Suppose that the garbage collector determines at a certain point in time that an object is softly reachable. At that time it may choose to clear atomically all soft references to that object and all soft references to any other softly-reachable objects from which that object is reachable through a chain of strong references. At the same time or at some later time it will enqueue those newly-cleared soft references that are registered with reference queues.
All soft references to softly-reachable objects are guaranteed to have
been cleared before the virtual machine throws an
OutOfMemoryError. Otherwise no constraints are placed upon the
time at which a soft reference will be cleared or the order in which a set
of such references to different objects will be cleared. Virtual machine
implementations are, however, encouraged to bias against clearing
recently-created or recently-used soft references.
Direct instances of this class may be used to implement simple caches; this class or derived subclasses may also be used in larger data structures to implement more sophisticated caches. As long as the referent of a soft reference is strongly reachable, that is, is actually in use, the soft reference will not be cleared. Thus a sophisticated cache can, for example, prevent its most recently used entries from being discarded by keeping strong referents to those entries, leaving the remaining entries to be discarded at the discretion of the garbage collector.
| Modifier and Type | Field and Description |
|---|---|
| private static long | clock
Timestamp clock, updated by the garbage collector |
| private long | timestamp
Timestamp updated by each invocation of the get method. |
| Access | Constructor and Description |
|---|---|
| public | SoftReference(T
object the new soft reference will refer to referent)Creates a new soft reference that refers to the given object. |
| public | SoftReference(T
object the new soft reference will refer to referent, ReferenceQueue<? super T> the queue with which the reference is to be registered,
or q)null if registration is not requiredCreates a new soft reference that refers to the given object and is registered with the given queue. |
| Modifier and Type | Method and Description |
|---|---|
| public T | Returns: The object to which this reference refers, ornull if this reference object has been clearedOverrides java. Returns this reference object's referent. |
| clock | back to summary |
|---|---|
| private static long clock Timestamp clock, updated by the garbage collector | |
| timestamp | back to summary |
|---|---|
| private long timestamp Timestamp updated by each invocation of the get method. The VM may use this field when selecting soft references to be cleared, but it is not required to do so. | |
| SoftReference | back to summary |
|---|---|
| public SoftReference(T referent) Creates a new soft reference that refers to the given object. The new reference is not registered with any queue.
| |
| SoftReference | back to summary |
|---|---|
| public SoftReference(T referent, ReferenceQueue<? super T> q) Creates a new soft reference that refers to the given object and is registered with the given queue.
| |
| get | back to summary |
|---|---|
| public T get() Overrides java. Returns this reference object's referent. If this reference object has
been cleared, either by the program or by the garbage collector, then
this method returns
| |