class UnknownFieldSetLite {
     }
     return this;
   }
+
+  @CanIgnoreReturnValue
+  UnknownFieldSetLite mergeFrom(UnknownFieldSetLite other) {
+    if (other.equals(getDefaultInstance())) {
+      return this;
+    }
+
+    checkMutable();
+    int newCount = this.count + other.count;
+    ensureCapacity(newCount);
+    System.arraycopy(other.tags, 0, tags, this.count, other.count);
+    System.arraycopy(other.objects, 0, objects, this.count, other.count);
+    this.count = newCount;
+    return this;
+  }
 }
diff --git a/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLiteSchema.java b/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLiteSchema.java
index ffd723230..2cfdeca24 100644
--- a/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLiteSchema.java
+++ b/java/core/src/main/java/com/google/protobuf/UnknownFieldSetLiteSchema.java
@@ -122,10 +122,14 @@ class UnknownFieldSetLiteSchema
   }
 
   @Override
-  UnknownFieldSetLite merge(UnknownFieldSetLite message, UnknownFieldSetLite other) {
-    return other.equals(UnknownFieldSetLite.getDefaultInstance())
-        ? message
-        : UnknownFieldSetLite.mutableCopyOf(message, other);
+  UnknownFieldSetLite merge(UnknownFieldSetLite target, UnknownFieldSetLite source) {
+    if (UnknownFieldSetLite.getDefaultInstance().equals(source)) {
+      return target;
+    }
+    if (UnknownFieldSetLite.getDefaultInstance().equals(target)) {
+      return UnknownFieldSetLite.mutableCopyOf(target, source);
+    }
+    return target.mergeFrom(source);
   }
 
   @Override
diff --git a/java/lite/src/test/java/com/google/protobuf/LiteTest.java b/java/lite/src/test/java/com/google/protobuf/LiteTest.java
index b0972111a..6535a1bf6 100644
--- a/java/lite/src/test/java/com/google/protobuf/LiteTest.java
+++ b/java/lite/src/test/java/com/google/protobuf/LiteTest.java
@@ -190,19 +190,28 @@ public class LiteTest {
 
   @Test
   public void testMemoization() throws Exception {
-    TestAllExtensionsLite message = TestUtilLite.getAllLiteExtensionsSet();
+    GeneratedMessageLite<?, ?> message = TestUtilLite.getAllLiteExtensionsSet();
+
+    // This built message should not be mutable
+    assertThat(message.isMutable()).isFalse();
 
     // Test serialized size is memoized
-    message.memoizedSerializedSize = -1;
+    assertThat(message.getMemoizedSerializedSize())
+        .isEqualTo(GeneratedMessageLite.UNINITIALIZED_SERIALIZED_SIZE);
     int size = message.getSerializedSize();
     assertThat(size).isGreaterThan(0);
-    assertThat(message.memoizedSerializedSize).isEqualTo(size);
+    assertThat(message.getMemoizedSerializedSize()).isEqualTo(size);
+    message.clearMemoizedSerializedSize();
+    assertThat(message.getMemoizedSerializedSize())
+        .isEqualTo(GeneratedMessageLite.UNINITIALIZED_SERIALIZED_SIZE);
 
     // Test hashCode is memoized
-    assertThat(message.memoizedHashCode).isEqualTo(0);
+    assertThat(message.hashCodeIsNotMemoized()).isTrue();
     int hashCode = message.hashCode();
-    assertThat(hashCode).isNotEqualTo(0);
-    assertThat(hashCode).isEqualTo(message.memoizedHashCode);
+    assertThat(message.hashCodeIsNotMemoized()).isFalse();
+    assertThat(message.getMemoizedHashCode()).isEqualTo(hashCode);
+    message.clearMemoizedHashCode();
+    assertThat(message.hashCodeIsNotMemoized()).isTrue();
 
     // Test isInitialized is memoized
     Field memo = message.getClass().getDeclaredField("memoizedIsInitialized");
diff --git a/java/util/src/main/java/com/google/protobuf/util/FieldMaskTree.java b/java/util/src/main/java/com/google/protobuf/util/FieldMaskTree.java
index 352376e01..2aa0916f0 100644
--- a/java/util/src/main/java/com/google/protobuf/util/FieldMaskTree.java
+++ b/java/util/src/main/java/com/google/protobuf/util/FieldMaskTree.java
@@ -35,6 +35,7 @@ import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor;
 import com.google.protobuf.FieldMask;
+import com.google.protobuf.GeneratedMessage;
 import com.google.protobuf.Message;
 import java.util.ArrayList;
 import java.util.List;
@@ -304,9 +305,12 @@ final class FieldMaskTree {
           // so we don't create unnecessary empty messages.
           continue;
         }
-        String childPath = path.isEmpty() ? entry.getKey() : path + "." + entry.getKey();
-        Message.Builder childBuilder = ((Message) destination.getField(field)).toBuilder();
-        merge(entry.getValue(), childPath, (Message) source.getField(field), childBuilder, options);
+        // This is a mess because of java proto API 1 still hanging around.
+        Message.Builder childBuilder =
+            destination instanceof GeneratedMessage.Builder
+                ? destination.getFieldBuilder(field)
+                : ((Message) destination.getField(field)).toBuilder();
+        merge(entry.getValue(), path, (Message) source.getField(field), childBuilder, options);
         destination.setField(field, childBuilder.buildPartial());
         continue;
       }
diff --git a/src/google/protobuf/compiler/java/java_enum_field.cc b/src/google/protobuf/compiler/java/java_enum_field.cc
index 0dad42ada..eed701ee0 100644
--- a/src/google/protobuf/compiler/java/java_enum_field.cc
+++ b/src/google/protobuf/compiler/java/java_enum_field.cc
@@ -118,13 +118,6 @@ void SetEnumVariables(const FieldDescriptor* descriptor, int messageBitIndex,
   (*variables)["set_mutable_bit_builder"] = GenerateSetBit(builderBitIndex);
   (*variables)["clear_mutable_bit_builder"] = GenerateClearBit(builderBitIndex);
 
-  // For repeated fields, one bit is used for whether the array is immutable
-  // in the parsing constructor.
-  (*variables)["get_mutable_bit_parser"] =
-      GenerateGetBitMutableLocal(builderBitIndex);
-  (*variables)["set_mutable_bit_parser"] =
-      GenerateSetBitMutableLocal(builderBitIndex);
-
   (*variables)["get_has_field_bit_from_local"] =
       GenerateGetBitFromLocal(builderBitIndex);
   (*variables)["set_has_field_bit_to_local"] =
@@ -351,32 +344,26 @@ void ImmutableEnumFieldGenerator::GenerateBuildingCode(
   printer->Print(variables_, "result.$name$_ = $name$_;\n");
 }
 
-void ImmutableEnumFieldGenerator::GenerateParsingCode(
+void ImmutableEnumFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   if (SupportUnknownEnumValue(descriptor_->file())) {
     printer->Print(variables_,
-                   "int rawValue = input.readEnum();\n"
-                   "$set_has_field_bit_message$\n"
-                   "$name$_ = rawValue;\n");
+                   "$name$_ = input.readEnum();\n"
+                   "$set_has_field_bit_builder$\n");
   } else {
     printer->Print(variables_,
-                   "int rawValue = input.readEnum();\n"
-                   "  @SuppressWarnings(\"deprecation\")\n"
-                   "$type$ value = $type$.$for_number$(rawValue);\n"
-                   "if (value == null) {\n"
-                   "  unknownFields.mergeVarintField($number$, rawValue);\n"
+                   "int tmpRaw = input.readEnum();\n"
+                   "$type$ tmpValue =\n"
+                   "    $type$.forNumber(tmpRaw);\n"
+                   "if (tmpValue == null) {\n"
+                   "  mergeUnknownVarintField($number$, tmpRaw);\n"
                    "} else {\n"
-                   "  $set_has_field_bit_message$\n"
-                   "  $name$_ = rawValue;\n"
+                   "  $name$_ = tmpRaw;\n"
+                   "  $set_has_field_bit_builder$\n"
                    "}\n");
   }
 }
 
-void ImmutableEnumFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  // noop for enums
-}
-
 void ImmutableEnumFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) const {
   printer->Print(variables_,
@@ -537,6 +524,11 @@ void ImmutableEnumOneofFieldGenerator::GenerateBuilderMembers(
   printer->Annotate("{", "}", descriptor_);
 }
 
+void ImmutableEnumOneofFieldGenerator::GenerateBuilderClearCode(
+    io::Printer* printer) const {
+  // No-op: Enum fields in oneofs are correctly cleared by clearing the oneof
+}
+
 void ImmutableEnumOneofFieldGenerator::GenerateBuildingCode(
     io::Printer* printer) const {
   printer->Print(variables_,
@@ -557,7 +549,7 @@ void ImmutableEnumOneofFieldGenerator::GenerateMergingCode(
   }
 }
 
-void ImmutableEnumOneofFieldGenerator::GenerateParsingCode(
+void ImmutableEnumOneofFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   if (SupportUnknownEnumValue(descriptor_->file())) {
     printer->Print(variables_,
@@ -567,10 +559,10 @@ void ImmutableEnumOneofFieldGenerator::GenerateParsingCode(
   } else {
     printer->Print(variables_,
                    "int rawValue = input.readEnum();\n"
-                   "@SuppressWarnings(\"deprecation\")\n"
-                   "$type$ value = $type$.$for_number$(rawValue);\n"
+                   "$type$ value =\n"
+                   "    $type$.forNumber(rawValue);\n"
                    "if (value == null) {\n"
-                   "  unknownFields.mergeVarintField($number$, rawValue);\n"
+                   "  mergeUnknownVarintField($number$, rawValue);\n"
                    "} else {\n"
                    "  $set_oneof_case_message$;\n"
                    "  $oneof_name$_ = rawValue;\n"
@@ -949,36 +941,29 @@ void RepeatedImmutableEnumFieldGenerator::GenerateBuildingCode(
       "result.$name$_ = $name$_;\n");
 }
 
-void RepeatedImmutableEnumFieldGenerator::GenerateParsingCode(
+void RepeatedImmutableEnumFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   // Read and store the enum
   if (SupportUnknownEnumValue(descriptor_->file())) {
     printer->Print(variables_,
-                   "int rawValue = input.readEnum();\n"
-                   "if (!$get_mutable_bit_parser$) {\n"
-                   "  $name$_ = new java.util.ArrayList<java.lang.Integer>();\n"
-                   "  $set_mutable_bit_parser$;\n"
-                   "}\n"
-                   "$name$_.add(rawValue);\n");
+                   "int tmpRaw = input.readEnum();\n"
+                   "ensure$capitalized_name$IsMutable();\n"
+                   "$name$_.add(tmpRaw);\n");
   } else {
-    printer->Print(
-        variables_,
-        "int rawValue = input.readEnum();\n"
-        "@SuppressWarnings(\"deprecation\")\n"
-        "$type$ value = $type$.$for_number$(rawValue);\n"
-        "if (value == null) {\n"
-        "  unknownFields.mergeVarintField($number$, rawValue);\n"
-        "} else {\n"
-        "  if (!$get_mutable_bit_parser$) {\n"
-        "    $name$_ = new java.util.ArrayList<java.lang.Integer>();\n"
-        "    $set_mutable_bit_parser$;\n"
-        "  }\n"
-        "  $name$_.add(rawValue);\n"
-        "}\n");
+    printer->Print(variables_,
+                   "int tmpRaw = input.readEnum();\n"
+                   "$type$ tmpValue =\n"
+                   "    $type$.forNumber(tmpRaw);\n"
+                   "if (tmpValue == null) {\n"
+                   "  mergeUnknownVarintField($number$, tmpRaw);\n"
+                   "} else {\n"
+                   "  ensure$capitalized_name$IsMutable();\n"
+                   "  $name$_.add(tmpRaw);\n"
+                   "}\n");
   }
 }
 
-void RepeatedImmutableEnumFieldGenerator::GenerateParsingCodeFromPacked(
+void RepeatedImmutableEnumFieldGenerator::GenerateBuilderParsingCodeFromPacked(
     io::Printer* printer) const {
   // Wrap GenerateParsingCode's contents with a while loop.
 
@@ -988,7 +973,7 @@ void RepeatedImmutableEnumFieldGenerator::GenerateParsingCodeFromPacked(
                  "while(input.getBytesUntilLimit() > 0) {\n");
   printer->Indent();
 
-  GenerateParsingCode(printer);
+  GenerateBuilderParsingCode(printer);
 
   printer->Outdent();
   printer->Print(variables_,
@@ -996,15 +981,6 @@ void RepeatedImmutableEnumFieldGenerator::GenerateParsingCodeFromPacked(
                  "input.popLimit(oldLimit);\n");
 }
 
-void RepeatedImmutableEnumFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  printer->Print(
-      variables_,
-      "if ($get_mutable_bit_parser$) {\n"
-      "  $name$_ = java.util.Collections.unmodifiableList($name$_);\n"
-      "}\n");
-}
-
 void RepeatedImmutableEnumFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) const {
   if (descriptor_->is_packed()) {
diff --git a/src/google/protobuf/compiler/java/java_enum_field.h b/src/google/protobuf/compiler/java/java_enum_field.h
index 82dbd9e4a..aafc26ced 100644
--- a/src/google/protobuf/compiler/java/java_enum_field.h
+++ b/src/google/protobuf/compiler/java/java_enum_field.h
@@ -71,10 +71,9 @@ class ImmutableEnumFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderMembers(io::Printer* printer) const override;
   void GenerateInitializationCode(io::Printer* printer) const override;
   void GenerateBuilderClearCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
@@ -103,9 +102,10 @@ class ImmutableEnumOneofFieldGenerator : public ImmutableEnumFieldGenerator {
 
   void GenerateMembers(io::Printer* printer) const override;
   void GenerateBuilderMembers(io::Printer* printer) const override;
+  void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateEqualsCode(io::Printer* printer) const override;
@@ -132,9 +132,9 @@ class RepeatedImmutableEnumFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingCodeFromPacked(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCodeFromPacked(
+      io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
diff --git a/src/google/protobuf/compiler/java/java_field.cc b/src/google/protobuf/compiler/java/java_field.cc
index 8916d1385..0c69a53bf 100644
--- a/src/google/protobuf/compiler/java/java_field.cc
+++ b/src/google/protobuf/compiler/java/java_field.cc
@@ -185,7 +185,7 @@ static inline void ReportUnexpectedPackedFieldsCall(io::Printer* printer) {
   //     but this method should be overridden.
   //   - This FieldGenerator doesn't support packing, and this method
   //     should never have been called.
-  GOOGLE_LOG(FATAL) << "GenerateParsingCodeFromPacked() "
+  GOOGLE_LOG(FATAL) << "GenerateBuilderParsingCodeFromPacked() "
              << "called on field generator that does not support packing.";
 }
 
@@ -193,7 +193,7 @@ static inline void ReportUnexpectedPackedFieldsCall(io::Printer* printer) {
 
 ImmutableFieldGenerator::~ImmutableFieldGenerator() {}
 
-void ImmutableFieldGenerator::GenerateParsingCodeFromPacked(
+void ImmutableFieldGenerator::GenerateBuilderParsingCodeFromPacked(
     io::Printer* printer) const {
   ReportUnexpectedPackedFieldsCall(printer);
 }
diff --git a/src/google/protobuf/compiler/java/java_field.h b/src/google/protobuf/compiler/java/java_field.h
index a7c995c4e..a570cd61b 100644
--- a/src/google/protobuf/compiler/java/java_field.h
+++ b/src/google/protobuf/compiler/java/java_field.h
@@ -77,9 +77,8 @@ class ImmutableFieldGenerator {
   virtual void GenerateBuilderClearCode(io::Printer* printer) const = 0;
   virtual void GenerateMergingCode(io::Printer* printer) const = 0;
   virtual void GenerateBuildingCode(io::Printer* printer) const = 0;
-  virtual void GenerateParsingCode(io::Printer* printer) const = 0;
-  virtual void GenerateParsingCodeFromPacked(io::Printer* printer) const;
-  virtual void GenerateParsingDoneCode(io::Printer* printer) const = 0;
+  virtual void GenerateBuilderParsingCode(io::Printer* printer) const = 0;
+  virtual void GenerateBuilderParsingCodeFromPacked(io::Printer* printer) const;
   virtual void GenerateSerializationCode(io::Printer* printer) const = 0;
   virtual void GenerateSerializedSizeCode(io::Printer* printer) const = 0;
   virtual void GenerateFieldBuilderInitializationCode(
diff --git a/src/google/protobuf/compiler/java/java_map_field.cc b/src/google/protobuf/compiler/java/java_map_field.cc
index 8a8910071..d5c76f161 100644
--- a/src/google/protobuf/compiler/java/java_map_field.cc
+++ b/src/google/protobuf/compiler/java/java_map_field.cc
@@ -158,13 +158,6 @@ void SetMessageVariables(const FieldDescriptor* descriptor, int messageBitIndex,
           : "";
   (*variables)["on_changed"] = "onChanged();";
 
-  // For repeated fields, one bit is used for whether the array is immutable
-  // in the parsing constructor.
-  (*variables)["get_mutable_bit_parser"] =
-      GenerateGetBitMutableLocal(builderBitIndex);
-  (*variables)["set_mutable_bit_parser"] =
-      GenerateSetBitMutableLocal(builderBitIndex);
-
   (*variables)["default_entry"] =
       (*variables)["capitalized_name"] + "DefaultEntryHolder.defaultEntry";
   (*variables)["map_field_parameter"] = (*variables)["default_entry"];
@@ -784,27 +777,19 @@ void ImmutableMapFieldGenerator::GenerateBuildingCode(
                  "result.$name$_.makeImmutable();\n");
 }
 
-void ImmutableMapFieldGenerator::GenerateParsingCode(
+void ImmutableMapFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
-  printer->Print(variables_,
-                 "if (!$get_mutable_bit_parser$) {\n"
-                 "  $name$_ = com.google.protobuf.MapField.newMapField(\n"
-                 "      $map_field_parameter$);\n"
-                 "  $set_mutable_bit_parser$;\n"
-                 "}\n");
   if (!SupportUnknownEnumValue(descriptor_->file()) &&
       GetJavaType(ValueField(descriptor_)) == JAVATYPE_ENUM) {
     printer->Print(
         variables_,
         "com.google.protobuf.ByteString bytes = input.readBytes();\n"
         "com.google.protobuf.MapEntry<$type_parameters$>\n"
-        "$name$__ = $default_entry$.getParserForType().parseFrom(bytes);\n");
-    printer->Print(
-        variables_,
+        "$name$__ = $default_entry$.getParserForType().parseFrom(bytes);\n"
         "if ($value_enum_type$.forNumber($name$__.getValue()) == null) {\n"
-        "  unknownFields.mergeLengthDelimitedField($number$, bytes);\n"
+        "  mergeUnknownLengthDelimitedField($number$, bytes);\n"
         "} else {\n"
-        "  $name$_.getMutableMap().put(\n"
+        "  internalGetMutable$capitalized_name$().getMutableMap().put(\n"
         "      $name$__.getKey(), $name$__.getValue());\n"
         "}\n");
   } else {
@@ -813,16 +798,11 @@ void ImmutableMapFieldGenerator::GenerateParsingCode(
         "com.google.protobuf.MapEntry<$type_parameters$>\n"
         "$name$__ = input.readMessage(\n"
         "    $default_entry$.getParserForType(), extensionRegistry);\n"
-        "$name$_.getMutableMap().put(\n"
+        "internalGetMutable$capitalized_name$().getMutableMap().put(\n"
         "    $name$__.getKey(), $name$__.getValue());\n");
   }
 }
 
-void ImmutableMapFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  // Nothing to do here.
-}
-
 void ImmutableMapFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) const {
   printer->Print(variables_,
diff --git a/src/google/protobuf/compiler/java/java_map_field.h b/src/google/protobuf/compiler/java/java_map_field.h
index 98d9249a7..7262edf22 100644
--- a/src/google/protobuf/compiler/java/java_map_field.h
+++ b/src/google/protobuf/compiler/java/java_map_field.h
@@ -55,8 +55,7 @@ class ImmutableMapFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
diff --git a/src/google/protobuf/compiler/java/java_message.cc b/src/google/protobuf/compiler/java/java_message.cc
index 27d1014f6..d8c09ae44 100644
--- a/src/google/protobuf/compiler/java/java_message.cc
+++ b/src/google/protobuf/compiler/java/java_message.cc
@@ -52,6 +52,7 @@
 #include <google/protobuf/descriptor.pb.h>
 #include <google/protobuf/io/coded_stream.h>
 #include <google/protobuf/io/printer.h>
+#include <google/protobuf/descriptor.h>
 #include <google/protobuf/wire_format.h>
 #include <google/protobuf/stubs/strutil.h>
 #include <google/protobuf/stubs/substitute.h>
@@ -376,6 +377,7 @@ void ImmutableMessageGenerator::Generate(io::Printer* printer) {
                  "}\n"
                  "\n");
 
+  // TODO(b/248149118): Remove this superfluous override.
   printer->Print(
       "@java.lang.Override\n"
       "public final com.google.protobuf.UnknownFieldSet\n"
@@ -383,10 +385,6 @@ void ImmutableMessageGenerator::Generate(io::Printer* printer) {
       "  return this.unknownFields;\n"
       "}\n");
 
-  if (context_->HasGeneratedMethods(descriptor_)) {
-    GenerateParsingConstructor(printer);
-  }
-
   GenerateDescriptorMethods(printer);
 
   // Nested types
@@ -635,9 +633,9 @@ void ImmutableMessageGenerator::GenerateMessageSerializationMethods(
   }
 
   if (descriptor_->options().message_set_wire_format()) {
-    printer->Print("unknownFields.writeAsMessageSetTo(output);\n");
+    printer->Print("getUnknownFields().writeAsMessageSetTo(output);\n");
   } else {
-    printer->Print("unknownFields.writeTo(output);\n");
+    printer->Print("getUnknownFields().writeTo(output);\n");
   }
 
   printer->Outdent();
@@ -666,9 +664,10 @@ void ImmutableMessageGenerator::GenerateMessageSerializationMethods(
   }
 
   if (descriptor_->options().message_set_wire_format()) {
-    printer->Print("size += unknownFields.getSerializedSizeAsMessageSet();\n");
+    printer->Print(
+        "size += getUnknownFields().getSerializedSizeAsMessageSet();\n");
   } else {
-    printer->Print("size += unknownFields.getSerializedSize();\n");
+    printer->Print("size += getUnknownFields().getSerializedSize();\n");
   }
 
   printer->Print(
@@ -1065,7 +1064,8 @@ void ImmutableMessageGenerator::GenerateEqualsAndHashCode(
   // false for non-canonical ordering when running in LITE_RUNTIME but it's
   // the best we can do.
   printer->Print(
-      "if (!unknownFields.equals(other.unknownFields)) return false;\n");
+      "if (!getUnknownFields().equals(other.getUnknownFields())) return "
+      "false;\n");
   if (descriptor_->extension_range_count() > 0) {
     printer->Print(
         "if (!getExtensionFields().equals(other.getExtensionFields()))\n"
@@ -1139,7 +1139,7 @@ void ImmutableMessageGenerator::GenerateEqualsAndHashCode(
     printer->Print("hash = hashFields(hash, getExtensionFields());\n");
   }
 
-  printer->Print("hash = (29 * hash) + unknownFields.hashCode();\n");
+  printer->Print("hash = (29 * hash) + getUnknownFields().hashCode();\n");
   printer->Print(
       "memoizedHashCode = hash;\n"
       "return hash;\n");
@@ -1165,185 +1165,32 @@ void ImmutableMessageGenerator::GenerateExtensionRegistrationCode(
 }
 
 // ===================================================================
-void ImmutableMessageGenerator::GenerateParsingConstructor(
-    io::Printer* printer) {
-  std::unique_ptr<const FieldDescriptor*[]> sorted_fields(
-      SortFieldsByNumber(descriptor_));
-
-  printer->Print(
-      "private $classname$(\n"
-      "    com.google.protobuf.CodedInputStream input,\n"
-      "    com.google.protobuf.ExtensionRegistryLite extensionRegistry)\n"
-      "    throws com.google.protobuf.InvalidProtocolBufferException {\n",
-      "classname", descriptor_->name());
-  printer->Indent();
-
-  // Initialize all fields to default.
-  printer->Print(
-      "this();\n"
-      "if (extensionRegistry == null) {\n"
-      "  throw new java.lang.NullPointerException();\n"
-      "}\n");
-
-  // Use builder bits to track mutable repeated fields.
-  int totalBuilderBits = 0;
-  for (int i = 0; i < descriptor_->field_count(); i++) {
-    const ImmutableFieldGenerator& field =
-        field_generators_.get(descriptor_->field(i));
-    totalBuilderBits += field.GetNumBitsForBuilder();
-  }
-  int totalBuilderInts = (totalBuilderBits + 31) / 32;
-  for (int i = 0; i < totalBuilderInts; i++) {
-    printer->Print("int mutable_$bit_field_name$ = 0;\n", "bit_field_name",
-                   GetBitFieldName(i));
-  }
-
-  printer->Print(
-      "com.google.protobuf.UnknownFieldSet.Builder unknownFields =\n"
-      "    com.google.protobuf.UnknownFieldSet.newBuilder();\n");
-
-  printer->Print("try {\n");
-  printer->Indent();
-
-  printer->Print(
-      "boolean done = false;\n"
-      "while (!done) {\n");
-  printer->Indent();
-
-  printer->Print(
-      "int tag = input.readTag();\n"
-      "switch (tag) {\n");
-  printer->Indent();
-
-  printer->Print(
-      "case 0:\n"  // zero signals EOF / limit reached
-      "  done = true;\n"
-      "  break;\n");
-
-  for (int i = 0; i < descriptor_->field_count(); i++) {
-    const FieldDescriptor* field = sorted_fields[i];
-    uint32_t tag = WireFormatLite::MakeTag(
-        field->number(), WireFormat::WireTypeForFieldType(field->type()));
-
-    printer->Print("case $tag$: {\n", "tag",
-                   StrCat(static_cast<int32_t>(tag)));
-    printer->Indent();
-
-    field_generators_.get(field).GenerateParsingCode(printer);
-
-    printer->Outdent();
-    printer->Print(
-        "  break;\n"
-        "}\n");
-
-    if (field->is_packable()) {
-      // To make packed = true wire compatible, we generate parsing code from a
-      // packed version of this field regardless of field->options().packed().
-      uint32_t packed_tag = WireFormatLite::MakeTag(
-          field->number(), WireFormatLite::WIRETYPE_LENGTH_DELIMITED);
-      printer->Print("case $tag$: {\n", "tag",
-                     StrCat(static_cast<int32_t>(packed_tag)));
-      printer->Indent();
-
-      field_generators_.get(field).GenerateParsingCodeFromPacked(printer);
-
-      printer->Outdent();
-      printer->Print(
-          "  break;\n"
-          "}\n");
-    }
-  }
-
-  printer->Print(
-      "default: {\n"
-      "  if (!parseUnknownField(\n"
-      "      input, unknownFields, extensionRegistry, tag)) {\n"
-      "    done = true;\n"  // it's an endgroup tag
-      "  }\n"
-      "  break;\n"
-      "}\n");
-
-  printer->Outdent();
-  printer->Outdent();
-  printer->Print(
-      "  }\n"  // switch (tag)
-      "}\n");  // while (!done)
-
-  printer->Outdent();
-  printer->Print(
-      "} catch (com.google.protobuf.InvalidProtocolBufferException e) {\n"
-      "  throw e.setUnfinishedMessage(this);\n"
-      "} catch (java.io.IOException e) {\n"
-      "  throw new com.google.protobuf.InvalidProtocolBufferException(\n"
-      "      e).setUnfinishedMessage(this);\n"
-      "} finally {\n");
-  printer->Indent();
-
-  // Make repeated field list immutable.
-  for (int i = 0; i < descriptor_->field_count(); i++) {
-    const FieldDescriptor* field = sorted_fields[i];
-    field_generators_.get(field).GenerateParsingDoneCode(printer);
-  }
-
-  // Make unknown fields immutable.
-  printer->Print("this.unknownFields = unknownFields.build();\n");
-
-  // Make extensions immutable.
-  printer->Print("makeExtensionsImmutable();\n");
-
-  printer->Outdent();
-  printer->Outdent();
-  printer->Print(
-      "  }\n"  // finally
-      "}\n");
-}
-
-// ===================================================================
 void ImmutableMessageGenerator::GenerateParser(io::Printer* printer) {
   printer->Print(
       "$visibility$ static final com.google.protobuf.Parser<$classname$>\n"
-      "    PARSER = new com.google.protobuf.AbstractParser<$classname$>() {\n",
-      "visibility",
-      ExposePublicParser(descriptor_->file()) ? "@java.lang.Deprecated public"
-                                              : "private",
-      "classname", descriptor_->name());
-  printer->Indent();
-  printer->Print(
-      "@java.lang.Override\n"
-      "public $classname$ parsePartialFrom(\n"
-      "    com.google.protobuf.CodedInputStream input,\n"
-      "    com.google.protobuf.ExtensionRegistryLite extensionRegistry)\n"
-      "    throws com.google.protobuf.InvalidProtocolBufferException {\n",
-      "classname", descriptor_->name());
-  if (context_->HasGeneratedMethods(descriptor_)) {
-    printer->Print("  return new $classname$(input, extensionRegistry);\n",
-                   "classname", descriptor_->name());
-  } else {
-    // When parsing constructor isn't generated, use builder to parse
-    // messages. Note, will fallback to use reflection based mergeFieldFrom()
-    // in AbstractMessage.Builder.
-    printer->Indent();
-    printer->Print(
-        "Builder builder = newBuilder();\n"
-        "try {\n"
-        "  builder.mergeFrom(input, extensionRegistry);\n"
-        "} catch (com.google.protobuf.InvalidProtocolBufferException e) {\n"
-        "  throw e.setUnfinishedMessage(builder.buildPartial());\n"
-        "} catch (java.io.IOException e) {\n"
-        "  throw new com.google.protobuf.InvalidProtocolBufferException(\n"
-        "      e.getMessage()).setUnfinishedMessage(\n"
-        "          builder.buildPartial());\n"
-        "}\n"
-        "return builder.buildPartial();\n");
-    printer->Outdent();
-  }
-  printer->Print("}\n");
-  printer->Outdent();
-  printer->Print(
+      "    PARSER = new com.google.protobuf.AbstractParser<$classname$>() {\n"
+      "  @java.lang.Override\n"
+      "  public $classname$ parsePartialFrom(\n"
+      "      com.google.protobuf.CodedInputStream input,\n"
+      "      com.google.protobuf.ExtensionRegistryLite extensionRegistry)\n"
+      "      throws com.google.protobuf.InvalidProtocolBufferException {\n"
+      "    Builder builder = newBuilder();\n"
+      "    try {\n"
+      "      builder.mergeFrom(input, extensionRegistry);\n"
+      "    } catch (com.google.protobuf.InvalidProtocolBufferException e) {\n"
+      "      throw e.setUnfinishedMessage(builder.buildPartial());\n"
+      "    } catch (com.google.protobuf.UninitializedMessageException e) {\n"
+      "      throw "
+      "e.asInvalidProtocolBufferException().setUnfinishedMessage(builder."
+      "buildPartial());\n"
+      "    } catch (java.io.IOException e) {\n"
+      "      throw new com.google.protobuf.InvalidProtocolBufferException(e)\n"
+      "          .setUnfinishedMessage(builder.buildPartial());\n"
+      "    }\n"
+      "    return builder.buildPartial();\n"
+      "  }\n"
       "};\n"
-      "\n");
-
-  printer->Print(
+      "\n"
       "public static com.google.protobuf.Parser<$classname$> parser() {\n"
       "  return PARSER;\n"
       "}\n"
@@ -1353,6 +1200,9 @@ void ImmutableMessageGenerator::GenerateParser(io::Printer* printer) {
       "  return PARSER;\n"
       "}\n"
       "\n",
+      "visibility",
+      ExposePublicParser(descriptor_->file()) ? "@java.lang.Deprecated public"
+                                              : "private",
       "classname", descriptor_->name());
 }
 
diff --git a/src/google/protobuf/compiler/java/java_message_builder.cc b/src/google/protobuf/compiler/java/java_message_builder.cc
index 320852b1b..24ea648c7 100644
--- a/src/google/protobuf/compiler/java/java_message_builder.cc
+++ b/src/google/protobuf/compiler/java/java_message_builder.cc
@@ -58,6 +58,9 @@ namespace protobuf {
 namespace compiler {
 namespace java {
 
+using internal::WireFormat;
+using internal::WireFormatLite;
+
 namespace {
 std::string MapValueImmutableClassdName(const Descriptor* descriptor,
                                         ClassNameResolver* name_resolver) {
@@ -285,43 +288,63 @@ void MessageBuilderGenerator::GenerateDescriptorMethods(io::Printer* printer) {
 
 void MessageBuilderGenerator::GenerateCommonBuilderMethods(
     io::Printer* printer) {
+  // Decide if we really need to have the "maybeForceBuilderInitialization()"
+  // method.
+  // TODO(b/249158148): Remove the need for this entirely
+  bool need_maybe_force_builder_init = false;
+  for (int i = 0; i < descriptor_->field_count(); i++) {
+    if (descriptor_->field(i)->message_type() != nullptr &&
+        !IsRealOneof(descriptor_->field(i)) &&
+        HasHasbit(descriptor_->field(i))) {
+      need_maybe_force_builder_init = true;
+      break;
+    }
+  }
+
+  const char* force_builder_init = need_maybe_force_builder_init
+                                       ? "  maybeForceBuilderInitialization();"
+                                       : "";
+
   printer->Print(
       "// Construct using $classname$.newBuilder()\n"
       "private Builder() {\n"
-      "  maybeForceBuilderInitialization();\n"
+      "$force_builder_init$\n"
       "}\n"
       "\n",
-      "classname", name_resolver_->GetImmutableClassName(descriptor_));
+      "classname", name_resolver_->GetImmutableClassName(descriptor_),
+      "force_builder_init", force_builder_init);
 
   printer->Print(
       "private Builder(\n"
       "    com.google.protobuf.GeneratedMessage$ver$.BuilderParent parent) {\n"
       "  super(parent);\n"
-      "  maybeForceBuilderInitialization();\n"
+      "$force_builder_init$\n"
       "}\n",
       "classname", name_resolver_->GetImmutableClassName(descriptor_), "ver",
-      GeneratedCodeVersionSuffix());
+      GeneratedCodeVersionSuffix(), "force_builder_init", force_builder_init);
 
-  printer->Print(
-      "private void maybeForceBuilderInitialization() {\n"
-      "  if (com.google.protobuf.GeneratedMessage$ver$\n"
-      "          .alwaysUseFieldBuilders) {\n",
-      "ver", GeneratedCodeVersionSuffix());
+  if (need_maybe_force_builder_init) {
+    printer->Print(
+        "private void maybeForceBuilderInitialization() {\n"
+        "  if (com.google.protobuf.GeneratedMessage$ver$\n"
+        "          .alwaysUseFieldBuilders) {\n",
+        "ver", GeneratedCodeVersionSuffix());
 
-  printer->Indent();
-  printer->Indent();
-  for (int i = 0; i < descriptor_->field_count(); i++) {
-    if (!IsRealOneof(descriptor_->field(i))) {
-      field_generators_.get(descriptor_->field(i))
-          .GenerateFieldBuilderInitializationCode(printer);
+    printer->Indent();
+    printer->Indent();
+    for (int i = 0; i < descriptor_->field_count(); i++) {
+      if (!IsRealOneof(descriptor_->field(i))) {
+        field_generators_.get(descriptor_->field(i))
+            .GenerateFieldBuilderInitializationCode(printer);
+      }
     }
-  }
-  printer->Outdent();
-  printer->Outdent();
+    printer->Outdent();
+    printer->Outdent();
 
-  printer->Print(
-      "  }\n"
-      "}\n");
+    printer->Print(
+        "  }\n"
+        "}\n");
+  }
 
   printer->Print(
       "@java.lang.Override\n"
@@ -331,10 +354,8 @@ void MessageBuilderGenerator::GenerateCommonBuilderMethods(
   printer->Indent();
 
   for (int i = 0; i < descriptor_->field_count(); i++) {
-    if (!IsRealOneof(descriptor_->field(i))) {
-      field_generators_.get(descriptor_->field(i))
-          .GenerateBuilderClearCode(printer);
-    }
+    field_generators_.get(descriptor_->field(i))
+        .GenerateBuilderClearCode(printer);
   }
 
   for (auto oneof : oneofs_) {
@@ -575,7 +596,7 @@ void MessageBuilderGenerator::GenerateCommonBuilderMethods(
       printer->Print("  this.mergeExtensionFields(other);\n");
     }
 
-    printer->Print("  this.mergeUnknownFields(other.unknownFields);\n");
+    printer->Print("  this.mergeUnknownFields(other.getUnknownFields());\n");
 
     printer->Print("  onChanged();\n");
 
@@ -596,20 +617,92 @@ void MessageBuilderGenerator::GenerateBuilderParsingMethods(
       "    com.google.protobuf.CodedInputStream input,\n"
       "    com.google.protobuf.ExtensionRegistryLite extensionRegistry)\n"
       "    throws java.io.IOException {\n"
-      "  $classname$ parsedMessage = null;\n"
+      "  if (extensionRegistry == null) {\n"
+      "    throw new java.lang.NullPointerException();\n"
+      "  }\n"
       "  try {\n"
-      "    parsedMessage = PARSER.parsePartialFrom(input, extensionRegistry);\n"
+      "    boolean done = false;\n"
+      "    while (!done) {\n"
+      "      int tag = input.readTag();\n"
+      "      switch (tag) {\n"
+      "        case 0:\n"  // zero signals EOF / limit reached
+      "          done = true;\n"
+      "          break;\n");
+  printer->Indent();  // method
+  printer->Indent();  // try
+  printer->Indent();  // while
+  printer->Indent();  // switch
+  GenerateBuilderFieldParsingCases(printer);
+  printer->Outdent();  // switch
+  printer->Outdent();  // while
+  printer->Outdent();  // try
+  printer->Outdent();  // method
+  printer->Print(
+      "        default: {\n"
+      "          if (!super.parseUnknownField(input, extensionRegistry, tag)) "
+      "{\n"
+      "            done = true; // was an endgroup tag\n"
+      "          }\n"
+      "          break;\n"
+      "        } // default:\n"
+      "      } // switch (tag)\n"
+      "    } // while (!done)\n"
       "  } catch (com.google.protobuf.InvalidProtocolBufferException e) {\n"
-      "    parsedMessage = ($classname$) e.getUnfinishedMessage();\n"
       "    throw e.unwrapIOException();\n"
       "  } finally {\n"
-      "    if (parsedMessage != null) {\n"
-      "      mergeFrom(parsedMessage);\n"
-      "    }\n"
-      "  }\n"
+      "    onChanged();\n"
+      "  } // finally\n"
       "  return this;\n"
-      "}\n",
-      "classname", name_resolver_->GetImmutableClassName(descriptor_));
+      "}\n");
+}
+
+void MessageBuilderGenerator::GenerateBuilderFieldParsingCases(
+    io::Printer* printer) {
+  std::unique_ptr<const FieldDescriptor*[]> sorted_fields(
+      SortFieldsByNumber(descriptor_));
+  for (int i = 0; i < descriptor_->field_count(); i++) {
+    const FieldDescriptor* field = sorted_fields[i];
+    GenerateBuilderFieldParsingCase(printer, field);
+    if (field->is_packable()) {
+      GenerateBuilderPackedFieldParsingCase(printer, field);
+    }
+  }
+}
+
+void MessageBuilderGenerator::GenerateBuilderFieldParsingCase(
+    io::Printer* printer, const FieldDescriptor* field) {
+  uint32_t tag = WireFormatLite::MakeTag(
+      field->number(), WireFormat::WireTypeForFieldType(field->type()));
+  std::string tagString = StrCat(static_cast<int32_t>(tag));
+  printer->Print("case $tag$: {\n", "tag", tagString);
+  printer->Indent();
+
+  field_generators_.get(field).GenerateBuilderParsingCode(printer);
+
+  printer->Outdent();
+  printer->Print(
+      "  break;\n"
+      "} // case $tag$\n",
+      "tag", tagString);
+}
+
+void MessageBuilderGenerator::GenerateBuilderPackedFieldParsingCase(
+    io::Printer* printer, const FieldDescriptor* field) {
+  // To make packed = true wire compatible, we generate parsing code from a
+  // packed version of this field regardless of field->options().packed().
+  uint32_t tag = WireFormatLite::MakeTag(
+      field->number(), WireFormatLite::WIRETYPE_LENGTH_DELIMITED);
+  std::string tagString = StrCat(static_cast<int32_t>(tag));
+  printer->Print("case $tag$: {\n", "tag", tagString);
+  printer->Indent();
+
+  field_generators_.get(field).GenerateBuilderParsingCodeFromPacked(printer);
+
+  printer->Outdent();
+  printer->Print(
+      "  break;\n"
+      "} // case $tag$\n",
+      "tag", tagString);
 }
 
 // ===================================================================
diff --git a/src/google/protobuf/compiler/java/java_message_builder.h b/src/google/protobuf/compiler/java/java_message_builder.h
index fcd73b343..96f289a83 100644
--- a/src/google/protobuf/compiler/java/java_message_builder.h
+++ b/src/google/protobuf/compiler/java/java_message_builder.h
@@ -70,6 +70,11 @@ class MessageBuilderGenerator {
   void GenerateCommonBuilderMethods(io::Printer* printer);
   void GenerateDescriptorMethods(io::Printer* printer);
   void GenerateBuilderParsingMethods(io::Printer* printer);
+  void GenerateBuilderFieldParsingCases(io::Printer* printer);
+  void GenerateBuilderFieldParsingCase(io::Printer* printer,
+                                       const FieldDescriptor* field);
+  void GenerateBuilderPackedFieldParsingCase(io::Printer* printer,
+                                             const FieldDescriptor* field);
   void GenerateIsInitialized(io::Printer* printer);
 
   const Descriptor* descriptor_;
diff --git a/src/google/protobuf/compiler/java/java_message_field.cc b/src/google/protobuf/compiler/java/java_message_field.cc
index 8aae96145..24592082f 100644
--- a/src/google/protobuf/compiler/java/java_message_field.cc
+++ b/src/google/protobuf/compiler/java/java_message_field.cc
@@ -109,13 +109,6 @@ void SetMessageVariables(const FieldDescriptor* descriptor, int messageBitIndex,
   (*variables)["set_mutable_bit_builder"] = GenerateSetBit(builderBitIndex);
   (*variables)["clear_mutable_bit_builder"] = GenerateClearBit(builderBitIndex);
 
-  // For repeated fields, one bit is used for whether the array is immutable
-  // in the parsing constructor.
-  (*variables)["get_mutable_bit_parser"] =
-      GenerateGetBitMutableLocal(builderBitIndex);
-  (*variables)["set_mutable_bit_parser"] =
-      GenerateSetBitMutableLocal(builderBitIndex);
-
   (*variables)["get_has_field_bit_from_local"] =
       GenerateGetBitFromLocal(builderBitIndex);
   (*variables)["set_has_field_bit_to_local"] =
@@ -490,35 +483,21 @@ void ImmutableMessageFieldGenerator::GenerateBuildingCode(
   }
 }
 
-void ImmutableMessageFieldGenerator::GenerateParsingCode(
+void ImmutableMessageFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
-  printer->Print(variables_,
-                 "$type$.Builder subBuilder = null;\n"
-                 "if ($is_field_present_message$) {\n"
-                 "  subBuilder = $name$_.toBuilder();\n"
-                 "}\n");
-
   if (GetType(descriptor_) == FieldDescriptor::TYPE_GROUP) {
     printer->Print(variables_,
-                   "$name$_ = input.readGroup($number$, $type$.$get_parser$,\n"
-                   "    extensionRegistry);\n");
+                   "input.readGroup($number$,\n"
+                   "    get$capitalized_name$FieldBuilder().getBuilder(),\n"
+                   "    extensionRegistry);\n"
+                   "$set_has_field_bit_builder$\n");
   } else {
     printer->Print(variables_,
-                   "$name$_ = input.readMessage($type$.$get_parser$, "
-                   "extensionRegistry);\n");
+                   "input.readMessage(\n"
+                   "    get$capitalized_name$FieldBuilder().getBuilder(),\n"
+                   "    extensionRegistry);\n"
+                   "$set_has_field_bit_builder$\n");
   }
-
-  printer->Print(variables_,
-                 "if (subBuilder != null) {\n"
-                 "  subBuilder.mergeFrom($name$_);\n"
-                 "  $name$_ = subBuilder.buildPartial();\n"
-                 "}\n"
-                 "$set_has_field_bit_message$\n");
-}
-
-void ImmutableMessageFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  // noop for messages.
 }
 
 void ImmutableMessageFieldGenerator::GenerateSerializationCode(
@@ -770,6 +749,15 @@ void ImmutableMessageOneofFieldGenerator::GenerateBuilderMembers(
   printer->Annotate("{", "}", descriptor_);
 }
 
+void ImmutableMessageOneofFieldGenerator::GenerateBuilderClearCode(
+    io::Printer* printer) const {
+  // Make sure the builder gets cleared.
+  printer->Print(variables_,
+                 "if ($name$Builder_ != null) {\n"
+                 "  $name$Builder_.clear();\n"
+                 "}\n");
+}
+
 void ImmutableMessageOneofFieldGenerator::GenerateBuildingCode(
     io::Printer* printer) const {
   printer->Print(variables_, "if ($has_oneof_case_message$) {\n");
@@ -790,32 +778,21 @@ void ImmutableMessageOneofFieldGenerator::GenerateMergingCode(
                  "merge$capitalized_name$(other.get$capitalized_name$());\n");
 }
 
-void ImmutableMessageOneofFieldGenerator::GenerateParsingCode(
+void ImmutableMessageOneofFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
-  printer->Print(variables_,
-                 "$type$.Builder subBuilder = null;\n"
-                 "if ($has_oneof_case_message$) {\n"
-                 "  subBuilder = (($type$) $oneof_name$_).toBuilder();\n"
-                 "}\n");
-
   if (GetType(descriptor_) == FieldDescriptor::TYPE_GROUP) {
-    printer->Print(
-        variables_,
-        "$oneof_name$_ = input.readGroup($number$, $type$.$get_parser$,\n"
-        "    extensionRegistry);\n");
+    printer->Print(variables_,
+                   "input.readGroup($number$,\n"
+                   "    get$capitalized_name$FieldBuilder().getBuilder(),\n"
+                   "    extensionRegistry);\n"
+                   "$set_oneof_case_message$;\n");
   } else {
-    printer->Print(
-        variables_,
-        "$oneof_name$_ =\n"
-        "    input.readMessage($type$.$get_parser$, extensionRegistry);\n");
+    printer->Print(variables_,
+                   "input.readMessage(\n"
+                   "    get$capitalized_name$FieldBuilder().getBuilder(),\n"
+                   "    extensionRegistry);\n"
+                   "$set_oneof_case_message$;\n");
   }
-
-  printer->Print(variables_,
-                 "if (subBuilder != null) {\n"
-                 "  subBuilder.mergeFrom(($type$) $oneof_name$_);\n"
-                 "  $oneof_name$_ = subBuilder.buildPartial();\n"
-                 "}\n");
-  printer->Print(variables_, "$set_oneof_case_message$;\n");
 }
 
 void ImmutableMessageOneofFieldGenerator::GenerateSerializationCode(
@@ -1266,10 +1243,12 @@ void RepeatedImmutableMessageFieldGenerator::GenerateInitializationCode(
 void RepeatedImmutableMessageFieldGenerator::GenerateBuilderClearCode(
     io::Printer* printer) const {
   PrintNestedBuilderCondition(printer,
-                              "$name$_ = java.util.Collections.emptyList();\n"
-                              "$clear_mutable_bit_builder$;\n",
+                              "$name$_ = java.util.Collections.emptyList();\n",
 
+                              "$name$_ = null;\n"
                               "$name$Builder_.clear();\n");
+
+  printer->Print(variables_, "$clear_mutable_bit_builder$;\n");
 }
 
 void RepeatedImmutableMessageFieldGenerator::GenerateMergingCode(
@@ -1324,34 +1303,25 @@ void RepeatedImmutableMessageFieldGenerator::GenerateBuildingCode(
       "result.$name$_ = $name$Builder_.build();\n");
 }
 
-void RepeatedImmutableMessageFieldGenerator::GenerateParsingCode(
+void RepeatedImmutableMessageFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
-  printer->Print(variables_,
-                 "if (!$get_mutable_bit_parser$) {\n"
-                 "  $name$_ = new java.util.ArrayList<$type$>();\n"
-                 "  $set_mutable_bit_parser$;\n"
-                 "}\n");
-
   if (GetType(descriptor_) == FieldDescriptor::TYPE_GROUP) {
-    printer->Print(
-        variables_,
-        "$name$_.add(input.readGroup($number$, $type$.$get_parser$,\n"
-        "    extensionRegistry));\n");
+    printer->Print(variables_,
+                   "$type$ m =\n"
+                   "    input.readGroup($number$,\n"
+                   "        $type$.$get_parser$,\n"
+                   "        extensionRegistry);\n");
   } else {
-    printer->Print(
-        variables_,
-        "$name$_.add(\n"
-        "    input.readMessage($type$.$get_parser$, extensionRegistry));\n");
+    printer->Print(variables_,
+                   "$type$ m =\n"
+                   "    input.readMessage(\n"
+                   "        $type$.$get_parser$,\n"
+                   "        extensionRegistry);\n");
   }
-}
-
-void RepeatedImmutableMessageFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  printer->Print(
-      variables_,
-      "if ($get_mutable_bit_parser$) {\n"
-      "  $name$_ = java.util.Collections.unmodifiableList($name$_);\n"
-      "}\n");
+  PrintNestedBuilderCondition(printer,
+                              "ensure$capitalized_name$IsMutable();\n"
+                              "$name$_.add(m);\n",
+                              "$name$Builder_.addMessage(m);\n");
 }
 
 void RepeatedImmutableMessageFieldGenerator::GenerateSerializationCode(
diff --git a/src/google/protobuf/compiler/java/java_message_field.h b/src/google/protobuf/compiler/java/java_message_field.h
index 8588100b6..172a45210 100644
--- a/src/google/protobuf/compiler/java/java_message_field.h
+++ b/src/google/protobuf/compiler/java/java_message_field.h
@@ -74,8 +74,7 @@ class ImmutableMessageFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
@@ -114,9 +113,10 @@ class ImmutableMessageOneofFieldGenerator
 
   void GenerateMembers(io::Printer* printer) const override;
   void GenerateBuilderMembers(io::Printer* printer) const override;
+  void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
 
@@ -141,8 +141,7 @@ class RepeatedImmutableMessageFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
diff --git a/src/google/protobuf/compiler/java/java_primitive_field.cc b/src/google/protobuf/compiler/java/java_primitive_field.cc
index f67f6d39e..ec040047f 100644
--- a/src/google/protobuf/compiler/java/java_primitive_field.cc
+++ b/src/google/protobuf/compiler/java/java_primitive_field.cc
@@ -186,13 +186,6 @@ void SetPrimitiveVariables(const FieldDescriptor* descriptor,
   (*variables)["set_mutable_bit_builder"] = GenerateSetBit(builderBitIndex);
   (*variables)["clear_mutable_bit_builder"] = GenerateClearBit(builderBitIndex);
 
-  // For repeated fields, one bit is used for whether the array is immutable
-  // in the parsing constructor.
-  (*variables)["get_mutable_bit_parser"] =
-      GenerateGetBitMutableLocal(builderBitIndex);
-  (*variables)["set_mutable_bit_parser"] =
-      GenerateSetBitMutableLocal(builderBitIndex);
-
   (*variables)["get_has_field_bit_from_local"] =
       GenerateGetBitFromLocal(builderBitIndex);
   (*variables)["set_has_field_bit_to_local"] =
@@ -400,16 +393,11 @@ void ImmutablePrimitiveFieldGenerator::GenerateBuildingCode(
   }
 }
 
-void ImmutablePrimitiveFieldGenerator::GenerateParsingCode(
+void ImmutablePrimitiveFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   printer->Print(variables_,
-                 "$set_has_field_bit_message$\n"
-                 "$name$_ = input.read$capitalized_type$();\n");
-}
-
-void ImmutablePrimitiveFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  // noop for primitives.
+                 "$name$_ = input.read$capitalized_type$();\n"
+                 "$set_has_field_bit_builder$\n");
 }
 
 void ImmutablePrimitiveFieldGenerator::GenerateSerializationCode(
@@ -614,6 +602,12 @@ void ImmutablePrimitiveOneofFieldGenerator::GenerateBuilderMembers(
   printer->Annotate("{", "}", descriptor_);
 }
 
+void ImmutablePrimitiveOneofFieldGenerator::GenerateBuilderClearCode(
+    io::Printer* printer) const {
+  // No-Op: When a primitive field is in a oneof, clearing the oneof clears that
+  // field.
+}
+
 void ImmutablePrimitiveOneofFieldGenerator::GenerateBuildingCode(
     io::Printer* printer) const {
   printer->Print(variables_,
@@ -628,7 +622,7 @@ void ImmutablePrimitiveOneofFieldGenerator::GenerateMergingCode(
                  "set$capitalized_name$(other.get$capitalized_name$());\n");
 }
 
-void ImmutablePrimitiveOneofFieldGenerator::GenerateParsingCode(
+void ImmutablePrimitiveOneofFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   printer->Print(variables_,
                  "$oneof_name$_ = input.read$capitalized_type$();\n"
@@ -982,38 +976,24 @@ void RepeatedImmutablePrimitiveFieldGenerator::GenerateBuildingCode(
                  "result.$name$_ = $name$_;\n");
 }
 
-void RepeatedImmutablePrimitiveFieldGenerator::GenerateParsingCode(
+void RepeatedImmutablePrimitiveFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   printer->Print(variables_,
-                 "if (!$get_mutable_bit_parser$) {\n"
-                 "  $name$_ = $create_list$;\n"
-                 "  $set_mutable_bit_parser$;\n"
-                 "}\n"
-                 "$repeated_add$(input.read$capitalized_type$());\n");
+                 "$type$ v = input.read$capitalized_type$();\n"
+                 "ensure$capitalized_name$IsMutable();\n"
+                 "$repeated_add$(v);\n");
 }
 
-void RepeatedImmutablePrimitiveFieldGenerator::GenerateParsingCodeFromPacked(
-    io::Printer* printer) const {
-  printer->Print(
-      variables_,
-      "int length = input.readRawVarint32();\n"
-      "int limit = input.pushLimit(length);\n"
-      "if (!$get_mutable_bit_parser$ && input.getBytesUntilLimit() > 0) {\n"
-      "  $name$_ = $create_list$;\n"
-      "  $set_mutable_bit_parser$;\n"
-      "}\n"
-      "while (input.getBytesUntilLimit() > 0) {\n"
-      "  $repeated_add$(input.read$capitalized_type$());\n"
-      "}\n"
-      "input.popLimit(limit);\n");
-}
-
-void RepeatedImmutablePrimitiveFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
+void RepeatedImmutablePrimitiveFieldGenerator::
+    GenerateBuilderParsingCodeFromPacked(io::Printer* printer) const {
   printer->Print(variables_,
-                 "if ($get_mutable_bit_parser$) {\n"
-                 "  $name_make_immutable$; // C\n"
-                 "}\n");
+                 "int length = input.readRawVarint32();\n"
+                 "int limit = input.pushLimit(length);\n"
+                 "ensure$capitalized_name$IsMutable();\n"
+                 "while (input.getBytesUntilLimit() > 0) {\n"
+                 "  $repeated_add$(input.read$capitalized_type$());\n"
+                 "}\n"
+                 "input.popLimit(limit);\n");
 }
 
 void RepeatedImmutablePrimitiveFieldGenerator::GenerateSerializationCode(
diff --git a/src/google/protobuf/compiler/java/java_primitive_field.h b/src/google/protobuf/compiler/java/java_primitive_field.h
index 1f0eb8c86..1f1aa0c41 100644
--- a/src/google/protobuf/compiler/java/java_primitive_field.h
+++ b/src/google/protobuf/compiler/java/java_primitive_field.h
@@ -74,8 +74,7 @@ class ImmutablePrimitiveFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
@@ -105,9 +104,10 @@ class ImmutablePrimitiveOneofFieldGenerator
 
   void GenerateMembers(io::Printer* printer) const override;
   void GenerateBuilderMembers(io::Printer* printer) const override;
+  void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
 
@@ -133,9 +133,9 @@ class RepeatedImmutablePrimitiveFieldGenerator
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingCodeFromPacked(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCodeFromPacked(
+      io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
diff --git a/src/google/protobuf/compiler/java/java_string_field.cc b/src/google/protobuf/compiler/java/java_string_field.cc
index 1a0959e40..7a3b458e9 100644
--- a/src/google/protobuf/compiler/java/java_string_field.cc
+++ b/src/google/protobuf/compiler/java/java_string_field.cc
@@ -128,13 +128,6 @@ void SetPrimitiveVariables(const FieldDescriptor* descriptor,
   (*variables)["set_mutable_bit_builder"] = GenerateSetBit(builderBitIndex);
   (*variables)["clear_mutable_bit_builder"] = GenerateClearBit(builderBitIndex);
 
-  // For repeated fields, one bit is used for whether the array is immutable
-  // in the parsing constructor.
-  (*variables)["get_mutable_bit_parser"] =
-      GenerateGetBitMutableLocal(builderBitIndex);
-  (*variables)["set_mutable_bit_parser"] =
-      GenerateSetBitMutableLocal(builderBitIndex);
-
   (*variables)["get_has_field_bit_from_local"] =
       GenerateGetBitFromLocal(builderBitIndex);
   (*variables)["set_has_field_bit_to_local"] =
@@ -452,26 +445,19 @@ void ImmutableStringFieldGenerator::GenerateBuildingCode(
   printer->Print(variables_, "result.$name$_ = $name$_;\n");
 }
 
-void ImmutableStringFieldGenerator::GenerateParsingCode(
+void ImmutableStringFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   if (CheckUtf8(descriptor_)) {
     printer->Print(variables_,
-                   "java.lang.String s = input.readStringRequireUtf8();\n"
-                   "$set_has_field_bit_message$\n"
-                   "$name$_ = s;\n");
+                   "$name$_ = input.readStringRequireUtf8();\n"
+                   "$set_has_field_bit_builder$\n");
   } else {
     printer->Print(variables_,
-                   "com.google.protobuf.ByteString bs = input.readBytes();\n"
-                   "$set_has_field_bit_message$\n"
-                   "$name$_ = bs;\n");
+                   "$name$_ = input.readBytes();\n"
+                   "$set_has_field_bit_builder$\n");
   }
 }
 
-void ImmutableStringFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  // noop for strings.
-}
-
 void ImmutableStringFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) const {
   printer->Print(variables_,
@@ -695,6 +681,11 @@ void ImmutableStringOneofFieldGenerator::GenerateBuilderMembers(
                  "}\n");
 }
 
+void ImmutableStringOneofFieldGenerator::GenerateBuilderClearCode(
+    io::Printer* printer) const {
+  // No-Op: String fields in oneofs are correctly cleared by clearing the oneof
+}
+
 void ImmutableStringOneofFieldGenerator::GenerateMergingCode(
     io::Printer* printer) const {
   // Allow a slight breach of abstraction here in order to avoid forcing
@@ -713,7 +704,7 @@ void ImmutableStringOneofFieldGenerator::GenerateBuildingCode(
                  "}\n");
 }
 
-void ImmutableStringOneofFieldGenerator::GenerateParsingCode(
+void ImmutableStringOneofFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   if (CheckUtf8(descriptor_)) {
     printer->Print(variables_,
@@ -1104,35 +1095,21 @@ void RepeatedImmutableStringFieldGenerator::GenerateBuildingCode(
                  "result.$name$_ = $name$_;\n");
 }
 
-void RepeatedImmutableStringFieldGenerator::GenerateParsingCode(
+void RepeatedImmutableStringFieldGenerator::GenerateBuilderParsingCode(
     io::Printer* printer) const {
   if (CheckUtf8(descriptor_)) {
     printer->Print(variables_,
-                   "java.lang.String s = input.readStringRequireUtf8();\n");
+                   "java.lang.String s = input.readStringRequireUtf8();\n"
+                   "ensure$capitalized_name$IsMutable();\n"
+                   "$name$_.add(s);\n");
   } else {
     printer->Print(variables_,
-                   "com.google.protobuf.ByteString bs = input.readBytes();\n");
-  }
-  printer->Print(variables_,
-                 "if (!$get_mutable_bit_parser$) {\n"
-                 "  $name$_ = new com.google.protobuf.LazyStringArrayList();\n"
-                 "  $set_mutable_bit_parser$;\n"
-                 "}\n");
-  if (CheckUtf8(descriptor_)) {
-    printer->Print(variables_, "$name$_.add(s);\n");
-  } else {
-    printer->Print(variables_, "$name$_.add(bs);\n");
+                   "com.google.protobuf.ByteString bs = input.readBytes();\n"
+                   "ensure$capitalized_name$IsMutable();\n"
+                   "$name$_.add(bs);\n");
   }
 }
 
-void RepeatedImmutableStringFieldGenerator::GenerateParsingDoneCode(
-    io::Printer* printer) const {
-  printer->Print(variables_,
-                 "if ($get_mutable_bit_parser$) {\n"
-                 "  $name$_ = $name$_.getUnmodifiableView();\n"
-                 "}\n");
-}
-
 void RepeatedImmutableStringFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) const {
   printer->Print(variables_,
diff --git a/src/google/protobuf/compiler/java/java_string_field.h b/src/google/protobuf/compiler/java/java_string_field.h
index efab5fee4..47517649e 100644
--- a/src/google/protobuf/compiler/java/java_string_field.h
+++ b/src/google/protobuf/compiler/java/java_string_field.h
@@ -74,8 +74,7 @@ class ImmutableStringFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(
@@ -106,9 +105,10 @@ class ImmutableStringOneofFieldGenerator
  private:
   void GenerateMembers(io::Printer* printer) const override;
   void GenerateBuilderMembers(io::Printer* printer) const override;
+  void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
 
@@ -132,8 +132,7 @@ class RepeatedImmutableStringFieldGenerator : public ImmutableFieldGenerator {
   void GenerateBuilderClearCode(io::Printer* printer) const override;
   void GenerateMergingCode(io::Printer* printer) const override;
   void GenerateBuildingCode(io::Printer* printer) const override;
-  void GenerateParsingCode(io::Printer* printer) const override;
-  void GenerateParsingDoneCode(io::Printer* printer) const override;
+  void GenerateBuilderParsingCode(io::Printer* printer) const override;
   void GenerateSerializationCode(io::Printer* printer) const override;
   void GenerateSerializedSizeCode(io::Printer* printer) const override;
   void GenerateFieldBuilderInitializationCode(