| /* | |
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| */ | |
| /** | |
| * Provides reference-object classes, which support a limited degree | |
| * of interaction with the garbage collector. A program may use a | |
| * reference object to maintain a reference to some other object in | |
| * such a way that the latter object may still be reclaimed by the | |
| * collector. A program may also arrange to be notified some time | |
| * after the collector has determined that the reachability of a given | |
| * object has changed. | |
| * | |
| * <h2>Reference Objects</h2> | |
| * | |
| * A <em>reference object</em> encapsulates a reference to some other | |
| * object so that the reference itself may be examined and manipulated | |
| * like any other object. Three types of reference objects are | |
| * provided, each weaker than the last: <em>soft</em>, <em>weak</em>, | |
| * and <em>phantom</em>. Each type corresponds to a different level | |
| * of reachability, as defined below. Soft references are for | |
| * implementing memory-sensitive caches, weak references are for | |
| * implementing canonicalizing mappings that do not prevent their keys | |
| * (or values) from being reclaimed, and phantom references are for | |
| * scheduling post-mortem cleanup actions. | |
| * Post-mortem cleanup actions can also be registered and managed by a | |
| * {@link java.lang.ref.Cleaner}. | |
| * | |
| * <p> Each reference-object type is implemented by a subclass of the | |
| * abstract base {@link java.lang.ref.Reference} class. | |
| * An instance of one of these subclasses encapsulates a single | |
| * reference to a particular object, called the <em>referent</em>. | |
| * Every reference object provides methods for getting and clearing | |
| * the reference. Aside from the clearing operation reference objects | |
| * are otherwise immutable, so no {@code set} operation is | |
| * provided. A program may further subclass these subclasses, adding | |
| * whatever fields and methods are required for its purposes, or it | |
| * may use these subclasses without change. | |
| * | |
| * <a id="reachability"></a> | |
| * <h2>Reachability</h2> | |
| * | |
| * A <em>reachable</em> object is any object that can be accessed in any potential | |
| * continuing computation from any | |
| * {@linkplain java.lang.Thread#isAlive live thread} (as stated in JLS {@jls 12.6.1}). | |
| * | |
| * <p> Going from strongest to weakest, the different levels of | |
| * reachability reflect the life cycle of an object. They are | |
| * operationally defined as follows: | |
| * | |
| * <ul> | |
| * | |
| * <li> An object is <em>strongly reachable</em> if it is reachable and if it | |
| * can be accessed without traversing the referent of a Reference object. | |
| * | |
| * <li> An object is <em>softly reachable</em> if it is not strongly | |
| * reachable but can be reached by traversing a soft reference. | |
| * | |
| * <li> An object is <em>weakly reachable</em> if it is neither | |
| * strongly nor softly reachable but can be reached by traversing a | |
| * weak reference. When the weak references to a weakly-reachable | |
| * object are cleared, the object becomes eligible for finalization. | |
| * | |
| * <li> An object is <em>phantom reachable</em> if it is neither | |
| * strongly, softly, nor weakly reachable, it has been finalized, and | |
| * some phantom reference refers to it. | |
| * | |
| * <li> Finally, an object is <em>unreachable</em>, and therefore | |
| * eligible for reclamation, when it is not reachable in any of the | |
| * above ways. | |
| * | |
| * </ul> | |
| * | |
| * <h2>Notification</h2> | |
| * | |
| * A program may request to be notified of changes in an object's | |
| * reachability by <em>registering</em> an appropriate reference | |
| * object with a {@link java.lang.ref.ReferenceQueue}. | |
| * This is done by providing the reference queue as | |
| * a constructor argument when creating the reference object. | |
| * Some time after the garbage collector | |
| * determines that the reachability of the referent has changed to correspond | |
| * with the type of the reference, it will clear the | |
| * reference and add it to the associated queue. At this point, the | |
| * reference is considered to be <em>enqueued</em>. The program learns of the | |
| * referent's change in reachability when the associated reference becomes | |
| * available on the queue. The program may remove references from a queue | |
| * (that is, <em>dequeue</em> them) using the {@link ReferenceQueue#poll()} or | |
| * {@link ReferenceQueue#remove()} methods. Additional state needed to respond to a | |
| * referent's change in reachability can be stored in the fields of a custom | |
| * reference subclass, and accessed when the reference is returned from the | |
| * queue. | |
| * | |
| * <p> The relationship between a registered reference object and its | |
| * queue is one-sided. That is, a queue does not keep track of the | |
| * references that are registered with it. If a registered reference | |
| * becomes unreachable itself, then it will never be enqueued. It is | |
| * the responsibility of the program to ensure | |
| * that reference objects remain reachable for as long as the program is | |
| * interested in their referents. | |
| * | |
| * <p> While some programs will choose to dedicate a thread to | |
| * removing reference objects from one or more queues and processing | |
| * them, this is by no means necessary. A tactic that often works | |
| * well is to examine a reference queue in the course of performing | |
| * some other fairly-frequent action. For example, a hashtable that | |
| * uses weak references to implement weak keys could poll its | |
| * reference queue each time the table is accessed. This is how the | |
| * {@link java.util.WeakHashMap} class works. Because | |
| * the {@link java.lang.ref.ReferenceQueue#poll | |
| * ReferenceQueue.poll} method simply checks an internal data | |
| * structure, this check will add little overhead to the hashtable | |
| * access methods. | |
| * | |
| * <a id="MemoryConsistency"></a> | |
| * <h2>Memory Consistency Properties</h2> | |
| * Certain interactions between references, reference queues, and the garbage | |
| * collector form | |
| * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a> | |
| * relationships: | |
| * | |
| * <ul> | |
| * | |
| * <li>Actions in a thread prior to calling | |
| * {@link Reference#reachabilityFence Reference.reachabilityFence(x)} | |
| * <i>happen-before</i> the garbage collector clears any reference to {@code x}.</li> | |
| * | |
| * <li>The clearing of a reference by the garbage collector <i>happens-before</i> | |
| * the garbage collector enqueues the reference.</li> | |
| * | |
| * <li>The enqueueing of a reference (by the garbage collector, or | |
| * by a successful call to {@link Reference#enqueue}) <i>happens-before</i> | |
| * the reference is removed from the queue (<em>dequeued</em>).</li> | |
| * | |
| * <li>The dequeuing of a reference to a | |
| * {@linkplain Cleaner#register(Object object, Runnable action) registered} | |
| * object, by the Cleaner thread, <i>happens-before</i> the Cleaner thread runs | |
| * the cleaning action for that object.</li> | |
| * | |
| * </ul> | |
| * The above chain of <i>happens-before</i> edges ensures that actions in a | |
| * thread prior to a {@link Reference#reachabilityFence Reference.reachabilityFence(x)} | |
| * <i>happen-before</i> cleanup code for {@code x} runs on a Cleaner thread. | |
| * In particular, changes to the state of {@code x} made before | |
| * {@code reachabilityFence(x)} will be visible to the cleanup code running on | |
| * a Cleaner thread without additional synchronization. | |
| * See JLS {@jls 17.4.5}. | |
| * | |
| * <p> | |
| * The interaction between references, finalizers, and the garbage collector | |
| * also forms a <em>happens-before</em> relationship: | |
| * | |
| * <ul> | |
| * <li>Actions in a thread prior to calling | |
| * {@link Reference#reachabilityFence Reference.reachabilityFence(x)} | |
| * <i>happen-before</i> the finalizer for {@code x} is run by a finalizer thread.</li> | |
| * </ul> | |
| * | |
| * This ensures that actions in a thread prior to a | |
| * {@link Reference#reachabilityFence Reference.reachabilityFence(x)} | |
| * <i>happen-before</i> cleanup code for {@code x} runs on a finalizer thread. | |
| * In particular, changes to the state of {@code x} made before | |
| * {@code reachabilityFence(x)} will be visible to the cleanup code running on | |
| * a finalizer thread without additional synchronization. | |
| * See JLS {@jls 17.4.5}. | |
| * | |
| * @author Mark Reinhold | |
| * @since 1.2 | |
| */ | |
| package java.lang.ref; |