1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
|
/*
* Copyright (C) 2008 The Guava Authors
*
* Licensed 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 com.google.common.base.internal;
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* Thread that finalizes referents. All references should implement
* {@code com.google.common.base.FinalizableReference}.
*
* <p>While this class is public, we consider it to be *internal* and not part
* of our published API. It is public so we can access it reflectively across
* class loaders in secure environments.
*
* <p>This class can't depend on other Google Collections code. If we were
* to load this class in the same class loader as the rest of
* Google Collections, this thread would keep an indirect strong reference
* to the class loader and prevent it from being garbage collected. This
* poses a problem for environments where you want to throw away the class
* loader. For example, dynamically reloading a web application or unloading
* an OSGi bundle.
*
* <p>{@code com.google.common.base.FinalizableReferenceQueue} loads this class
* in its own class loader. That way, this class doesn't prevent the main
* class loader from getting garbage collected, and this class can detect when
* the main class loader has been garbage collected and stop itself.
*/
public class Finalizer implements Runnable {
private static final Logger logger
= Logger.getLogger(Finalizer.class.getName());
/** Name of FinalizableReference.class. */
private static final String FINALIZABLE_REFERENCE
= "com.google.common.base.FinalizableReference";
/**
* Starts the Finalizer thread. FinalizableReferenceQueue calls this method
* reflectively.
*
* @param finalizableReferenceClass FinalizableReference.class.
* @param queue a reference queue that the thread will poll.
* @param frqReference a phantom reference to the FinalizableReferenceQueue, which will be
* queued either when the FinalizableReferenceQueue is no longer referenced anywhere, or when
* its close() method is called.
*/
public static void startFinalizer(
Class<?> finalizableReferenceClass,
ReferenceQueue<Object> queue,
PhantomReference<Object> frqReference) {
/*
* We use FinalizableReference.class for two things:
*
* 1) To invoke FinalizableReference.finalizeReferent()
*
* 2) To detect when FinalizableReference's class loader has to be garbage
* collected, at which point, Finalizer can stop running
*/
if (!finalizableReferenceClass.getName().equals(FINALIZABLE_REFERENCE)) {
throw new IllegalArgumentException(
"Expected " + FINALIZABLE_REFERENCE + ".");
}
Finalizer finalizer = new Finalizer(finalizableReferenceClass, queue, frqReference);
Thread thread = new Thread(finalizer);
thread.setName(Finalizer.class.getName());
thread.setDaemon(true);
try {
if (inheritableThreadLocals != null) {
inheritableThreadLocals.set(thread, null);
}
} catch (Throwable t) {
logger.log(Level.INFO, "Failed to clear thread local values inherited"
+ " by reference finalizer thread.", t);
}
thread.start();
}
private final WeakReference<Class<?>> finalizableReferenceClassReference;
private final PhantomReference<Object> frqReference;
private final ReferenceQueue<Object> queue;
private static final Field inheritableThreadLocals
= getInheritableThreadLocalsField();
/** Constructs a new finalizer thread. */
private Finalizer(
Class<?> finalizableReferenceClass,
ReferenceQueue<Object> queue,
PhantomReference<Object> frqReference) {
this.queue = queue;
this.finalizableReferenceClassReference
= new WeakReference<Class<?>>(finalizableReferenceClass);
// Keep track of the FRQ that started us so we know when to stop.
this.frqReference = frqReference;
}
/**
* Loops continuously, pulling references off the queue and cleaning them up.
*/
@SuppressWarnings("InfiniteLoopStatement")
@Override
public void run() {
try {
while (true) {
try {
cleanUp(queue.remove());
} catch (InterruptedException e) { /* ignore */ }
}
} catch (ShutDown shutDown) { /* ignore */ }
}
/**
* Cleans up a single reference. Catches and logs all throwables.
*/
private void cleanUp(Reference<?> reference) throws ShutDown {
Method finalizeReferentMethod = getFinalizeReferentMethod();
do {
/*
* This is for the benefit of phantom references. Weak and soft
* references will have already been cleared by this point.
*/
reference.clear();
if (reference == frqReference) {
/*
* The client no longer has a reference to the
* FinalizableReferenceQueue. We can stop.
*/
throw new ShutDown();
}
try {
finalizeReferentMethod.invoke(reference);
} catch (Throwable t) {
logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
}
/*
* Loop as long as we have references available so as not to waste
* CPU looking up the Method over and over again.
*/
} while ((reference = queue.poll()) != null);
}
/**
* Looks up FinalizableReference.finalizeReferent() method.
*/
private Method getFinalizeReferentMethod() throws ShutDown {
Class<?> finalizableReferenceClass
= finalizableReferenceClassReference.get();
if (finalizableReferenceClass == null) {
/*
* FinalizableReference's class loader was reclaimed. While there's a
* chance that other finalizable references could be enqueued
* subsequently (at which point the class loader would be resurrected
* by virtue of us having a strong reference to it), we should pretty
* much just shut down and make sure we don't keep it alive any longer
* than necessary.
*/
throw new ShutDown();
}
try {
return finalizableReferenceClass.getMethod("finalizeReferent");
} catch (NoSuchMethodException e) {
throw new AssertionError(e);
}
}
public static Field getInheritableThreadLocalsField() {
try {
Field inheritableThreadLocals
= Thread.class.getDeclaredField("inheritableThreadLocals");
inheritableThreadLocals.setAccessible(true);
return inheritableThreadLocals;
} catch (Throwable t) {
logger.log(Level.INFO, "Couldn't access Thread.inheritableThreadLocals."
+ " Reference finalizer threads will inherit thread local"
+ " values.");
return null;
}
}
/** Indicates that it's time to shut down the Finalizer. */
@SuppressWarnings("serial") // Never serialized or thrown out of this class.
private static class ShutDown extends Exception {}
}
|
