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Chapter 9 of Inside the Java Virtual Machine
Garbage Collection
by Bill Venners

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Garbage Collection Algorithms

Any garbage collection algorithm must do two basic things. First, it must detect garbage objects. Second, it must reclaim the heap space used by the garbage objects and make the space available again to the program.

Garbage detection is ordinarily accomplished by defining a set of roots and determining reachability from the roots. An object is reachable if there is some path of references from the roots by which the executing program can access the object. The roots are always accessible to the program. Any objects that are reachable from the roots are considered "live." Objects that are not reachable are considered garbage, because they can no longer affect the future course of program execution.

The root set in a Java virtual machine is implementation dependent, but would always include any object references in the local variables and operand stack of any stack frame and any object references in any class variables. Another source of roots are any object references, such as strings, in the constant pool of loaded classes. The constant pool of a loaded class may refer to strings stored on the heap, such as the class name, superclass name, superinterface names, field names, field signatures, method names, and method signatures. Another source of roots may be any object references that were passed to native methods that either haven't been "released" by the native method. (Depending upon the native method interface, a native method may be able to release references by simply returning, by explicitly invoking a call back that releases passed references, or some combination of both.) Another potential source of roots is any part of the Java virtual machine's runtime data areas that are allocated from the garbage-collected heap. For example, the class data in the method area itself could be placed on the garbage-collected heap in some implementations, allowing the same garbage collection algorithm that frees objects to detect and unload unreferenced classes.

Any object referred to by a root is reachable and is therefore a live object. Additionally, any objects referred to by a live object are also reachable. The program is able to access any reachable objects, so these objects must remain on the heap. Any objects that are not reachable can be garbage collected because there is no way for the program to access them.

The Java virtual machine can be implemented such that the garbage collector knows the difference between a genuine object reference and a primitive type (for example, an int) that appears to be a valid object reference. (One example is an int that, if it were interpreted as a native pointer, would point to an object on the heap.) Some garbage collectors, however, may choose not to distinguish between genuine object references and look-alikes. Such garbage collectors are called conservative because they may not always free every unreferenced object. Sometimes a garbage object will be wrongly considered to be live by a conservative collector, because an object reference look-alike referred to it. Conservative collectors trade off an increase in garbage collection speed for occasionally not freeing some actual garbage.

Two basic approaches to distinguishing live objects from garbage are reference counting and tracing. Reference counting garbage collectors distinguish live objects from garbage objects by keeping a count for each object on the heap. The count keeps track of the number of references to that object. Tracing garbage collectors actually trace out the graph of references starting with the root nodes. Objects that are encountered during the trace are marked in some way. After the trace is complete, unmarked objects are known to be unreachable and can be garbage collected.

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