/**************************************************************
*
* 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;
}