Quoted from Java documentation, 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.
Just as with weak references, a soft reference can be created with an associated reference queue, and the reference is enqueued when it is cleared by the garbage collector. Reference queues are not as useful with soft references as with weak references, but they could be used to raise a management alert that the application is starting to run low on memory.
Soft references can be created using the SoftReference class. E.g.
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