Java - 对象不是“垃圾收集”和“垃圾收集”。在节目结束？

public class 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");
    }  
}

Runtime.addShutdownHook