Java - Objects are not "garbage collected" at the end of the program?

public cla开发者_StackOverflow社区ss Main {

    public static void main(String[] args) throws InterruptedException {
        ClassA a = new ClassA();
        a = null;

        //Runtime.getRuntime().gc();
        Thread.sleep(4000);
    }
}

public class ClassA {

    @Override
    public void finalize(){
        System.out.println("cleaned");
    }  
}

With the above code finalize() never executes. Nothing is printed to the console. When Removing comment from gc(), finalize() executes, and "cleaned" is printed to the console. Why do I have to call to the garbage collector explicitly ?

Finalization is not guaranteed to be executed with the virtual machine exit. There is no explicit GC for and the finalization is run in a dedicated thread which also exits. Link to explanation: http://download.oracle.com/javase/6/docs/api/java/lang/System.html#runFinalizersOnExit%28boolean%29

If you need clean up code use:
Runtime.addShutdownHook http://download.oracle.com/javase/6/docs/api/java/lang/Runtime.html#addShutdownHook%28java.lang.Thread%29