1 /*
2 * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
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24 */
25
26 package java.lang.ref;
27
28 import jdk.internal.ref.CleanerImpl;
29
30 import java.util.Objects;
31 import java.util.concurrent.ThreadFactory;
32 import java.util.function.Function;
33
34 /**
35 * {@code Cleaner} manages a set of object references and corresponding cleaning actions.
36 * <p>
37 * Cleaning actions are {@link #register(Object object, Runnable action) registered}
38 * to run after the cleaner is notified that the object has become
39 * phantom reachable.
40 * The cleaner uses {@link PhantomReference} and {@link ReferenceQueue} to be
41 * notified when the <a href="package-summary.html#reachability">reachability</a>
42 * changes.
43 * <p>
44 * Each cleaner operates independently, managing the pending cleaning actions
45 * and handling threading and termination when the cleaner is no longer in use.
46 * Registering an object reference and corresponding cleaning action returns
47 * a {@link Cleanable Cleanable}. The most efficient use is to explicitly invoke
48 * the {@link Cleanable#clean clean} method when the object is closed or
49 * no longer needed.
50 * The cleaning action is a {@link Runnable} to be invoked at most once when
51 * the object has become phantom reachable unless it has already been explicitly cleaned.
52 * Note that the cleaning action must not refer to the object being registered.
53 * If so, the object will not become phantom reachable and the cleaning action
54 * will not be invoked automatically.
55 * <p>
56 * The execution of the cleaning action is performed
57 * by a thread associated with the cleaner.
58 * All exceptions thrown by the cleaning action are ignored.
59 * The cleaner and other cleaning actions are not affected by
60 * exceptions in a cleaning action.
61 * The thread runs until all registered cleaning actions have
62 * completed and the cleaner itself is reclaimed by the garbage collector.
63 * <p>
64 * The behavior of cleaners during {@link System#exit(int) System.exit}
65 * is implementation specific. No guarantees are made relating
66 * to whether cleaning actions are invoked or not.
67 * <p>
68 * Unless otherwise noted, passing a {@code null} argument to a constructor or
69 * method in this class will cause a
70 * {@link java.lang.NullPointerException NullPointerException} to be thrown.
71 *
72 * @apiNote
73 * The cleaning action is invoked only after the associated object becomes
74 * phantom reachable, so it is important that the object implementing the
75 * cleaning action does not hold references to the object.
76 * In this example, a static class encapsulates the cleaning state and action.
77 * An "inner" class, anonymous or not, must not be used because it implicitly
78 * contains a reference to the outer instance, preventing it from becoming
79 * phantom reachable.
80 * The choice of a new cleaner or sharing an existing cleaner is determined
81 * by the use case.
82 * <p>
83 * If the CleaningExample is used in a try-finally block then the
84 * {@code close} method calls the cleaning action.
85 * If the {@code close} method is not called, the cleaning action is called
86 * by the Cleaner when the CleaningExample instance has become phantom reachable.
87 * <pre>{@code
88 * public class CleaningExample implements AutoCloseable {
89 * // A cleaner, preferably one shared within a library
90 * private static final Cleaner cleaner = <cleaner>;
91 *
92 * static class State implements Runnable {
93 *
94 * State(...) {
95 * // initialize State needed for cleaning action
96 * }
97 *
98 * public void run() {
99 * // cleanup action accessing State, executed at most once
100 * }
101 * }
102 *
103 * private final State;
104 * private final Cleaner.Cleanable cleanable
105 *
106 * public CleaningExample() {
107 * this.state = new State(...);
108 * this.cleanable = cleaner.register(this, state);
109 * }
110 *
111 * public void close() {
112 * cleanable.clean();
113 * }
114 * }
115 * }</pre>
116 * The cleaning action could be a lambda but all too easily will capture
117 * the object reference, by referring to fields of the object being cleaned,
118 * preventing the object from becoming phantom reachable.
119 * Using a static nested class, as above, will avoid accidentally retaining the
120 * object reference.
121 * <p>
122 * <a name="compatible-cleaners"></a>
123 * Cleaning actions should be prepared to be invoked concurrently with
124 * other cleaning actions.
125 * Typically the cleaning actions should be very quick to execute
126 * and not block. If the cleaning action blocks, it may delay processing
127 * other cleaning actions registered to the same cleaner.
128 * All cleaning actions registered to a cleaner should be mutually compatible.
129 * @since 9
130 */
131 public final class Cleaner {
132
133 /**
134 * The Cleaner implementation.
135 */
136 final CleanerImpl impl;
137
138 static {
139 CleanerImpl.setCleanerImplAccess(new Function<Cleaner, CleanerImpl>() {
140 @Override
141 public CleanerImpl apply(Cleaner cleaner) {
142 return cleaner.impl;
143 }
144 });
145 }
146
147 /**
148 * Construct a Cleaner implementation and start it.
149 */
150 private Cleaner() {
151 impl = new CleanerImpl();
152 }
153
154 /**
155 * Returns a new {@code Cleaner}.
156 * <p>
157 * The cleaner creates a {@link Thread#setDaemon(boolean) daemon thread}
158 * to process the phantom reachable objects and to invoke cleaning actions.
159 * The {@linkplain java.lang.Thread#getContextClassLoader context class loader}
160 * of the thread is set to the
161 * {@link ClassLoader#getSystemClassLoader() system class loader}.
162 * The thread has no permissions, enforced only if a
163 * {@link java.lang.System#setSecurityManager(SecurityManager) SecurityManager is set}.
164 * <p>
165 * The cleaner terminates when it is phantom reachable and all of the
166 * registered cleaning actions are complete.
167 *
168 * @return a new {@code Cleaner}
169 *
170 * @throws SecurityException if the current thread is not allowed to
171 * create or start the thread.
172 */
173 public static Cleaner create() {
174 Cleaner cleaner = new Cleaner();
175 cleaner.impl.start(cleaner, null);
176 return cleaner;
177 }
178
179 /**
180 * Returns a new {@code Cleaner} using a {@code Thread} from the {@code ThreadFactory}.
181 * <p>
182 * A thread from the thread factory's {@link ThreadFactory#newThread(Runnable) newThread}
183 * method is set to be a {@link Thread#setDaemon(boolean) daemon thread}
184 * and started to process phantom reachable objects and invoke cleaning actions.
185 * On each call the {@link ThreadFactory#newThread(Runnable) thread factory}
186 * must provide a Thread that is suitable for performing the cleaning actions.
187 * <p>
188 * The cleaner terminates when it is phantom reachable and all of the
189 * registered cleaning actions are complete.
190 *
191 * @param threadFactory a {@code ThreadFactory} to return a new {@code Thread}
192 * to process cleaning actions
193 * @return a new {@code Cleaner}
194 *
195 * @throws IllegalThreadStateException if the thread from the thread
196 * factory was {@link Thread.State#NEW not a new thread}.
197 * @throws SecurityException if the current thread is not allowed to
198 * create or start the thread.
199 */
200 public static Cleaner create(ThreadFactory threadFactory) {
201 Objects.requireNonNull(threadFactory, "threadFactory");
202 Cleaner cleaner = new Cleaner();
203 cleaner.impl.start(cleaner, threadFactory);
204 return cleaner;
205 }
206
207 /**
208 * Registers an object and a cleaning action to run when the object
209 * becomes phantom reachable.
210 * Refer to the <a href="#compatible-cleaners">API Note</a> above for
211 * cautions about the behavior of cleaning actions.
212 *
213 * @param obj the object to monitor
214 * @param action a {@code Runnable} to invoke when the object becomes phantom reachable
215 * @return a {@code Cleanable} instance
216 */
217 public Cleanable register(Object obj, Runnable action) {
218 Objects.requireNonNull(obj, "obj");
219 Objects.requireNonNull(action, "action");
220 return new CleanerImpl.PhantomCleanableRef(obj, this, action);
221 }
222
223 /**
224 * {@code Cleanable} represents an object and a
225 * cleaning action registered in a {@code Cleaner}.
226 * @since 9
227 */
228 public interface Cleanable {
229 /**
230 * Unregisters the cleanable and invokes the cleaning action.
231 * The cleanable's cleaning action is invoked at most once
232 * regardless of the number of calls to {@code clean}.
233 */
234 void clean();
235 }
236
237 }