| /************************************************************** |
| * |
| * 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 |
| * |
| * 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. |
| * |
| *************************************************************/ |
| |
| |
| |
| 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(); |
| ref = new PhantomReference(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(); |
| byte[] dummy = new byte[1024]; |
| } |
| } |
| |
| /** |
| * 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 { |
| public WaitThread(Object obj) { |
| super("ensureFinalization"); |
| unreachable = new WaitUnreachable(obj); |
| } |
| |
| public void run() { |
| unreachable.waitUnreachable(); |
| } |
| |
| private final WaitUnreachable unreachable; |
| } |
| new WaitThread(obj).start(); |
| } |
| |
| private Object obj; |
| private final ReferenceQueue queue; |
| private final PhantomReference ref; |
| } |