Programming Tutorials

Garbage Collection in Java

By: Ivan Lim in Java Tutorials on 2007-09-08  

Since objects are dynamically allocated by using the new operator, you might be wondering how such objects are destroyed and their memory released for later reallocation. In some languages, such as C++, dynamically allocated objects must be manually released by use of a delete operator. Java takes a different approach; it handles deallocation for you automatically. The technique that accomplishes this is called garbage collection. It works like this: when no references to an object exist, that object is assumed to be no longer needed, and the memory occupied by the object can be reclaimed. There is no explicit need to destroy objects as in C++. Garbage collection only occurs sporadically (if at all) during the execution of your program. It will not occur simply because one or more objects exist that are no longer used. Furthermore, different Java run-time implementations will take varying approaches to garbage collection, but for the most part, you should not have to think about it while writing your programs.

The finalize() Method

Sometimes an object will need to perform some action when it is destroyed. For example, if an object is holding some non-Java resource such as a file handle or window character font, then you might want to make sure these resources are freed before an object is destroyed. To handle such situations, Java provides a mechanism called finalization. By using finalization, you can define specific actions that will occur when an object is just about to be reclaimed by the garbage collector.

To add a finalizer to a class, you simply define the finalize() method. The Java run time calls that method whenever it is about to recycle an object of that class. Inside the finalize() method you will specify those actions that must be performed before an object is destroyed. The garbage collector runs periodically, checking for objects that are no longer referenced by any running state or indirectly through other referenced objects. Right before an asset is freed, the Java run time calls the finalize() method on the object. The finalize() method has this general form:

protected void finalize()
{
// finalization code here
}

Here, the keyword protected is a specifier that prevents access to finalize() by code defined outside its class. It is important to understand that finalize() is only called just prior to garbage collection. It is not called when an object goes out-of-scope, for example. This means that you cannot know when -or even if- finalize() will be executed. Therefore, your program should provide other means of releasing system resources, etc., used by the object. It must not rely on finalize() for normal program operation.

Note: If you are familiar with C++, then you know that C++ allows you to define a destructor for a class, which is called when an object goes out-of-scope. Java does not support this idea or provide for destructors. The finalize() method only approximates the function of a destructor. As you get more experienced with Java, you will see that the need for destructor functions is minimal because of Java's garbage collection subsystem.