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The JVM's heap stores all objects created by an executing Java program. Objects are created by Java's "new" operator, and memory for new objects is allocated on the heap at run time. Garbage collection is the process of automatically freeing objects that are no longer referenced by the program. This frees the programmer from having to keep track of when to free allocated memory, thereby preventing many potential bugs and headaches.
The name "garbage collection" implies that objects that are no longer needed by the program are "garbage" and can be thrown away. A more accurate and up-to-date metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects. The garbage collector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed.
In addition to freeing unreferenced objects, a garbage collector may also combat heap fragmentation. Heap fragmentation occurs through the course of normal program execution. New objects are allocated, and unreferenced objects are freed such that free blocks of heap memory are left in between blocks occupied by live objects. Requests to allocate new objects may have to be filled by extending the size of the heap even though there is enough total unused space in the existing heap. This will happen if there is not enough contiguous free heap space available into which the new object will fit. On a virtual memory system, the extra paging required to service an ever growing heap can degrade the performance of the executing program.
This article does not describe an official Java garbage-collected heap, because none exists. The JVM specification says only that the heap of the Java virtual machine must be garbage collected. The specification does not define how the garbage collector must work. The designer of each JVM must decide how to implement the garbage-collected heap. This article describes various garbage collection techniques that have been developed and demonstrates a particular garbage collection technique in an applet.
Garbage collection relieves programmers from the burden of freeing allocated memory. Knowing when to explicitly free allocated memory can be very tricky. Giving this job to the JVM has several advantages. First, it can make programmers more productive. When programming in non-garbage-collected languages the programmer can spend many late hours (or days or weeks) chasing down an elusive memory problem. When programming in Java the programmer can use that time more advantageously by getting ahead of schedule or simply going home to have a life.