| Soft reference objects, which are cleared at the discretion of the garbage
collector in response to memory demand. Soft references are most often used
to implement memory-sensitive caches.
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.
version:
1.41, 05/05/07
author:
Mark Reinhold
since:
1.2 |