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
class. 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
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
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
class works. Because the
checks an internal data structure, this check will add little overhead to the
hashtable access methods.
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:
- An object is strongly reachable if it can be reached by some thread without traversing any reference objects. A newly-created object is strongly reachable by the thread that created it.
- An object is softly reachable if it is not strongly reachable but can be reached by traversing a soft reference.
- An object is weakly reachable 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.
- An object is phantom reachable if it is neither strongly, softly, nor weakly reachable, it has been finalized, and some phantom reference refers to it.
- Finally, an object is unreachable, and therefore eligible for reclamation, when it is not reachable in any of the above ways.
|PhantomReference<T>||Implements a phantom reference, which is the weakest of the three types of references.|
|Reference<T>||Provides an abstract class which describes behavior common to all reference objects.|
|SoftReference<T>||A reference that is cleared when its referent is not strongly reachable and there is memory pressure.|
|WeakReference<T>||Implements a weak reference, which is the middle of the three types of references.|