ShutdownHook作用

public class TestRuntimeShutdownHook {  
    public static void main(String[] args) {  
  
        Thread shutdownHookOne = new Thread() {  
            public void run() {  
                System.out.println("shutdownHook one...");  
            }  
        };  
        Runtime.getRuntime().addShutdownHook(shutdownHookOne);  
  
        Runnable threadOne = new Runnable() {  
            public void run() {  
                try {  
                    Thread.sleep(1000);  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
                System.out.println("thread one doing something...");  
            }  
        };  
  
        Runnable threadTwo = new Thread() {  
            public void run() {  
                try {  
                    Thread.sleep(2000);  
                } catch (InterruptedException e) {  
                    e.printStackTrace();  
                }  
                System.out.println("thread two doing something...");  
            }  
        };  
  
        threadOne.run();  
        threadTwo.run();  
    }  
}