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14  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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26 
27 package java.lang.ref;
28 
29 import 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 
42 public 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      */
get()97     public T get() {
98         return getReferent();
99     }
100 
101     @FastNative
getReferent()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      */
clear()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
clearReferent()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      */
isEnqueued()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      */
enqueue()150     public boolean enqueue() {
151        return queue != null && queue.enqueue(this);
152     }
153 
154 
155     /* -- Constructors -- */
156 
Reference(T referent)157     Reference(T referent) {
158         this(referent, null);
159     }
160 
Reference(T referent, ReferenceQueue<? super T> queue)161     Reference(T referent, ReferenceQueue<? super T> queue) {
162         this.referent = referent;
163         this.queue = queue;
164     }
165 }
166