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/*
* Copyright (C) 2014 The Android Open Source Project
* Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.lang.ref;
import dalvik.annotation.optimization.FastNative;
/**
* Abstract base class for reference objects. This class defines the
* operations common to all reference objects. Because reference objects are
* implemented in close cooperation with the garbage collector, this class may
* not be subclassed directly.
*
* @author Mark Reinhold
* @since 1.2
*/
// Android-changed: Major parts of the code below were changed to accomodate a
// different GC and compiler. ClassLinker knows about the fields of this class.
public abstract class Reference {
/**
* Forces JNI path.
* If GC is not in progress (ie: not going through slow path), the referent
* can be quickly returned through intrinsic without passing through JNI.
* This flag forces the JNI path so that it can be tested and benchmarked.
*/
private static boolean disableIntrinsic = false;
/**
* Slow path flag for the reference processor.
* Used by the reference processor to determine whether or not the referent
* can be immediately returned. Because the referent might get swept during
* GC, the slow path, which passes through JNI, must be taken.
*/
private static boolean slowPathEnabled = false;
// Treated specially by GC. ART's ClassLinker::LinkFields() knows this is the
// alphabetically last non-static field.
volatile T referent;
final ReferenceQueue queue;
/*
* This field forms a singly-linked list of reference objects that have
* been enqueued. The queueNext field is non-null if and only if this
* reference has been enqueued. After this reference has been enqueued and
* before it has been removed from its queue, the queueNext field points
* to the next reference on the queue. The last reference on a queue
* points to itself. Once this reference has been removed from the
* reference queue, the queueNext field points to the
* ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the
* rest of this reference's lifetime.
*
* Access to the queueNext field is guarded by synchronization on a lock
* internal to 'queue'.
*/
Reference queueNext;
/**
* The pendingNext field is initially set by the GC. After the GC forms a
* complete circularly linked list, the list is handed off to the
* ReferenceQueueDaemon using the ReferenceQueue.class lock. The
* ReferenceQueueDaemon can then read the pendingNext fields without
* additional synchronization.
*/
Reference pendingNext;
/* -- Referent accessor and setters -- */
/**
* Returns this reference object's referent. If this reference object has
* been cleared, either by the program or by the garbage collector, then
* this method returns null.
*
* @return The object to which this reference refers, or
* null if this reference object has been cleared
*/
public T get() {
return getReferent();
}
@FastNative
private final native T getReferent();
/**
* Clears this reference object. Invoking this method will not cause this
* object to be enqueued.
*
*
This method is invoked only by Java code; when the garbage collector
* clears references it does so directly, without invoking this method.
*/
public void clear() {
clearReferent();
}
// Direct access to the referent is prohibited, clearReferent blocks and set
// the referent to null when it is safe to do so.
@FastNative
native void clearReferent();
/* -- Queue operations -- */
/**
* Tells whether or not this reference object has been enqueued, either by
* the program or by the garbage collector. If this reference object was
* not registered with a queue when it was created, then this method will
* always return false.
*
* @return true if and only if this reference object has
* been enqueued
*/
public boolean isEnqueued() {
// Contrary to what the documentation says, this method returns false
// after this reference object has been removed from its queue
// (b/26647823). ReferenceQueue.isEnqueued preserves this historically
// incorrect behavior.
return queue != null && queue.isEnqueued(this);
}
/**
* Adds this reference object to the queue with which it is registered,
* if any.
*
*
This method is invoked only by Java code; when the garbage collector
* enqueues references it does so directly, without invoking this method.
*
* @return true if this reference object was successfully
* enqueued; false if it was already enqueued or if
* it was not registered with a queue when it was created
*/
public boolean enqueue() {
return queue != null && queue.enqueue(this);
}
/* -- Constructors -- */
Reference(T referent) {
this(referent, null);
}
Reference(T referent, ReferenceQueue queue) {
this.referent = referent;
this.queue = queue;
}
// BEGIN Android-added: reachabilityFence() documentation from upstream OpenJDK9+181
/**
* Ensures that the object referenced by the given reference remains
* strongly reachable,
* regardless of any prior actions of the program that might otherwise cause
* the object to become unreachable; thus, the referenced object is not
* reclaimable by garbage collection at least until after the invocation of
* this method. Invocation of this method does not itself initiate garbage
* collection or finalization.
*
*
This method establishes an ordering for
* strong reachability
* with respect to garbage collection. It controls relations that are
* otherwise only implicit in a program -- the reachability conditions
* triggering garbage collection. This method is designed for use in
* uncommon situations of premature finalization where using
* {@code synchronized} blocks or methods, or using other synchronization
* facilities are not possible or do not provide the desired control. This
* method is applicable only when reclamation may have visible effects,
* which is possible for objects with finalizers (See
*
* Section 12.6 17 of The Java™ Language Specification)
* that are implemented in ways that rely on ordering control for correctness.
*
* @apiNote
* Finalization may occur whenever the virtual machine detects that no
* reference to an object will ever be stored in the heap: The garbage
* collector may reclaim an object even if the fields of that object are
* still in use, so long as the object has otherwise become unreachable.
* This may have surprising and undesirable effects in cases such as the
* following example in which the bookkeeping associated with a class is
* managed through array indices. Here, method {@code action} uses a
* {@code reachabilityFence} to ensure that the {@code Resource} object is
* not reclaimed before bookkeeping on an associated
* {@code ExternalResource} has been performed; in particular here, to
* ensure that the array slot holding the {@code ExternalResource} is not
* nulled out in method {@link Object#finalize}, which may otherwise run
* concurrently.
*
*
{@code
* class Resource {
* private static ExternalResource[] externalResourceArray = ...
*
* int myIndex;
* Resource(...) {
* myIndex = ...
* externalResourceArray[myIndex] = ...;
* ...
* }
* protected void finalize() {
* externalResourceArray[myIndex] = null;
* ...
* }
* public void action() {
* try {
* // ...
* int i = myIndex;
* Resource.update(externalResourceArray[i]);
* } finally {
* Reference.reachabilityFence(this);
* }
* }
* private static void update(ExternalResource ext) {
* ext.status = ...;
* }
* }}
*
* Here, the invocation of {@code reachabilityFence} is nonintuitively
* placed after the call to {@code update}, to ensure that the
* array slot is not nulled out by {@link Object#finalize} before the
* update, even if the call to {@code action} was the last use of this
* object. This might be the case if, for example a usage in a user program
* had the form {@code new Resource().action();} which retains no other
* reference to this {@code Resource}. While probably overkill here,
* {@code reachabilityFence} is placed in a {@code finally} block to ensure
* that it is invoked across all paths in the method. In a method with more
* complex control paths, you might need further precautions to ensure that
* {@code reachabilityFence} is encountered along all of them.
*
* It is sometimes possible to better encapsulate use of
* {@code reachabilityFence}. Continuing the above example, if it were
* acceptable for the call to method {@code update} to proceed even if the
* finalizer had already executed (nulling out slot), then you could
* localize use of {@code reachabilityFence}:
*
*
{@code
* public void action2() {
* // ...
* Resource.update(getExternalResource());
* }
* private ExternalResource getExternalResource() {
* ExternalResource ext = externalResourceArray[myIndex];
* Reference.reachabilityFence(this);
* return ext;
* }}
*
* Method {@code reachabilityFence} is not required in constructions
* that themselves ensure reachability. For example, because objects that
* are locked cannot, in general, be reclaimed, it would suffice if all
* accesses of the object, in all methods of class {@code Resource}
* (including {@code finalize}) were enclosed in {@code synchronized (this)}
* blocks. (Further, such blocks must not include infinite loops, or
* themselves be unreachable, which fall into the corner case exceptions to
* the "in general" disclaimer.) However, method {@code reachabilityFence}
* remains a better option in cases where this approach is not as efficient,
* desirable, or possible; for example because it would encounter deadlock.
*
* @param ref the reference. If {@code null}, this method has no effect.
* @since 9
*/
// END Android-added: reachabilityFence() documentation from upstream OpenJDK9+181
// Android-changed: reachabilityFence implementation differs from OpenJDK9.
public static void reachabilityFence(Object ref) {
// This code is usually replaced by much faster intrinsic implementations.
// It will be executed for tests run with the access checks interpreter in
// ART, e.g. with --verify-soft-fail. Since this is a volatile store, it
// cannot easily be moved up past prior accesses, even if this method is
// inlined.
SinkHolder.sink = ref;
// Leaving SinkHolder set to ref is unpleasant, since it keeps ref live
// until the next reachabilityFence call. This causes e.g. 036-finalizer
// to fail. Clear it again in a way that's unlikely to be optimizable.
// The fact that finalize_count is volatile makes it hard to move the test up.
if (SinkHolder.finalize_count == 0) {
SinkHolder.sink = null;
}
}
private static class SinkHolder {
static volatile Object sink;
// Ensure that sink looks live to even a reasonably clever compiler.
private static volatile int finalize_count = 0;
private static Object sinkUser = new Object() {
protected void finalize() {
if (sink == null && finalize_count > 0) {
throw new AssertionError("Can't get here");
}
finalize_count++;
}
};
}
}