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 23 * questions. 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 33 /** 34 * {@code Cleaner} manages a set of object references and corresponding cleaning actions. 35 * <p> 36 * Cleaning actions are {@link #register(Object object, Runnable action) registered} 37 * to run after the cleaner is notified that the object has become 38 * phantom reachable. 39 * The cleaner uses {@link PhantomReference} and {@link ReferenceQueue} to be 40 * notified when the <a href="package-summary.html#reachability">reachability</a> 41 * changes. 42 * <p> 43 * Each cleaner operates independently, managing the pending cleaning actions 44 * and handling threading and termination when the cleaner is no longer in use. 45 * Registering an object reference and corresponding cleaning action returns 46 * a {@link Cleanable Cleanable}. The most efficient use is to explicitly invoke 47 * the {@link Cleanable#clean clean} method when the object is closed or 48 * no longer needed. 49 * The cleaning action is a {@link Runnable} to be invoked at most once when 50 * the object has become phantom reachable unless it has already been explicitly cleaned. 51 * Note that the cleaning action must not refer to the object being registered. 52 * If so, the object will not become phantom reachable and the cleaning action 53 * will not be invoked automatically. 54 * <p> 55 * The execution of the cleaning action is performed 56 * by a thread associated with the cleaner. 57 * All exceptions thrown by the cleaning action are ignored. 58 * The cleaner and other cleaning actions are not affected by 59 * exceptions in a cleaning action. 60 * The thread runs until all registered cleaning actions have 61 * completed and the cleaner itself is reclaimed by the garbage collector. 62 * <p> 63 * The behavior of cleaners during {@link System#exit(int) System.exit} 64 * is implementation specific. No guarantees are made relating 65 * to whether cleaning actions are invoked or not. 66 * <p> 67 * Unless otherwise noted, passing a {@code null} argument to a constructor or 68 * method in this class will cause a 69 * {@link java.lang.NullPointerException NullPointerException} to be thrown. 70 * 71 * @apiNote 72 * The cleaning action is invoked only after the associated object becomes 73 * phantom reachable, so it is important that the object implementing the 74 * cleaning action does not hold references to the object. 75 * In this example, a static class encapsulates the cleaning state and action. 76 * An "inner" class, anonymous or not, must not be used because it implicitly 77 * contains a reference to the outer instance, preventing it from becoming 78 * phantom reachable. 79 * The choice of a new cleaner or sharing an existing cleaner is determined 80 * by the use case. 81 * <p> 82 * If the CleaningExample is used in a try-finally block then the 83 * {@code close} method calls the cleaning action. 84 * If the {@code close} method is not called, the cleaning action is called 85 * by the Cleaner when the CleaningExample instance has become phantom reachable. 86 * <pre>{@code 87 * public class CleaningExample implements AutoCloseable { 88 * // A cleaner, preferably one shared within a library 89 * private static final Cleaner cleaner = <cleaner>; 90 * 91 * static class State implements Runnable { 92 * 93 * State(...) { 94 * // initialize State needed for cleaning action 95 * } 96 * 97 * public void run() { 98 * // cleanup action accessing State, executed at most once 99 * } 100 * } 101 * 102 * private final State; 103 * private final Cleaner.Cleanable cleanable 104 * 105 * public CleaningExample() { 106 * this.state = new State(...); 107 * this.cleanable = cleaner.register(this, state); 108 * } 109 * 110 * public void close() { 111 * cleanable.clean(); 112 * } 113 * } 114 * }</pre> 115 * The cleaning action could be a lambda but all too easily will capture 116 * the object reference, by referring to fields of the object being cleaned, 117 * preventing the object from becoming phantom reachable. 118 * Using a static nested class, as above, will avoid accidentally retaining the 119 * object reference. 120 * <p> 121 * <a name="compatible-cleaners"></a> 122 * Cleaning actions should be prepared to be invoked concurrently with 123 * other cleaning actions. 124 * Typically the cleaning actions should be very quick to execute 125 * and not block. If the cleaning action blocks, it may delay processing 126 * other cleaning actions registered to the same cleaner. 127 * All cleaning actions registered to a cleaner should be mutually compatible. 128 * @since 9 129 */ 130 public interface Cleaner { 131 132 /** 133 * Returns a new {@code Cleaner}. 134 * <p> 135 * The cleaner creates a {@link Thread#setDaemon(boolean) daemon thread} 136 * to process the phantom reachable objects and to invoke cleaning actions. 137 * The {@linkplain java.lang.Thread#getContextClassLoader context class loader} 138 * of the thread is set to the 139 * {@link ClassLoader#getSystemClassLoader() system class loader}. 140 * The thread has no permissions, enforced only if a 141 * {@link java.lang.System#setSecurityManager(SecurityManager) SecurityManager is set}. 142 * <p> 143 * The cleaner terminates when it is phantom reachable and all of the 144 * registered cleaning actions are complete. 145 * 146 * @return a new {@code Cleaner} 147 * 148 * @throws SecurityException if the current thread is not allowed to 149 * create or start the thread. 150 */ 151 static Cleaner create() { 152 return new CleanerImpl(null); 153 } 154 155 /** 156 * Returns a new {@code Cleaner} using a {@code Thread} from the {@code ThreadFactory}. 157 * <p> 158 * A thread from the thread factory's {@link ThreadFactory#newThread(Runnable) newThread} 159 * method is set to be a {@link Thread#setDaemon(boolean) daemon thread} 160 * and started to process phantom reachable objects and invoke cleaning actions. 161 * On each call the {@link ThreadFactory#newThread(Runnable) thread factory} 162 * must provide a Thread that is suitable for performing the cleaning actions. 163 * <p> 164 * The cleaner terminates when it is phantom reachable and all of the 165 * registered cleaning actions are complete. 166 * 167 * @param threadFactory a {@code ThreadFactory} to return a new {@code Thread} 168 * to process cleaning actions 169 * @return a new {@code Cleaner} 170 * 171 * @throws IllegalThreadStateException if the thread from the thread 172 * factory was {@link Thread.State#NEW not a new thread}. 173 * @throws SecurityException if the current thread is not allowed to 174 * create or start the thread. 175 */ 176 static Cleaner create(ThreadFactory threadFactory) { 177 Objects.requireNonNull(threadFactory, "threadFactory"); 178 return new CleanerImpl(threadFactory); 179 } 180 181 /** 182 * Registers an object and a cleaning action to run when the object 183 * becomes phantom reachable. 184 * Refer to the <a href="#compatible-cleaners">API Note</a> above for 185 * cautions about the behavior of cleaning actions. 186 * 187 * @param obj the object to monitor 188 * @param action a {@code Runnable} to invoke when the object becomes phantom reachable 189 * @return a {@code Cleanable} instance 190 */ 191 Cleanable register(Object obj, Runnable action); 192 193 /** 194 * {@code Cleanable} represents an object and a 195 * cleaning action registered in a {@code Cleaner}. 196 * @since 9 197 */ 198 interface Cleanable { 199 /** 200 * Unregisters the cleanable and invokes the cleaning action. 201 * The cleanable's cleaning action is invoked at most once 202 * regardless of the number of calls to {@code clean}. 203 */ 204 void clean(); 205 } 206 207 }