You want to have some action taken when your objects are removed from service.

Use finalize( ) but don’t trust it; or, write your own end-of-life method.

Developers coming from a C++ background tend to form a mental map that has a line of equivalency drawn from C++ destructors to Java finalizers. In C++, destructors are called automatically when you delete an object. Java, though, has no such operator as delete; objects are freed automatically by a part of the Java runtime called the garbage collector, or GC. GC runs as a background thread in Java processes and looks around every so often to see if there are any objects that are no longer referred to by any reference variable. When it runs, as it frees objects, it calls their finalize( ) methods.

For example, what if you (or some code you called) invoke System.exit( ) ? In this case the entire JVM will cease to exists (assuming there isn’t an applet-style security manager to deny it permission to do so) and the finalizer is never run. Similarly, a “memory leak” or mistakenly held reference to your object will also prevent finalizers from running.

Can’t you just ensure that all finalizers get run simply by calling System.runFinalizersOnExit(true) ? Not really! This method is deprecated (see Section 1.10); the documentation notes:

So what if you need some kind of cleanup? You must take responsibility for defining a method and invoking it before you let any object of that class go out of reference. You might call such a method cleanUp( ).

Java 2 SDK 1.3 introduced the runtime method addShutdownHook( ) , to which you pass a non-started Thread subclass object; if the virtual machine has a chance, it runs your shutdown hook code as part of termination. This will normally work, unless the VM was terminated abruptly as by a kill signal on Unix or a KillProcess on Win32, or the VM aborts due to detecting internal corruption of its data structures.

The bottom line? There’s no guarantee, but finalizers and shutdown hooks both have pretty good odds of being run.