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Java Virtual Machine - The Heap

Whenever a class instance or array is created in a running Java Application, the memory for the new object is allocated from a single heap. As there is only one heap inside a java Virtual Machine instance, all threads share it. Because a java Application runs inside its "own" exlcusive java Virtual Machine instance, there is a separate heap for every individual running application.

One possible heap design divides the heap into two parts: a handle pool and an object pool. An object reference is a native pointer to a handle pool entry. A handle pool entry has two components: a pointer to instance data in the object pool and a pointer to class data in the method area. The advantage of this scheme is that it makes it easy for the virtual machine to combat heap fragmentation. When the virtual machine moves an object in the object pool, it need only update one pointer with the object's new address : the relevant pointer in the handle pool. The disadvantage of this approach is that every access to an object's instance data requires dereferencing two pinters - ptr into object pool and ptr to class data.

When a running program attempts to cast an object reference to another type, the virtual machine must check to see if the type being cast to is the actual class of the referenced object or one of its supertypes.

Garbage Collection: a garbage collector's primary function is to automatically reclaim the memory used by objects that are no longer referenced by the running application. It may also move objects as the application runs to reduce heap fragmentation.



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