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/*
* Copyright (C) 2009 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import java.lang.ref.WeakReference;
public class Main {
public static void main(String[] args) {
staleStackTest();
}
public static void staleStackTest() {
WeakReference wrefs[] = new WeakReference[10];
populate(wrefs);
check(wrefs);
}
static void populate(WeakReference[] wrefs) {
/*
* Get a bunch of non-constant String objects into registers. These
* should be the first locals declared.
*/
String str0 = generateString("String", 0);
String str1 = generateString("String", 1);
String str2 = generateString("String", 2);
String str3 = generateString("String", 3);
String str4 = generateString("String", 4);
String str5 = generateString("String", 5);
String str6 = generateString("String", 6);
String str7 = generateString("String", 7);
String str8 = generateString("String", 8);
String str9 = generateString("String", 9);
/* stuff them into the weak references array */
wrefs[0] = new WeakReference(str0);
wrefs[1] = new WeakReference(str1);
wrefs[2] = new WeakReference(str2);
wrefs[3] = new WeakReference(str3);
wrefs[4] = new WeakReference(str4);
wrefs[5] = new WeakReference(str5);
wrefs[6] = new WeakReference(str6);
wrefs[7] = new WeakReference(str7);
wrefs[8] = new WeakReference(str8);
wrefs[9] = new WeakReference(str9);
}
static String generateString(String base, int num) {
return base + num;
}
static void check(WeakReference[] wrefs) {
/*
* Declare locals so that our stack overlaps the same region
* that populate() did.
*/
String str0;
String str1;
String str2;
String str3;
String str4;
String str5;
String str6;
String str7;
String str8;
String str9;
int numValid = 0;
/*
* This *should* blow out all the weakly-reference objects. If
* we still have stale copies of references on the stack, a
* conservative GC will try to hold on to those objects and the
* count will be nonzero.
*
* Getting a zero result here isn't conclusive, but it's a strong
* indicator that precise GC is having an impact.
*/
System.gc();
for (int i = 0; i < wrefs.length; i++) {
if (wrefs[i].get() != null)
numValid++;
}
System.out.println("Valid refs: " + numValid);
/* use the locals in case the compiler gets smart */
str0 = generateString("String", 0);
str1 = generateString("String", 1);
str2 = generateString("String", 2);
str3 = generateString("String", 3);
str4 = generateString("String", 4);
str5 = generateString("String", 5);
str6 = generateString("String", 6);
str7 = generateString("String", 7);
str8 = generateString("String", 8);
str9 = generateString("String", 9);
System.out.println(str0+str1+str2+str3+str4+str5+str6+str7+str8+str9);
}
}
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