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/*
* This file is part of the LibreOffice project.
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*
* This file incorporates work covered by the following license notice:
*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed
* with this work for additional information regarding copyright
* ownership. The ASF licenses this file to you 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 .
*/
package util;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
/**
* Wait until an object has become unreachable.
*
* <p>Instances of this class will typically be used as either:</p>
* <pre>
* SomeType o = new SomeType(...);
* ... // use "o"
* WaitUnreachable u = new WaitUnreachable(o);
* o = null;
* u.waitUnreachable();
* </pre>
* <p>or:</p>
* <pre>
* WaitUnreachable u = new WaitUnreachable(new SomeType(...));
* ... // use "(SomeType) u.get()"
* u.waitUnreachable();
* </pre>
*/
public final class WaitUnreachable {
/**
* Creates a new waiter.
*
* @param obj the object on which to wait
*/
public WaitUnreachable(Object obj) {
this.obj = obj;
queue = new ReferenceQueue<Object>();
ref = new PhantomReference<Object>(obj, queue);
}
/**
* Gets the object on which to wait.
*
* @return the object on which to wait, or <code>null</code> if
* <code>waitUnreachable</code> has already been called
*/
public synchronized Object get() {
return obj;
}
/**
* Starts waiting for the object to become unreachable.
*
* <p>This blocks the current thread until the object has become
* unreachable.</p>
*
* <p>Actually, this method waits until the JVM has <em>detected</em> that
* the object has become unreachable. This is not deterministic, but this
* methods makes a best effort to cause the JVM to eventually detect the
* situation. With a typical JVM, this should suffice.</p>
*/
public void waitUnreachable() {
synchronized (this) {
obj = null;
}
System.out.println("waiting for gc");
while (queue.poll() == null) {
System.gc();
System.runFinalization();
}
}
/**
* Ensures that an object will be finalized as soon as possible.
*
* <p>This does not block the current thread. Instead, a new thread is
* spawned that busy waits for the given object to become unreachable.</p>
*
* <p>This method cannot guarantee that the given object is eventually
* finalized, but it makes a best effort. With a typical JVM, this should
* suffice.</p>
*
* @param obj the object of which to ensure finalization
*/
public static void ensureFinalization(final Object obj) {
final class WaitThread extends Thread {
private WaitThread(Object obj) {
super("ensureFinalization");
unreachable = new WaitUnreachable(obj);
}
@Override
public void run() {
unreachable.waitUnreachable();
}
private final WaitUnreachable unreachable;
}
new WaitThread(obj).start();
}
private Object obj;
private final ReferenceQueue<Object> queue;
@SuppressWarnings("unused")
private final PhantomReference<Object> ref;
}
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