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 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 id="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 { CleanerImpl.setCleanerImplAccess(new Function<Cleaner, CleanerImpl>() { @Override public CleanerImpl apply(Cleaner cleaner) { return cleaner.impl; } })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 */ Cleaner()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 */ create()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 */ create(ThreadFactory threadFactory)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 */ register(Object obj, Runnable action)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 */ clean()234 void clean(); 235 } 236 237 } 238