path: root/java/lang/ref/Reference.java

/*
 * 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<T> {
    /**
     * 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<? super T> 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.
     * <p>
     * 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 <code>null</code>.
     *
     * @return   The object to which this reference refers, or
     *           <code>null</code> 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.
     *
     * <p> 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 <code>false</code>.
     *
     * @return   <code>true</code> 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.
     *
     * <p> This method is invoked only by Java code; when the garbage collector
     * enqueues references it does so directly, without invoking this method.
     *
     * @return   <code>true</code> if this reference object was successfully
     *           enqueued; <code>false</code> 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<? super T> 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
     * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
     * 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.
     *
     * <p> This method establishes an ordering for
     * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
     * 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
     * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
     * Section 12.6 17 of <cite>The Java&trade; Language Specification</cite></a>)
     * 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.
     *
     * <pre> {@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 = ...;
     *   }
     * }}</pre>
     *
     * Here, the invocation of {@code reachabilityFence} is nonintuitively
     * placed <em>after</em> 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.
     *
     * <p> 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}:
     *
     * <pre> {@code
     * public void action2() {
     *   // ...
     *   Resource.update(getExternalResource());
     * }
     * private ExternalResource getExternalResource() {
     *   ExternalResource ext = externalResourceArray[myIndex];
     *   Reference.reachabilityFence(this);
     *   return ext;
     * }}</pre>
     *
     * <p> 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) {
        SinkHolder.sink = ref;
        // TODO: This is a horrible implementation. Fix it. Remove SinkHolder.
        // b/72698200 .
    }

    private static class SinkHolder {
        static volatile Object sink;

        // Ensure that sink looks live to even a reasonably clever compiler.
        private static 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++;
            }
        };
    }
}