001 /*
002 * Copyright (C) 2008 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.base.internal;
018
019 import java.lang.ref.PhantomReference;
020 import java.lang.ref.Reference;
021 import java.lang.ref.ReferenceQueue;
022 import java.lang.ref.WeakReference;
023 import java.lang.reflect.Field;
024 import java.lang.reflect.Method;
025 import java.util.logging.Level;
026 import java.util.logging.Logger;
027
028 /**
029 * Thread that finalizes referents. All references should implement
030 * {@code com.google.common.base.FinalizableReference}.
031 *
032 * <p>While this class is public, we consider it to be *internal* and not part
033 * of our published API. It is public so we can access it reflectively across
034 * class loaders in secure environments.
035 *
036 * <p>This class can't depend on other Google Collections code. If we were
037 * to load this class in the same class loader as the rest of
038 * Google Collections, this thread would keep an indirect strong reference
039 * to the class loader and prevent it from being garbage collected. This
040 * poses a problem for environments where you want to throw away the class
041 * loader. For example, dynamically reloading a web application or unloading
042 * an OSGi bundle.
043 *
044 * <p>{@code com.google.common.base.FinalizableReferenceQueue} loads this class
045 * in its own class loader. That way, this class doesn't prevent the main
046 * class loader from getting garbage collected, and this class can detect when
047 * the main class loader has been garbage collected and stop itself.
048 */
049 public class Finalizer extends Thread {
050
051 private static final Logger logger
052 = Logger.getLogger(Finalizer.class.getName());
053
054 /** Name of FinalizableReference.class. */
055 private static final String FINALIZABLE_REFERENCE
056 = "com.google.common.base.FinalizableReference";
057
058 /**
059 * Starts the Finalizer thread. FinalizableReferenceQueue calls this method
060 * reflectively.
061 *
062 * @param finalizableReferenceClass FinalizableReference.class
063 * @param frq reference to instance of FinalizableReferenceQueue that started
064 * this thread
065 * @return ReferenceQueue which Finalizer will poll
066 */
067 public static ReferenceQueue<Object> startFinalizer(
068 Class<?> finalizableReferenceClass, Object frq) {
069 /*
070 * We use FinalizableReference.class for two things:
071 *
072 * 1) To invoke FinalizableReference.finalizeReferent()
073 *
074 * 2) To detect when FinalizableReference's class loader has to be garbage
075 * collected, at which point, Finalizer can stop running
076 */
077 if (!finalizableReferenceClass.getName().equals(FINALIZABLE_REFERENCE)) {
078 throw new IllegalArgumentException(
079 "Expected " + FINALIZABLE_REFERENCE + ".");
080 }
081
082 Finalizer finalizer = new Finalizer(finalizableReferenceClass, frq);
083 finalizer.start();
084 return finalizer.queue;
085 }
086
087 private final WeakReference<Class<?>> finalizableReferenceClassReference;
088 private final PhantomReference<Object> frqReference;
089 private final ReferenceQueue<Object> queue = new ReferenceQueue<Object>();
090
091 private static final Field inheritableThreadLocals
092 = getInheritableThreadLocalsField();
093
094 /** Constructs a new finalizer thread. */
095 private Finalizer(Class<?> finalizableReferenceClass, Object frq) {
096 super(Finalizer.class.getName());
097
098 this.finalizableReferenceClassReference
099 = new WeakReference<Class<?>>(finalizableReferenceClass);
100
101 // Keep track of the FRQ that started us so we know when to stop.
102 this.frqReference = new PhantomReference<Object>(frq, queue);
103
104 setDaemon(true);
105
106 try {
107 if (inheritableThreadLocals != null) {
108 inheritableThreadLocals.set(this, null);
109 }
110 } catch (Throwable t) {
111 logger.log(Level.INFO, "Failed to clear thread local values inherited"
112 + " by reference finalizer thread.", t);
113 }
114
115 // TODO(user): Priority?
116 }
117
118 /**
119 * Loops continuously, pulling references off the queue and cleaning them up.
120 */
121 @SuppressWarnings("InfiniteLoopStatement")
122 @Override
123 public void run() {
124 try {
125 while (true) {
126 try {
127 cleanUp(queue.remove());
128 } catch (InterruptedException e) { /* ignore */ }
129 }
130 } catch (ShutDown shutDown) { /* ignore */ }
131 }
132
133 /**
134 * Cleans up a single reference. Catches and logs all throwables.
135 */
136 private void cleanUp(Reference<?> reference) throws ShutDown {
137 Method finalizeReferentMethod = getFinalizeReferentMethod();
138 do {
139 /*
140 * This is for the benefit of phantom references. Weak and soft
141 * references will have already been cleared by this point.
142 */
143 reference.clear();
144
145 if (reference == frqReference) {
146 /*
147 * The client no longer has a reference to the
148 * FinalizableReferenceQueue. We can stop.
149 */
150 throw new ShutDown();
151 }
152
153 try {
154 finalizeReferentMethod.invoke(reference);
155 } catch (Throwable t) {
156 logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
157 }
158
159 /*
160 * Loop as long as we have references available so as not to waste
161 * CPU looking up the Method over and over again.
162 */
163 } while ((reference = queue.poll()) != null);
164 }
165
166 /**
167 * Looks up FinalizableReference.finalizeReferent() method.
168 */
169 private Method getFinalizeReferentMethod() throws ShutDown {
170 Class<?> finalizableReferenceClass
171 = finalizableReferenceClassReference.get();
172 if (finalizableReferenceClass == null) {
173 /*
174 * FinalizableReference's class loader was reclaimed. While there's a
175 * chance that other finalizable references could be enqueued
176 * subsequently (at which point the class loader would be resurrected
177 * by virtue of us having a strong reference to it), we should pretty
178 * much just shut down and make sure we don't keep it alive any longer
179 * than necessary.
180 */
181 throw new ShutDown();
182 }
183 try {
184 return finalizableReferenceClass.getMethod("finalizeReferent");
185 } catch (NoSuchMethodException e) {
186 throw new AssertionError(e);
187 }
188 }
189
190 public static Field getInheritableThreadLocalsField() {
191 try {
192 Field inheritableThreadLocals
193 = Thread.class.getDeclaredField("inheritableThreadLocals");
194 inheritableThreadLocals.setAccessible(true);
195 return inheritableThreadLocals;
196 } catch (Throwable t) {
197 logger.log(Level.INFO, "Couldn't access Thread.inheritableThreadLocals."
198 + " Reference finalizer threads will inherit thread local"
199 + " values.");
200 return null;
201 }
202 }
203
204 /** Indicates that it's time to shut down the Finalizer. */
205 @SuppressWarnings("serial") // Never serialized or thrown out of this class.
206 private static class ShutDown extends Exception { }
207 }