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/*
* Copyright (c) 2014, 2018, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package gc.g1.unloading.keepref;
import gc.g1.unloading.check.cleanup.UnusedThreadKiller;
/**
* This holder prevents class from being unloaded by keeping reference in stack local.
*
*/
public class InStackLocalHolder implements RefHolder {
public static class AuxiliaryThread extends Thread {
private static final int STACK_DEPTH = 300;
private ReferenceHolder referenceHolder;
public AuxiliaryThread(ReferenceHolder referenceHolder) {
this.referenceHolder = referenceHolder;
}
synchronized public void finishThread() {
notifyAll();
}
@Override
synchronized public void run() {
new A().call(referenceHolder, STACK_DEPTH);
}
}
private static class A {
public void call(ReferenceHolder holder, int stackDepth) {
if (stackDepth > 0) {
new A().call(holder, stackDepth - 1);
} else {
Object ref = holder.obtainAndClear();
synchronized (Thread.currentThread()) {
try {
Thread.currentThread().wait();
if (ref.hashCode() == 42) {
System.out.println("This clause is made to prevent compiler and javac optimizations from eliminating local reference \"ref\".");
}
} catch (InterruptedException e) {
new RuntimeException("Unexpected InterruptedException");
}
}
}
}
}
private static class ReferenceHolder {
private Object reference;
public ReferenceHolder(Object reference) {
this.reference = reference;
}
public Object obtainAndClear() {
Object returnValue = reference;
reference = new Object();
return returnValue;
}
}
@Override
public Object hold(Object object) {
Thread thread = new AuxiliaryThread(new ReferenceHolder(object));
thread.setDaemon(true);
thread.start();
return new UnusedThreadKiller(thread.getId());
}
}
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