ShutdownHook作用

源地址：http://kim-miao.iteye.com/blog/1662550

void java.lang.Runtime.addShutdownHook(Thread hook)

该方法用来在jvm中增加一个关闭的钩子。当程序正常退出,系统调用 System.exit方法或虚拟机被关闭时才会执行添加的shutdownHook线程。其中shutdownHook是一个已初始化但并不有启动的线程，当jvm关闭的时候，会执行系统中已经设置的所有通过方法addShutdownHook添加的钩子，当系统执行完这些钩子后，jvm才会关闭。所以可通过这些钩子在jvm关闭的时候进行内存清理、资源回收等工作。  

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();  
    }  
}