Garbage Collection and Method finalize

Every class in Java has the methods of class Object (package java.lang), one of which is the finalize method. This method is rarely used. In fact, we searched over 6500 source-code files for the Java API classes and found fewer than 50 declarations of the finalize method. Nevertheless, because this method is part of every class, we discuss it here to help you understand its intended purpose in case you encounter it in your studies or in industry. The complete details of the finalize method are beyond the scope of this book, and most programmers should not use ityou'll soon see why. You will learn more about class Object in Chapter 9, Object-Oriented Programming: Inheritance.

Every object you create uses various system resources, such as memory. We need a disciplined way to give resources back to the system when they are no longer needed to avoid "resource leaks." The Java Virtual Machine (JVM) performs automatic garbage collection to reclaim the memory occupied by objects that are no longer in use. When there are no more references to an object, the object is marked for garbage collection by the JVM. The memory for such an object can be reclaimed when the JVM executes its garbage collector, which is responsible for retrieving the memory of objects that are no longer used so the memory can be used for other objects. Therefore, memory leaks that are common in other languages like C and C++ (because memory is not automatically reclaimed in those languages) are less likely in Java (but some can still happen in subtle ways). Other types of resource leaks can occur. For example, an application could open a file on disk to modify the file's contents. If the application does not close the file, no other application can use the file until the application that opened the file completes.

The finalize method is called by the garbage collector to perform termination housekeeping on an object just before the garbage collector reclaims the object's memory. Method finalize does not take parameters and has return type void. A problem with method finalize is that the garbage collector is not guaranteed to execute at a specified time. In fact, the garbage collector may never execute before a program terminates. Thus, it is unclear if, or when, method finalize will be called. For this reason, most programmers should avoid method finalize. In Section 8.11, we demonstrate a situation in which method finalize is called by the garbage collector.

Software Engineering Observation 8.10

A class that uses system resources, such as files on disk, should provide a method to eventually release the resources. Many Java API classes provide close or dispose methods for this purpose. For example, class Scanner (java.sun.com/j2se/5.0/docs/api/java/util/Scanner.html) has a close method.