1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 * Copyright (c) 1997, 2013, 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
29import dalvik.annotation.optimization.FastNative;
30
31
32/**
33 * Abstract base class for reference objects.  This class defines the
34 * operations common to all reference objects.  Because reference objects are
35 * implemented in close cooperation with the garbage collector, this class may
36 * not be subclassed directly.
37 *
38 * @author   Mark Reinhold
39 * @since    1.2
40 */
41
42public abstract class Reference<T> {
43    /**
44     * Forces JNI path.
45     * If GC is not in progress (ie: not going through slow path), the referent
46     * can be quickly returned through intrinsic without passing through JNI.
47     * This flag forces the JNI path so that it can be tested and benchmarked.
48     */
49    private static boolean disableIntrinsic = false;
50
51    /**
52     * Slow path flag for the reference processor.
53     * Used by the reference processor to determine whether or not the referent
54     * can be immediately returned. Because the referent might get swept during
55     * GC, the slow path, which passes through JNI, must be taken.
56     */
57    private static boolean slowPathEnabled = false;
58
59    volatile T referent;         /* Treated specially by GC */
60    final ReferenceQueue<? super T> queue;
61
62    /*
63     * This field forms a singly-linked list of reference objects that have
64     * been enqueued. The queueNext field is non-null if and only if this
65     * reference has been enqueued. After this reference has been enqueued and
66     * before it has been removed from its queue, the queueNext field points
67     * to the next reference on the queue. The last reference on a queue
68     * points to itself. Once this reference has been removed from the
69     * reference queue, the queueNext field points to the
70     * ReferenceQueue.sQueueNextUnenqueued sentinel reference object for the
71     * rest of this reference's lifetime.
72     * <p>
73     * Access to the queueNext field is guarded by synchronization on a lock
74     * internal to 'queue'.
75     */
76    Reference queueNext;
77
78    /**
79     * The pendingNext field is initially set by the GC. After the GC forms a
80     * complete circularly linked list, the list is handed off to the
81     * ReferenceQueueDaemon using the ReferenceQueue.class lock. The
82     * ReferenceQueueDaemon can then read the pendingNext fields without
83     * additional synchronization.
84     */
85    Reference<?> pendingNext;
86
87    /* -- Referent accessor and setters -- */
88
89    /**
90     * Returns this reference object's referent.  If this reference object has
91     * been cleared, either by the program or by the garbage collector, then
92     * this method returns <code>null</code>.
93     *
94     * @return   The object to which this reference refers, or
95     *           <code>null</code> if this reference object has been cleared
96     */
97    public T get() {
98        return getReferent();
99    }
100
101    @FastNative
102    private final native T getReferent();
103
104    /**
105     * Clears this reference object.  Invoking this method will not cause this
106     * object to be enqueued.
107     *
108     * <p> This method is invoked only by Java code; when the garbage collector
109     * clears references it does so directly, without invoking this method.
110     */
111    public void clear() {
112        clearReferent();
113    }
114
115    // Direct access to the referent is prohibited, clearReferent blocks and set
116    // the referent to null when it is safe to do so.
117    @FastNative
118    native void clearReferent();
119
120    /* -- Queue operations -- */
121
122    /**
123     * Tells whether or not this reference object has been enqueued, either by
124     * the program or by the garbage collector.  If this reference object was
125     * not registered with a queue when it was created, then this method will
126     * always return <code>false</code>.
127     *
128     * @return   <code>true</code> if and only if this reference object has
129     *           been enqueued
130     */
131    public boolean isEnqueued() {
132        // Contrary to what the documentation says, this method returns false
133        // after this reference object has been removed from its queue
134        // (b/26647823). ReferenceQueue.isEnqueued preserves this historically
135        // incorrect behavior.
136        return queue != null && queue.isEnqueued(this);
137    }
138
139    /**
140     * Adds this reference object to the queue with which it is registered,
141     * if any.
142     *
143     * <p> This method is invoked only by Java code; when the garbage collector
144     * enqueues references it does so directly, without invoking this method.
145     *
146     * @return   <code>true</code> if this reference object was successfully
147     *           enqueued; <code>false</code> if it was already enqueued or if
148     *           it was not registered with a queue when it was created
149     */
150    public boolean enqueue() {
151       return queue != null && queue.enqueue(this);
152    }
153
154
155    /* -- Constructors -- */
156
157    Reference(T referent) {
158        this(referent, null);
159    }
160
161    Reference(T referent, ReferenceQueue<? super T> queue) {
162        this.referent = referent;
163        this.queue = queue;
164    }
165}
166