/* * 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. * *

Instances of this class will typically be used as either:

*
 *   SomeType o = new SomeType(...);
 *   ... // use "o"
 *   WaitUnreachable u = new WaitUnreachable(o);
 *   o = null;
 *   u.waitUnreachable();
 * 
*

or:

*
 *   WaitUnreachable u = new WaitUnreachable(new SomeType(...));
 *   ... // use "(SomeType) u.get()"
 *   u.waitUnreachable();
 * 
*/ 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(); ref = new PhantomReference(obj, queue); } /** * Gets the object on which to wait. * * @return the object on which to wait, or null if * waitUnreachable has already been called */ public synchronized Object get() { return obj; } /** * Starts waiting for the object to become unreachable. * *

This blocks the current thread until the object has become * unreachable.

* *

Actually, this method waits until the JVM has detected 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.

*/ 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. * *

This does not block the current thread. Instead, a new thread is * spawned that busy waits for the given object to become unreachable.

* *

This method cannot guarantee that the given object is eventually * finalized, but it makes a best effort. With a typical JVM, this should * suffice.

* * @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 queue; @SuppressWarnings("unused") private final PhantomReference ref; }