如何使用Runtime.addShutdownHook

根据 Java API, 所谓 shutdown hook 就是已经初始化但尚未开始执行的线程对象。在
Runtime 注册后，如果 jvm 要停止前，这些 shutdown hook 便开始执行。

有什么用呢？就是在你的程序结束前，执行一些清理工作，尤其是没有用户界面的程序。

很明显，这些 shutdown hook 都是些线程对象，因此，你的清理工作要写在 run() 里。
根据 Java API，你的清理工作不能太重了，要尽快结束。但仍然可以对数据库进行操作。

举例如下：

package john2;

/**

* test shutdown hook

* All rights released and correctness not guaranteed.

*/

public class ShutdownHook implements Runnable {

public ShutdownHook() {

// register a shutdown hook for this class.

// a shutdown hook is an initialzed but not started thread, which will get up and run

// when the JVM is about to exit. this is used for short clean up tasks.

Runtime.getRuntime().addShutdownHook(new Thread(this));

System.out.println(">>> shutdown hook registered");

}

// this method will be executed of course, since it's a Runnable.

// tasks should not be light and short, accessing database is alright though.

public void run() {

System.out.println("/n>>> About to execute: " + ShutdownHook.class.getName() + ".run() to clean up before JVM exits.");

this.cleanUp();

System.out.println(">>> Finished execution: " + ShutdownHook.class.getName() + ".run()");

}

// (-: a very simple task to execute

private void cleanUp() {

for(int i=0; i < 7; i++) {

System.out.println(i);

}

}

/**

* there're couple of cases that JVM will exit, according to the Java api doc.

* typically:

* 1. method called: System.exit(int)

* 2. ctrl-C pressed on the console.

* 3. the last non-daemon thread exits.

* 4. user logoff or system shutdown.

* @param args

*/

public static void main(String[] args) {

new ShutdownHook();

System.out.println(">>> Sleeping for 5 seconds, try ctrl-C now if you like.");

try {

Thread.sleep(5000); // (-: give u the time to try ctrl-C

} catch (InterruptedException ie) {

ie.printStackTrace();

}

System.out.println(">>> Slept for 10 seconds and the main thread exited.");

}

}

参考资料：
1. Java API Documentation
2. http://java.sun.com/j2se/1.3/docs/guide/lang/hook-design.html