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Java's garbage-collected heap

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

By Bill Venners, JavaWorld.com, 08/01/96

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Welcome to another installment of "Under The Hood." This column gives Java developers a glimpse of what is going on underneath their running Java programs. This month's article takes a look at the garbage-collected heap of the Java virtual machine (JVM).

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.

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.

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Good description Bill, I read many articles and books but the way u have described is really good...By Anonymous on February 3, 2010, 10:54 pmGood description Bill, I read many articles and books but the way u have described is really good. u made it so simple to understand. after going through this article...

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Resources
  • "The Java Virtual Machine Specification," the official word from Sun, can be downloaded in PDF, HTML, and PostScript format.
    http://java.sun.com/doc
  • When it comes out, the book The Java Virtual Machine Specification, by Tim Lindholm and Frank Yellin (ISBN 0-201-63452-X), part of The Java Series, from Addison-Wesley, will likely be the best JVM reference.
    http://www.aw.com/cp/lindholm-yellin.html
    http://www.aw.com/cp/javaseries.html
  • Andrew W. Appel, "Garbage collection," in P. Lee (ed.), Advanced Language Implementations, MIT Press, 1991, chapter 4, pp. 89-100.
  • Jacques Cohen, 'Garbage collection of linked data structures', Computing Surveys, 13(3), 1981, pp. 341-367.
  • Paul R. Wilson, "Uniprocessor garbage collection techniques," Yves Bekkers and Jacques Cohen (eds.), Memory Management - International Workshop IWMM 92, St. Malo, France, September 1992, pp. 1-42.
  • June, 1996 Under The HoodThe lean, mean virtual machine -- Gives an introduction to the Java virtual machine. Look here to see how the garbage-collected heap fits in with the other parts of the JVM.
  • July, 1996 Under The HoodThe Java class file lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.