Java의 다양한 레퍼런스 타입에 따른 Reachability

Reachability
 

Going from strongest to weakest, the different levels of reachability reflect the life cycle of an object. They are operationally defined as follows:

 An object is strongly reachable if it can be reached by some thread without traversing any reference objects. A newly-created object is strongly reachable by the thread that created it.

An object is softly reachable if it is not strongly reachable but can be reached by traversing a soft reference.

An object is weakly reachable if it is neither strongly nor softly reachable but can be reached by traversing a weak reference. When the weak references to a weakly-reachable object are cleared, the object becomes eligible for finalization.

An object is phantom reachable if it is neither strongly, softly, nor weakly reachable, it has been finalized, and some phantom reference refers to it.
Finally, an object is unreachable, and therefore eligible for reclamation, when it is not reachable in any of the above ways.

출처: http://download.oracle.com/javase/6/docs/api/java/lang/ref/package-summary.html