钩子(hook)，是一种具有 既定生命周期 的框架工具，在生命周期的各个阶段预留给用户执行一些特定操作的口子，其实是一种 面向切面 编程。

例如当 JVM 程序即将退出时，会执行我们注入的 钩子线程。

 public static void main(String[] args) {
     Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
         @Override
         public void run() {
             System.out.println("The hook thread 1 is running.");
             try {
                 TimeUnit.SECONDS.sleep(2);
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println("The hook thread 1 is exit.");
         }
     }));

     Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
         @Override
         public void run() {
             System.out.println("The hook thread 2 is running.");
             try {
                 TimeUnit.SECONDS.sleep(2);
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println("The hook thread 2 is exit.");
         }
     }));
 }

当该方法运行结束时，会调用这两个 hook 线程。

应用场景：防止应用程序重复启动

public static void main(String[] args) {
    File file = new File("./.lock");
    if(file.exists()){
        throw new RuntimeException("The program already running.");
    }
    try {
        file.createNewFile();
    } catch (IOException e) {
        e.printStackTrace();
    }

    Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("The program received kill signal.");
            file.delete();
        }
    }));

    while(true){
        try {
            TimeUnit.SECONDS.sleep(1);
            System.out.println("The program is running.");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}