1. What is Weak reference and Soft reference in java?

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  • Jun 14th, 2006

Soft Reference :-->

An object is softly reachable if it is not strongly reachable and there is a path to it with no weak or phantom references, but one or more soft references. The garbage collector might or might not reclaim a softly reachable object depending on how recently the object was created or accessed, but is required to clear all soft references before throwing an OutOfMemoryError.

If heap memory is running low, the garbage collector may, at its own discretion, find softly reachable objects that have not been accessed in the longest time and clear them (set their reference field to null).

Weak Reference:-->

An object is weakly reachable when the garbage collector finds no strong or soft references, but at least one path to the object with a weak reference. Weakly reachable objects are finalized some time after their weak references have been cleared. The only real difference between a soft reference and a weak reference is that the garbage collector uses algorithms to decide whether or not to reclaim a softly reachable object, but always reclaims a weakly reachable object.























































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  • Jun 19th, 2006

A "weak reference" is a holder for a reference to an object, called the referent. With weak references, you can maintain a reference to the referent without preventing it from being garbage collected. When the garbage collector traces the heap, if the only outstanding references to an object are weak references, then the referent becomes a candidate for GC as if there were no outstanding references, and any outstanding weak references are cleared. (An object that is only referenced by weak references is called weakly reachable.)

Weak references are most useful for building weak collections, such as those that store metadata about objects only for as long as the rest of the application uses those objects -- which is exactly what the SocketManager class is supposed to do.

Soft references are a way of saying to the garbage collector, "As long as memory isn't too tight, I'd like to keep this object around. But if memory gets really tight, go ahead and collect it and I'll deal with that." The garbage collector is required to clear all soft references before it can throw OutOfMemoryError.

Where weak references allow the application to create references that do not interfere with garbage collection, soft references allow the application to enlist the aid of the garbage collector by designating some objects as "expendable."

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