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| java.lang.ref | ||
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.
Package SpecificationA reference object 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: soft, weak, and phantom. 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 pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism. Each reference-object type is implemented by a subclass of the abstract
base NotificationA program may request to be notified of changes in an object's reachability by registering an appropriate reference object with a reference queue at the time the reference object is created. Some time after the garbage collector determines that the reachability of the referent has changed to the value corresponding to the type of the reference, it will add the reference to the associated queue. At this point, the reference is considered to be enqueued. The program may remove references from a queue either by polling or by blocking until a reference becomes available. Reference queues are implemented by the{@link java.lang.ref.ReferenceQueue}
class.
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 using reference objects to ensure that the objects remain reachable for as long as the program is interested in their referents. 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 Automatically-cleared referencesSoft and weak references are automatically cleared by the collector before being added to the queues with which they are registered, if any. Therefore soft and weak references need not be registered with a queue in order to be useful, while phantom references do. An object that is reachable via phantom references will remain so until all such references are cleared or themselves become unreachable.ReachabilityGoing from strongest to weakest, the different levels of reachability reflect the life cycle of an object. They are operationally defined as follows:
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| Java Source File Name | Type | Comment |
| Finalizer.java | Class | |
| FinalReference.java | Class | |
| PhantomReference.java | Class | Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. |
| Reference.java | Class | Abstract base class for reference objects. |
| ReferenceQueue.java | Class | Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected. |
| SoftReference.java | Class | Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand. |
| WeakReference.java | Class | Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed. |