Java.lang.ref.SoftReference
WeakReference 보다는 약간 강한 참조이다. Soft reference 만 남아있는 객체는 메모리의 요청에 의한 gc 의 대상이 된다. 메모리의 남은 공간이 넉넉하다면 gc되지 않을 것이며, 메모리의 남은 공간이 얼마 없거나 하다면 gc 의 대상이 될 것이다. 확실한건 vm 이 OutOfMemoryError 를 발생시키기 전에는 Soft reference 만 남아있는 객체가 gc의 대상이 된다는 점 정도고, 그 이외에는 vm 의 구현에 따라 다를 수 있다.
Original Reference
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.