解析Runtime中shutdown hook的使用详解

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。声明：Runtime.addShutdownHook(Thread t)
举例如下：


代码如下:

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.");
    }

}

也许有人会担心性能问题,shutdown hook会不会占用太多的VM的资源，答案是shutdown hook不会占用VM太多的资源，因为shutdown hook 只是一个已初始化但尚未启动的线程。这意味着它只在程序关闭的时候才会启动，而不是在程序一开始运行时就启动。而在大多数的Java平台中，如果一个线程没有启动(即没有调用线程的start()函数)VM不会分配资源给线程。因此维护一群没有启动的线程不会给VM带来太大的负担.
最后还要注意以下两点：
如果VM crash,那么不能保证关闭挂钩(shutdown hooks)能运行.试想一下如果Windows XP突然蓝屏了那么本来计划在关机之前的更新也就无法进行了.
如果调用Runtime.halt()方法来结束程序的话，那么关闭挂钩(shutdown hooks)也不会执行