根據 Java API, 所謂 shutdown hook 就是已經初始化但尚未開始執行的線程對象。在Runtime 注冊後，如果 jvm 要停止前，這些 shutdown hook 便開始執行。聲明：Runtime.addShutdownHook(Thread t)
舉例如下：
代碼如下：
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.");
}
}
也許有人會擔心性能問題,shutdown hook會不會占用太多的VM的資源，答案是shutdown hook不會占用VM太多的資源，因為shutdown hook 只是一個已初始化但尚未啟動的線程。這意味著它只在程序關閉的時候才會啟動，而不是在程序一開始運行時就啟動。而在大多數的Java平台中，如果一個線程沒有啟動(即沒有調用線程的start()函數)VM不會分配資源給線程。因此維護一群沒有啟動的線程不會給VM帶來太大的負擔.
最後還要注意以下兩點：
如果VM crash,那麼不能保證關閉掛鉤(shutdown hooks)能運行.試想一下如果Windows XP突然藍屏了那麼本來計劃在關機之前的更新也就無法進行了.
如果調用Runtime.halt()方法來結束程序的話，那麼關閉掛鉤(shutdown hooks)也不會執行
