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26 package java.lang.ref;
30 * Abstract base class for reference objects. This class defines the
31 * operations common to all reference objects. Because reference objects are
32 * implemented in close cooperation with the garbage collector, this class may
33 * not be subclassed directly.
35 * @author Mark Reinhold
39 public abstract class Reference<T> {
41 /* A Reference instance is in one of four possible internal states:
43 * Active: Subject to special treatment by the garbage collector. Some
44 * time after the collector detects that the reachability of the
45 * referent has changed to the appropriate state, it changes the
46 * instance's state to either Pending or Inactive, depending upon
47 * whether or not the instance was registered with a queue when it was
48 * created. In the former case it also adds the instance to the
49 * pending-Reference list. Newly-created instances are Active.
51 * Pending: An element of the pending-Reference list, waiting to be
52 * enqueued by the Reference-handler thread. Unregistered instances
53 * are never in this state.
55 * Enqueued: An element of the queue with which the instance was
56 * registered when it was created. When an instance is removed from
57 * its ReferenceQueue, it is made Inactive. Unregistered instances are
58 * never in this state.
60 * Inactive: Nothing more to do. Once an instance becomes Inactive its
61 * state will never change again.
63 * The state is encoded in the queue and next fields as follows:
65 * Active: queue = ReferenceQueue with which instance is registered, or
66 * ReferenceQueue.NULL if it was not registered with a queue; next =
69 * Pending: queue = ReferenceQueue with which instance is registered;
70 * next = Following instance in queue, or this if at end of list.
72 * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
73 * in queue, or this if at end of list.
75 * Inactive: queue = ReferenceQueue.NULL; next = this.
77 * With this scheme the collector need only examine the next field in order
78 * to determine whether a Reference instance requires special treatment: If
79 * the next field is null then the instance is active; if it is non-null,
80 * then the collector should treat the instance normally.
82 * To ensure that concurrent collector can discover active Reference
83 * objects without interfering with application threads that may apply
84 * the enqueue() method to those objects, collectors should link
85 * discovered objects through the discovered field.
88 private T referent; /* Treated specially by GC */
90 ReferenceQueue<? super T> queue;
93 transient private Reference<T> discovered; /* used by VM */
96 /* Object used to synchronize with the garbage collector. The collector
97 * must acquire this lock at the beginning of each collection cycle. It is
98 * therefore critical that any code holding this lock complete as quickly
99 * as possible, allocate no new objects, and avoid calling user code.
101 static private class Lock { };
102 private static Lock lock = new Lock();
105 /* List of References waiting to be enqueued. The collector adds
106 * References to this list, while the Reference-handler thread removes
107 * them. This list is protected by the above lock object.
109 private static Reference pending = null;
113 /* -- Referent accessor and setters -- */
116 * Returns this reference object's referent. If this reference object has
117 * been cleared, either by the program or by the garbage collector, then
118 * this method returns <code>null</code>.
120 * @return The object to which this reference refers, or
121 * <code>null</code> if this reference object has been cleared
124 return this.referent;
128 * Clears this reference object. Invoking this method will not cause this
129 * object to be enqueued.
131 * <p> This method is invoked only by Java code; when the garbage collector
132 * clears references it does so directly, without invoking this method.
134 public void clear() {
135 this.referent = null;
139 /* -- Queue operations -- */
142 * Tells whether or not this reference object has been enqueued, either by
143 * the program or by the garbage collector. If this reference object was
144 * not registered with a queue when it was created, then this method will
145 * always return <code>false</code>.
147 * @return <code>true</code> if and only if this reference object has
150 public boolean isEnqueued() {
151 /* In terms of the internal states, this predicate actually tests
152 whether the instance is either Pending or Enqueued */
153 synchronized (this) {
154 return (this.queue != ReferenceQueue.NULL) && (this.next != null);
159 * Adds this reference object to the queue with which it is registered,
162 * <p> This method is invoked only by Java code; when the garbage collector
163 * enqueues references it does so directly, without invoking this method.
165 * @return <code>true</code> if this reference object was successfully
166 * enqueued; <code>false</code> if it was already enqueued or if
167 * it was not registered with a queue when it was created
169 public boolean enqueue() {
170 return this.queue.enqueue(this);
174 /* -- Constructors -- */
176 Reference(T referent) {
177 this(referent, null);
180 Reference(T referent, ReferenceQueue<? super T> queue) {
181 this.referent = referent;
182 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;