1ef39d40dSAndrew Rist /************************************************************** 2cdf0e10cSrcweir * 3ef39d40dSAndrew Rist * Licensed to the Apache Software Foundation (ASF) under one 4ef39d40dSAndrew Rist * or more contributor license agreements. See the NOTICE file 5ef39d40dSAndrew Rist * distributed with this work for additional information 6ef39d40dSAndrew Rist * regarding copyright ownership. The ASF licenses this file 7ef39d40dSAndrew Rist * to you under the Apache License, Version 2.0 (the 8ef39d40dSAndrew Rist * "License"); you may not use this file except in compliance 9ef39d40dSAndrew Rist * with the License. You may obtain a copy of the License at 10ef39d40dSAndrew Rist * 11ef39d40dSAndrew Rist * http://www.apache.org/licenses/LICENSE-2.0 12ef39d40dSAndrew Rist * 13ef39d40dSAndrew Rist * Unless required by applicable law or agreed to in writing, 14ef39d40dSAndrew Rist * software distributed under the License is distributed on an 15ef39d40dSAndrew Rist * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY 16ef39d40dSAndrew Rist * KIND, either express or implied. See the License for the 17ef39d40dSAndrew Rist * specific language governing permissions and limitations 18ef39d40dSAndrew Rist * under the License. 19ef39d40dSAndrew Rist * 20ef39d40dSAndrew Rist *************************************************************/ 21ef39d40dSAndrew Rist 22ef39d40dSAndrew Rist 23cdf0e10cSrcweir 24cdf0e10cSrcweir package util; 25cdf0e10cSrcweir 26cdf0e10cSrcweir import java.lang.ref.PhantomReference; 27cdf0e10cSrcweir import java.lang.ref.ReferenceQueue; 28cdf0e10cSrcweir 29cdf0e10cSrcweir /** 30cdf0e10cSrcweir * Wait until an object has become unreachable. 31cdf0e10cSrcweir * 32cdf0e10cSrcweir * <p>Instances of this class will typically be used as either:</p> 33cdf0e10cSrcweir * <pre> 34cdf0e10cSrcweir * SomeType o = new SomeType(...); 35cdf0e10cSrcweir * ... // use "o" 36cdf0e10cSrcweir * WaitUnreachable u = new WaitUnreachable(o); 37cdf0e10cSrcweir * o = null; 38cdf0e10cSrcweir * u.waitUnreachable(); 39cdf0e10cSrcweir * </pre> 40cdf0e10cSrcweir * <p>or:</p> 41cdf0e10cSrcweir * <pre> 42cdf0e10cSrcweir * WaitUnreachable u = new WaitUnreachable(new SomeType(...)); 43cdf0e10cSrcweir * ... // use "(SomeType) u.get()" 44cdf0e10cSrcweir * u.waitUnreachable(); 45cdf0e10cSrcweir * </pre> 46cdf0e10cSrcweir */ 47cdf0e10cSrcweir public final class WaitUnreachable { 48cdf0e10cSrcweir /** 49cdf0e10cSrcweir * Creates a new waiter. 50cdf0e10cSrcweir * 51cdf0e10cSrcweir * @param obj the object on which to wait 52cdf0e10cSrcweir */ WaitUnreachable(Object obj)53cdf0e10cSrcweir public WaitUnreachable(Object obj) { 54cdf0e10cSrcweir this.obj = obj; 55cdf0e10cSrcweir queue = new ReferenceQueue(); 56cdf0e10cSrcweir ref = new PhantomReference(obj, queue); 57cdf0e10cSrcweir } 58cdf0e10cSrcweir 59cdf0e10cSrcweir /** 60cdf0e10cSrcweir * Gets the object on which to wait. 61cdf0e10cSrcweir * 62cdf0e10cSrcweir * @return the object on which to wait, or <code>null</code> if 63cdf0e10cSrcweir * <code>waitUnreachable</code> has already been called 64cdf0e10cSrcweir */ get()65cdf0e10cSrcweir public synchronized Object get() { 66cdf0e10cSrcweir return obj; 67cdf0e10cSrcweir } 68cdf0e10cSrcweir 69cdf0e10cSrcweir /** 70cdf0e10cSrcweir * Starts waiting for the object to become unreachable. 71cdf0e10cSrcweir * 72cdf0e10cSrcweir * <p>This blocks the current thread until the object has become 73cdf0e10cSrcweir * unreachable.</p> 74cdf0e10cSrcweir * 75cdf0e10cSrcweir * <p>Actually, this method waits until the JVM has <em>detected</em> that 76cdf0e10cSrcweir * the object has become unreachable. This is not deterministic, but this 77cdf0e10cSrcweir * methods makes a best effort to cause the JVM to eventually detect the 78cdf0e10cSrcweir * situation. With a typical JVM, this should suffice.</p> 79cdf0e10cSrcweir */ waitUnreachable()80cdf0e10cSrcweir public void waitUnreachable() { 81cdf0e10cSrcweir synchronized (this) { 82cdf0e10cSrcweir obj = null; 83cdf0e10cSrcweir } 84cdf0e10cSrcweir System.out.println("waiting for gc"); 85cdf0e10cSrcweir while (queue.poll() == null) { 86cdf0e10cSrcweir System.gc(); 87cdf0e10cSrcweir System.runFinalization(); 88cdf0e10cSrcweir byte[] dummy = new byte[1024]; 89cdf0e10cSrcweir } 90cdf0e10cSrcweir } 91cdf0e10cSrcweir 92cdf0e10cSrcweir /** 93cdf0e10cSrcweir * Ensures that an object will be finalized as soon as possible. 94cdf0e10cSrcweir * 95cdf0e10cSrcweir * <p>This does not block the current thread. Instead, a new thread is 96cdf0e10cSrcweir * spawned that busy waits for the given object to become unreachable.</p> 97cdf0e10cSrcweir * 98cdf0e10cSrcweir * <p>This method cannot guarantee that the given object is eventually 99cdf0e10cSrcweir * finalized, but it makes a best effort. With a typical JVM, this should 100cdf0e10cSrcweir * suffice.</p> 101cdf0e10cSrcweir * 102cdf0e10cSrcweir * @param obj the object of which to ensure finalization 103cdf0e10cSrcweir */ ensureFinalization(final Object obj)104cdf0e10cSrcweir public static void ensureFinalization(final Object obj) { 105cdf0e10cSrcweir final class WaitThread extends Thread { 106cdf0e10cSrcweir public WaitThread(Object obj) { 107cdf0e10cSrcweir super("ensureFinalization"); 108cdf0e10cSrcweir unreachable = new WaitUnreachable(obj); 109cdf0e10cSrcweir } 110cdf0e10cSrcweir 111cdf0e10cSrcweir public void run() { 112cdf0e10cSrcweir unreachable.waitUnreachable(); 113cdf0e10cSrcweir } 114cdf0e10cSrcweir 115cdf0e10cSrcweir private final WaitUnreachable unreachable; 116*170fb961SPedro Giffuni } 117cdf0e10cSrcweir new WaitThread(obj).start(); 118cdf0e10cSrcweir } 119cdf0e10cSrcweir 120cdf0e10cSrcweir private Object obj; 121cdf0e10cSrcweir private final ReferenceQueue queue; 122cdf0e10cSrcweir private final PhantomReference ref; 123cdf0e10cSrcweir } 124