author | rriggs |
Mon, 21 Dec 2015 11:34:14 -0500 | |
changeset 34724 | e664c5672743 |
child 34942 | 4b462ee18ed9 |
permissions | -rw-r--r-- |
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/* |
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* Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved. |
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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* |
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* This code is free software; you can redistribute it and/or modify it |
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* under the terms of the GNU General Public License version 2 only, as |
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* published by the Free Software Foundation. Oracle designates this |
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* particular file as subject to the "Classpath" exception as provided |
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* by Oracle in the LICENSE file that accompanied this code. |
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* |
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* This code is distributed in the hope that it will be useful, but WITHOUT |
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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* version 2 for more details (a copy is included in the LICENSE file that |
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* accompanied this code). |
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* |
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* You should have received a copy of the GNU General Public License version |
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* 2 along with this work; if not, write to the Free Software Foundation, |
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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* |
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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* or visit www.oracle.com if you need additional information or have any |
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* questions. |
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*/ |
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|
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package java.lang.ref; |
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import java.util.Objects; |
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import java.util.concurrent.ThreadFactory; |
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import jdk.internal.misc.CleanerImpl; |
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/** |
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* {@code Cleaner} manages a set of object references and corresponding cleaning actions. |
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* <p> |
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* Cleaning actions are {@link #register(Object object, Runnable action) registered} |
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* to run after the cleaner is notified that the object has become |
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* phantom reachable. |
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* The cleaner uses {@link PhantomReference} and {@link ReferenceQueue} to be |
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* notified when the <a href="package-summary.html#reachability">reachability</a> |
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* changes. |
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* <p> |
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* Each cleaner operates independently, managing the pending cleaning actions |
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* and handling threading and termination when the cleaner is no longer in use. |
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* Registering an object reference and corresponding cleaning action returns |
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* a {@link Cleanable Cleanable}. The most efficient use is to explicitly invoke |
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* the {@link Cleanable#clean clean} method when the object is closed or |
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* no longer needed. |
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* The cleaning action is a {@link Runnable} to be invoked at most once when |
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* the object has become phantom reachable unless it has already been explicitly cleaned. |
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* Note that the cleaning action must not refer to the object being registered. |
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* If so, the object will not become phantom reachable and the cleaning action |
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* will not be invoked automatically. |
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* <p> |
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* The execution of the cleaning action is performed |
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* by a thread associated with the cleaner. |
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* All exceptions thrown by the cleaning action are ignored. |
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* The cleaner and other cleaning actions are not affected by |
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* exceptions in a cleaning action. |
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* The thread runs until all registered cleaning actions have |
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* completed and the cleaner itself is reclaimed by the garbage collector. |
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* <p> |
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* The behavior of cleaners during {@link System#exit(int) System.exit} |
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* is implementation specific. No guarantees are made relating |
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* to whether cleaning actions are invoked or not. |
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* <p> |
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* Unless otherwise noted, passing a {@code null} argument to a constructor or |
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* method in this class will cause a |
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* {@link java.lang.NullPointerException NullPointerException} to be thrown. |
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* |
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* @apiNote |
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* The cleaning action is invoked only after the associated object becomes |
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* phantom reachable, so it is important that the object implementing the |
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* cleaning action does not hold references to the object. |
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* In this example, a static class encapsulates the cleaning state and action. |
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* An "inner" class, anonymous or not, must not be used because it implicitly |
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* contains a reference to the outer instance, preventing it from becoming |
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* phantom reachable. |
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* The choice of a new cleaner or sharing an existing cleaner is determined |
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* by the use case. |
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* <p> |
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* If the CleaningExample is used in a try-finally block then the |
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* {@code close} method calls the cleaning action. |
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* If the {@code close} method is not called, the cleaning action is called |
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* by the Cleaner when the CleaningExample instance has become phantom reachable. |
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* <pre>{@code |
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* public class CleaningExample implements AutoCloseable { |
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* // A cleaner, preferably one shared within a library |
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* private static final Cleaner cleaner = <cleaner>; |
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* |
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* static class State implements Runnable { |
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* |
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* State(...) { |
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* // initialize State needed for cleaning action |
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* } |
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* |
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* public void run() { |
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* // cleanup action accessing State, executed at most once |
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* } |
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* } |
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* |
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* private final State; |
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* private final Cleaner.Cleanable cleanable |
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* |
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* public CleaningExample() { |
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* this.state = new State(...); |
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* this.cleanable = cleaner.register(this, state); |
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* } |
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* |
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* public void close() { |
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* cleanable.clean(); |
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* } |
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* } |
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* }</pre> |
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* The cleaning action could be a lambda but all too easily will capture |
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* the object reference, by referring to fields of the object being cleaned, |
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* preventing the object from becoming phantom reachable. |
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* Using a static nested class, as above, will avoid accidentally retaining the |
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* object reference. |
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* <p> |
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* <a name="compatible-cleaners"></a> |
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* Cleaning actions should be prepared to be invoked concurrently with |
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* other cleaning actions. |
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* Typically the cleaning actions should be very quick to execute |
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* and not block. If the cleaning action blocks, it may delay processing |
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* other cleaning actions registered to the same cleaner. |
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* All cleaning actions registered to a cleaner should be mutually compatible. |
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* @since 9 |
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*/ |
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public final class Cleaner { |
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|
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/** |
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* The Cleaner implementation. |
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*/ |
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final CleanerImpl impl; |
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|
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static { |
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CleanerImpl.setCleanerImplAccess((Cleaner c) -> c.impl); |
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} |
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|
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/** |
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* Construct a Cleaner implementation and start it. |
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*/ |
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private Cleaner() { |
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impl = new CleanerImpl(); |
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} |
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|
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/** |
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* Returns a new {@code Cleaner}. |
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* <p> |
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* The cleaner creates a {@link Thread#setDaemon(boolean) daemon thread} |
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* to process the phantom reachable objects and to invoke cleaning actions. |
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* The {@linkplain java.lang.Thread#getContextClassLoader context class loader} |
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* of the thread is set to the |
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* {@link ClassLoader#getSystemClassLoader() system class loader}. |
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* The thread has no permissions, enforced only if a |
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* {@link java.lang.System#setSecurityManager(SecurityManager) SecurityManager is set}. |
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* <p> |
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* The cleaner terminates when it is phantom reachable and all of the |
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* registered cleaning actions are complete. |
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* |
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* @return a new {@code Cleaner} |
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* |
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* @throws SecurityException if the current thread is not allowed to |
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* create or start the thread. |
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*/ |
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public static Cleaner create() { |
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Cleaner cleaner = new Cleaner(); |
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cleaner.impl.start(cleaner, null); |
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return cleaner; |
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} |
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|
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/** |
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* Returns a new {@code Cleaner} using a {@code Thread} from the {@code ThreadFactory}. |
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* <p> |
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* A thread from the thread factory's {@link ThreadFactory#newThread(Runnable) newThread} |
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* method is set to be a {@link Thread#setDaemon(boolean) daemon thread} |
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* and started to process phantom reachable objects and invoke cleaning actions. |
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* On each call the {@link ThreadFactory#newThread(Runnable) thread factory} |
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* must provide a Thread that is suitable for performing the cleaning actions. |
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* <p> |
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* The cleaner terminates when it is phantom reachable and all of the |
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* registered cleaning actions are complete. |
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* |
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* @param threadFactory a {@code ThreadFactory} to return a new {@code Thread} |
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* to process cleaning actions |
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* @return a new {@code Cleaner} |
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* |
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* @throws IllegalThreadStateException if the thread from the thread |
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* factory was {@link Thread.State#NEW not a new thread}. |
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* @throws SecurityException if the current thread is not allowed to |
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* create or start the thread. |
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*/ |
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public static Cleaner create(ThreadFactory threadFactory) { |
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Objects.requireNonNull(threadFactory, "threadFactory"); |
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Cleaner cleaner = new Cleaner(); |
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cleaner.impl.start(cleaner, threadFactory); |
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return cleaner; |
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} |
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|
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/** |
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* Registers an object and a cleaning action to run when the object |
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* becomes phantom reachable. |
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* Refer to the <a href="#compatible-cleaners">API Note</a> above for |
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* cautions about the behavior of cleaning actions. |
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* |
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* @param obj the object to monitor |
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* @param action a {@code Runnable} to invoke when the object becomes phantom reachable |
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* @return a {@code Cleanable} instance |
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*/ |
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public Cleanable register(Object obj, Runnable action) { |
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Objects.requireNonNull(obj, "obj"); |
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Objects.requireNonNull(action, "action"); |
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return new CleanerImpl.PhantomCleanableRef(obj, this, action); |
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} |
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|
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/** |
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* {@code Cleanable} represents an object and a |
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* cleaning action registered in a {@code Cleaner}. |
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* @since 9 |
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*/ |
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public interface Cleanable { |
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/** |
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* Unregisters the cleanable and invokes the cleaning action. |
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* The cleanable's cleaning action is invoked at most once |
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* regardless of the number of calls to {@code clean}. |
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*/ |
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void clean(); |
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} |
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|
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} |