Each reference-object type is implemented by a subclass of the
An instance of one of these subclasses encapsulates a single
reference to a particular object, called the referent.
Every reference object provides methods for getting and clearing
the reference. Aside from the clearing operation reference objects
are otherwise immutable, so no
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.
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
WeakHashMap class works. Because
ReferenceQueue.poll method simply checks an internal data
structure, this check will add little overhead to the hashtable
Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed.
Abstract base class for reference objects.
Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected.
Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand.
Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.
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For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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