The Reachability Lifecycle of Objects

In versions prior to 1.2, every object on the heap is in one of three states from the perspective of the garbage collector: reachable, resurrectable, or unreachable. An object is in the reachable state if the garbage collector can "reach" the object by tracing out the graph of object references starting with the root nodes. Every object begins its life in the reachable state, and stays reachable so long as the program maintains at least one reachable reference to the object. As soon as the program releases all references to an object, however, the object becomes resurrectable.

An object is in the resurrectable state if it is not currently reachable by tracing the graph of references starting with the root nodes, but could potentially be made reachable again later when the garbage collector executes some finalizer. All objects, not just objects that declare a finalize() method, pass through the resurrectable state. As mentioned in the previous section, the finalizer for an object may "resurrect" itself or any other resurrectable object by making the objects reachable again. Because any object in the resurrectable state could potentially be made reachable again by its own or some other object's finalize() method, the garbage collector cannot reclaim the memory occupied by a resurrectable object before it makes certain the object won't be brought back to life through the execution of a finalizer. By running the finalizers of all resurrectable objects that declare a finalize() method, the garbage collector will transform the state of all resurrectable objects, either back to the reachable state (for objects that get resurrected), or forward to the unreachable state.

The unreachable state indicates not only that an object is no longer reachable, but also that the object cannot be made reachable again through the execution of some finalizer. Unreachable objects can no longer have any affect on the running program. The garbage collector, therefore, is free to reclaim the memory they occupy.

In version 1.2, the three original reachability states -- reachable, resurrectable, and unreachable -- were augmented by three new states: softly, weakly, and phantom reachable. Because these three new states represent three new (progressively weaker) kinds of reachability, the state known simply as "reachable" in versions prior to 1.2 is called "strongly reachable" starting with 1.2. Any object referenced directly from a root node, such as a local variable, is strongly reachable. Likewise, any object referenced directly from an instance variable of a strongly reachable object is strongly reachable.