378 lines
16 KiB
Java
378 lines
16 KiB
Java
/*
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* Copyright (C) 2014 The Android Open Source Project
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* Copyright (c) 1997, 2017, 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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package java.lang.ref;
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import dalvik.annotation.optimization.FastNative;
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/**
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* Abstract base class for reference objects. This class defines the
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* operations common to all reference objects. Because reference objects are
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* implemented in close cooperation with the garbage collector, this class may
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* not be subclassed directly.
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*
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* @author Mark Reinhold
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* @since 1.2
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*/
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public abstract class Reference<T> {
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// BEGIN Android-changed: Reimplemented to accommodate a different GC and compiler.
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// ClassLinker knows about the fields of this class.
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// Backported refersTo() from OpenJDK 16.
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/**
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* Forces JNI path.
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* If GC is not in progress (ie: not going through slow path), the referent
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* can be quickly returned through intrinsic without passing through JNI.
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* This flag forces the JNI path so that it can be tested and benchmarked.
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*/
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private static boolean disableIntrinsic = false;
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/**
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* Slow path flag for the reference processor.
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* Used by the reference processor to determine whether or not the referent
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* can be immediately returned. Because the referent might get swept during
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* GC, the slow path, which passes through JNI, must be taken.
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* After initialization, this is only accessed by native code. It is not
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* used with the concurrent copying collector. It is enabled with mutators
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* suspended, but disabled asynchronously.
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*/
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private static boolean slowPathEnabled = false;
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// Treated specially by GC. ART's ClassLinker::LinkFieldsHelper::LinkFields()
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// knows this is the alphabetically last non-static field.
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// We assume that Reference.get() and Reference.clear() are intended to be
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// callable concurrently, and thus referent accesses should be treated as
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// volatile everywhere.
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volatile T referent;
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final ReferenceQueue<? super T> queue;
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/*
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* This field forms a singly-linked list of reference objects that have
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* been enqueued. The queueNext field is non-null if and only if this
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* reference has been enqueued. After this reference has been enqueued and
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* before it has been removed from its queue, the queueNext field points
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* to the next reference on the queue. The last reference on a queue
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* points to itself. Once this reference has been removed from the
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* reference queue, the queueNext field points to the
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* ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the
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* rest of this reference's lifetime.
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* <p>
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* Access to the queueNext field is guarded by synchronization on a lock
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* internal to 'queue'.
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*/
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Reference queueNext;
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/**
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* The pendingNext field is initially set by the GC. After the GC forms a
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* complete circularly linked list, the list is handed off to the
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* ReferenceQueueDaemon using the ReferenceQueue.class lock. The
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* ReferenceQueueDaemon can then read the pendingNext fields without
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* additional synchronization.
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*/
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Reference<?> pendingNext;
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/* -- Referent accessor and setters -- */
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/**
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* Returns this reference object's referent. If this reference object has
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* been cleared, either by the program or by the garbage collector, then
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* this method returns <code>null</code>.
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*
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* @return The object to which this reference refers, or
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* <code>null</code> if this reference object has been cleared
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*/
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public T get() {
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return getReferent();
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}
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@FastNative
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private final native T getReferent();
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/**
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* Tests if the referent of this reference object is {@code obj}.
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* Using a {@code null} {@code obj} returns {@code true} if the
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* reference object has been cleared. Prefer this to a comparison
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* with the result of {@code get}.
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*
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* @param obj the object to compare with this reference object's referent
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* @return {@code true} if {@code obj} is the referent of this reference object
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*/
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public final boolean refersTo(T obj) {
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return refersTo0(obj);
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}
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/* Implementation of refersTo(). */
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@FastNative
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private final native boolean refersTo0(Object o);
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/**
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* Clears this reference object. Invoking this method will not cause this
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* object to be enqueued.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* clears references it does so directly, without invoking this method.
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*/
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public void clear() {
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clearReferent();
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}
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// Direct access to the referent is prohibited, clearReferent blocks and set
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// the referent to null when it is safe to do so.
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@FastNative
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native void clearReferent();
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/* -- Queue operations -- */
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// Android-changed: deprecate since 9.
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// @Deprecated(since="16")
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/**
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* Tests if this reference object is in its associated queue, if any.
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* This method returns {@code true} only if all of the following conditions
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* are met:
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* <ul>
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* <li>this reference object was registered with a queue when it was created; and
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* <li>the garbage collector has added this reference object to the queue
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* or {@link #enqueue()} is called; and
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* <li>this reference object is not yet removed from the queue.
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* </ul>
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* Otherwise, this method returns {@code false}.
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* This method may return {@code false} if this reference object has been cleared
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* but not enqueued due to the race condition.
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*
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* @deprecated
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* This method was never implemented to test if a reference object has
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* been cleared and enqueued as it was previously specified since 1.2.
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* This method could be misused due to the inherent race condition
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* or without an associated {@code ReferenceQueue}.
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* An application relying on this method to release critical resources
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* could cause serious performance issue.
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* An application should use {@link ReferenceQueue} to reliably determine
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* what reference objects that have been enqueued or
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* {@code refersTo(null)} to determine if this reference
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* object has been cleared.
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*
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* @return {@code true} if and only if this reference object is
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* in its associated queue (if any).
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*/
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@Deprecated(since="9")
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public boolean isEnqueued() {
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// Contrary to what the documentation says, this method returns false
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// after this reference object has been removed from its queue
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// (b/26647823). ReferenceQueue.isEnqueued preserves this historically
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// incorrect behavior.
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return queue != null && queue.isEnqueued(this);
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}
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/**
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* Adds this reference object to the queue with which it is registered,
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* if any.
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*
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* <p> This method is invoked only by Java code; when the garbage collector
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* enqueues references it does so directly, without invoking this method.
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*
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* @return <code>true</code> if this reference object was successfully
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* enqueued; <code>false</code> if it was already enqueued or if
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* it was not registered with a queue when it was created
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*/
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public boolean enqueue() {
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return queue != null && queue.enqueue(this);
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}
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/**
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* Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
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* meaningfully cloned. Construct a new {@code Reference} instead.
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*
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* @return never returns normally
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* @throws CloneNotSupportedException always
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*
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* @since 11
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*/
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@Override
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protected Object clone() throws CloneNotSupportedException {
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throw new CloneNotSupportedException();
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}
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/* -- Constructors -- */
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Reference(T referent) {
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this(referent, null);
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}
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Reference(T referent, ReferenceQueue<? super T> queue) {
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this.referent = referent;
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this.queue = queue;
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}
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// END Android-changed: Reimplemented to accommodate a different GC and compiler.
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// BEGIN Android-added: reachabilityFence() from upstream OpenJDK9+181.
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// The actual implementation differs from OpenJDK9.
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/**
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* Ensures that the object referenced by the given reference remains
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* <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
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* regardless of any prior actions of the program that might otherwise cause
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* the object to become unreachable; thus, the referenced object is not
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* reclaimable by garbage collection at least until after the invocation of
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* this method. Invocation of this method does not itself initiate garbage
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* collection or finalization.
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*
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* <p> This method establishes an ordering for
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* <a href="package-summary.html#reachability"><em>strong reachability</em></a>
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* with respect to garbage collection. It controls relations that are
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* otherwise only implicit in a program -- the reachability conditions
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* triggering garbage collection. This method is designed for use in
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* uncommon situations of premature finalization where using
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* {@code synchronized} blocks or methods, or using other synchronization
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* facilities are not possible or do not provide the desired control. This
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* method is applicable only when reclamation may have visible effects,
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* which is possible for objects with finalizers (See
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* <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
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* Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
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* that are implemented in ways that rely on ordering control for correctness.
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*
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* @apiNote
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* Finalization may occur whenever the virtual machine detects that no
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* reference to an object will ever be stored in the heap: The garbage
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* collector may reclaim an object even if the fields of that object are
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* still in use, so long as the object has otherwise become unreachable.
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* This may have surprising and undesirable effects in cases such as the
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* following example in which the bookkeeping associated with a class is
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* managed through array indices. Here, method {@code action} uses a
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* {@code reachabilityFence} to ensure that the {@code Resource} object is
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* not reclaimed before bookkeeping on an associated
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* {@code ExternalResource} has been performed; in particular here, to
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* ensure that the array slot holding the {@code ExternalResource} is not
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* nulled out in method {@link Object#finalize}, which may otherwise run
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* concurrently.
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*
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* <pre> {@code
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* class Resource {
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* private static ExternalResource[] externalResourceArray = ...
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*
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* int myIndex;
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* Resource(...) {
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* myIndex = ...
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* externalResourceArray[myIndex] = ...;
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* ...
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* }
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* protected void finalize() {
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* externalResourceArray[myIndex] = null;
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* ...
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* }
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* public void action() {
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* try {
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* // ...
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* int i = myIndex;
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* Resource.update(externalResourceArray[i]);
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* } finally {
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* Reference.reachabilityFence(this);
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* }
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* }
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* private static void update(ExternalResource ext) {
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* ext.status = ...;
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* }
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* }}</pre>
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*
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* Here, the invocation of {@code reachabilityFence} is nonintuitively
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* placed <em>after</em> the call to {@code update}, to ensure that the
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* array slot is not nulled out by {@link Object#finalize} before the
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* update, even if the call to {@code action} was the last use of this
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* object. This might be the case if, for example a usage in a user program
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* had the form {@code new Resource().action();} which retains no other
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* reference to this {@code Resource}. While probably overkill here,
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* {@code reachabilityFence} is placed in a {@code finally} block to ensure
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* that it is invoked across all paths in the method. In a method with more
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* complex control paths, you might need further precautions to ensure that
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* {@code reachabilityFence} is encountered along all of them.
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*
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* <p> It is sometimes possible to better encapsulate use of
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* {@code reachabilityFence}. Continuing the above example, if it were
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* acceptable for the call to method {@code update} to proceed even if the
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* finalizer had already executed (nulling out slot), then you could
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* localize use of {@code reachabilityFence}:
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*
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* <pre> {@code
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* public void action2() {
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* // ...
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* Resource.update(getExternalResource());
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* }
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* private ExternalResource getExternalResource() {
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* ExternalResource ext = externalResourceArray[myIndex];
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* Reference.reachabilityFence(this);
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* return ext;
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* }}</pre>
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*
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* <p> Method {@code reachabilityFence} is not required in constructions
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* that themselves ensure reachability. For example, because objects that
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* are locked cannot, in general, be reclaimed, it would suffice if all
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* accesses of the object, in all methods of class {@code Resource}
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* (including {@code finalize}) were enclosed in {@code synchronized (this)}
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* blocks. (Further, such blocks must not include infinite loops, or
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* themselves be unreachable, which fall into the corner case exceptions to
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* the "in general" disclaimer.) However, method {@code reachabilityFence}
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* remains a better option in cases where this approach is not as efficient,
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* desirable, or possible; for example because it would encounter deadlock.
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*
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* @param ref the reference. If {@code null}, this method has no effect.
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* @since 9
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*/
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// @DontInline
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public static void reachabilityFence(Object ref) {
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// This code is usually replaced by much faster intrinsic implementations.
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// It will be executed for tests run with the access checks interpreter in
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// ART, e.g. with --verify-soft-fail. Since this is a volatile store, it
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// cannot easily be moved up past prior accesses, even if this method is
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// inlined.
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SinkHolder.sink = ref;
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// Leaving SinkHolder set to ref is unpleasant, since it keeps ref live
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// until the next reachabilityFence call. This causes e.g. 036-finalizer
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// to fail. Clear it again in a way that's unlikely to be optimizable.
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// The fact that finalize_count is volatile makes it hard to move the test up.
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if (SinkHolder.finalize_count == 0) {
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SinkHolder.sink = null;
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}
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}
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private static class SinkHolder {
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static volatile Object sink;
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// Ensure that sink looks live to even a reasonably clever compiler.
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private static volatile int finalize_count = 0;
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private static Object sinkUser = new Object() {
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protected void finalize() {
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if (sink == null && finalize_count > 0) {
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throw new AssertionError("Can't get here");
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}
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finalize_count++;
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}
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};
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}
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// END Android-added: reachabilityFence() from upstream OpenJDK9+181.
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}
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