Friday, October 3, 2008

Java's garbage-collected heap

An introduction to the garbage-collected
heap of the Java virtual machine


Why garbage collection?

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.

A second advantage of garbage collection is that it helps ensure program integrity. Garbage collection is an important part of Java's security strategy. Java programmers are unable to accidentally (or purposely) crash the JVM by incorrectly freeing memory.

A potential disadvantage of a garbage-collected heap is that it adds an overhead that can affect program performance. The JVM has to keep track of which objects are being referenced by the executing program, and finalize and free unreferenced objects on the fly. This activity will likely require more CPU time than would have been required if the program explicitly freed unnecessary memory. In addition, programmers in a garbage-collected environment have less control over the scheduling of CPU time devoted to freeing objects that are no longer needed.

Fortunately, very good garbage collection algorithms have been developed, and adequate performance can be achieved for all but the most demanding of applications. Because Java's garbage collector runs in its own thread, it will, in most cases, run transparently alongside the execution of the program. Plus, if a programmer really wants to explicitly request a garbage collection at some point, System.gc() or Runtime.gc() can be invoked, which will fire off a garbage collection at that time.

The Java programmer must keep in mind that it is the garbage collector that runs finalizers on objects. Because it is not generally possible to predict exactly when unreferenced objects will be garbage collected, it is not possible to predict when object finalizers will be run. Java programmers, therefore, should avoid writing code for which program correctness depends upon the timely finalization of objects. For example, if a finalizer of an unreferenced object releases a resource that is needed again later by the program, the resource will not be made available until after the garbage collector has run the object finalizer. If the program needs the resource before the garbage collector has gotten around to finalizing the unreferenced object, the program is out of luck.

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Another interesting article on Garbage Collection