1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.  Oracle designates this
9 * particular file as subject to the "Classpath" exception as provided
10 * by Oracle in the LICENSE file that accompanied this code.
11 *
12 * This code is distributed in the hope that it will be useful, but WITHOUT
13 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
14 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
15 * version 2 for more details (a copy is included in the LICENSE file that
16 * accompanied this code).
17 *
18 * You should have received a copy of the GNU General Public License version
19 * 2 along with this work; if not, write to the Free Software Foundation,
20 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
21 *
22 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
23 * or visit www.oracle.com if you need additional information or have any
24 * questions.
25 */
26
27package java.lang.ref;
28
29
30/**
31 * Abstract base class for reference objects.  This class defines the
32 * operations common to all reference objects.  Because reference objects are
33 * implemented in close cooperation with the garbage collector, this class may
34 * not be subclassed directly.
35 *
36 * @author   Mark Reinhold
37 * @since    1.2
38 */
39
40public abstract class Reference<T> {
41    /**
42     * Forces JNI path.
43     * If GC is not in progress (ie: not going through slow path), the referent
44     * can be quickly returned through intrinsic without passing through JNI.
45     * This flag forces the JNI path so that it can be tested and benchmarked.
46     */
47    private static boolean disableIntrinsic = false;
48
49    /**
50     * Slow path flag for the reference processor.
51     * Used by the reference processor to determine whether or not the referent
52     * can be immediately returned. Because the referent might get swept during
53     * GC, the slow path, which passes through JNI, must be taken.
54     */
55    private static boolean slowPathEnabled = false;
56
57    volatile T referent;         /* Treated specially by GC */
58    final ReferenceQueue<? super T> queue;
59
60    /*
61     * This field forms a singly-linked list of reference objects that have
62     * been enqueued. The queueNext field is non-null if and only if this
63     * reference has been enqueued. After this reference has been enqueued and
64     * before it has been removed from its queue, the queueNext field points
65     * to the next reference on the queue. The last reference on a queue
66     * points to itself. Once this reference has been removed from the
67     * reference queue, the queueNext field points to the
68     * ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the
69     * rest of this reference's lifetime.
70     * <p>
71     * Access to the queueNext field is guarded by synchronization on a lock
72     * internal to 'queue'.
73     */
74    Reference queueNext;
75
76    /**
77     * The pendingNext field is initially set by the GC. After the GC forms a
78     * complete circularly linked list, the list is handed off to the
79     * ReferenceQueueDaemon using the ReferenceQueue.class lock. The
80     * ReferenceQueueDaemon can then read the pendingNext fields without
81     * additional synchronization.
82     */
83    Reference<?> pendingNext;
84
85    /* -- Referent accessor and setters -- */
86
87    /**
88     * Returns this reference object's referent.  If this reference object has
89     * been cleared, either by the program or by the garbage collector, then
90     * this method returns <code>null</code>.
91     *
92     * @return   The object to which this reference refers, or
93     *           <code>null</code> if this reference object has been cleared
94     */
95    public T get() {
96        return getReferent();
97    }
98
99    private final native T getReferent();
100
101    /**
102     * Clears this reference object.  Invoking this method will not cause this
103     * object to be enqueued.
104     *
105     * <p> This method is invoked only by Java code; when the garbage collector
106     * clears references it does so directly, without invoking this method.
107     */
108    public void clear() {
109        this.referent = null;
110    }
111
112
113    /* -- Queue operations -- */
114
115    /**
116     * Tells whether or not this reference object has been enqueued, either by
117     * the program or by the garbage collector.  If this reference object was
118     * not registered with a queue when it was created, then this method will
119     * always return <code>false</code>.
120     *
121     * @return   <code>true</code> if and only if this reference object has
122     *           been enqueued
123     */
124    public boolean isEnqueued() {
125        // Contrary to what the documentation says, this method returns false
126        // after this reference object has been removed from its queue
127        // (b/26647823). ReferenceQueue.isEnqueued preserves this historically
128        // incorrect behavior.
129        return queue != null && queue.isEnqueued(this);
130    }
131
132    /**
133     * Adds this reference object to the queue with which it is registered,
134     * if any.
135     *
136     * <p> This method is invoked only by Java code; when the garbage collector
137     * enqueues references it does so directly, without invoking this method.
138     *
139     * @return   <code>true</code> if this reference object was successfully
140     *           enqueued; <code>false</code> if it was already enqueued or if
141     *           it was not registered with a queue when it was created
142     */
143    public boolean enqueue() {
144       return queue != null && queue.enqueue(this);
145    }
146
147
148    /* -- Constructors -- */
149
150    Reference(T referent) {
151        this(referent, null);
152    }
153
154    Reference(T referent, ReferenceQueue<? super T> queue) {
155        this.referent = referent;
156        this.queue = queue;
157    }
158}
159