java如何关闭钩子(ShutdownHook)

addShutdownHook

Java程序中可以通过添加关闭钩子，实现在程序退出时关闭资源、平滑退出的功能。
同理关闭钩子是removeShutdownHook
使用Runtime.addShutdownHook(Thread hook)方法，可以注册一个JVM关闭的钩子，这个钩子可以在以下几种场景被调用

1. 程序正常退出(比如main方法执行完毕)
2. 使用System.exit()
3. 终端使用Ctrl+C触发的中断
4. 系统关闭
5. 使用Kill -15 pid命令干掉进程

使用示例

public static void main(String[] args) {
    Thread thread = new Thread(() -> System.out.println("Do something1 in Shutdown Hook"), "dahai1");
    Runtime.getRuntime().addShutdownHook(thread);
    Runtime.getRuntime().addShutdownHook(new Thread(() -> System.out.println("Do something2 in Shutdown Hook2"), "dahai2"));
    System.out.println("hello world");
    Runtime.getRuntime().removeShutdownHook(thread);
}

匿名函数如何移除

像dubbo等第三方依赖，默认在静态代码块中添加了shutdown hook，还是匿名函数
这个时候removeShutdownHook就无法使用了

反射移除

倘若指定了线程的名称，那么可以通过反射来移除

public static void main(String[] args) throws NoSuchFieldException, ClassNotFoundException, IllegalAccessException {
    Runtime.getRuntime().addShutdownHook(new Thread(() -> System.out.println("Do something1 in Shutdown Hook1"), "dahai1"));
    Runtime.getRuntime().addShutdownHook(new Thread(() -> System.out.println("Do something2 in Shutdown Hook2"), "dahai2"));
    System.out.println("hello world");

    Class<?> cls = Class.forName("java.lang.ApplicationShutdownHooks");
    Field hooks = cls.getDeclaredField("hooks");
    hooks.setAccessible(true);
    IdentityHashMap<Thread, Thread> identityHashMap = (IdentityHashMap<Thread, Thread>) hooks.get(cls);
    Iterator<Thread> iterator = identityHashMap.keySet().iterator();
    while (iterator.hasNext()) {
        Thread next = iterator.next();
        if ("dahai1".equals(next.getName())) {
            iterator.remove();
        }
    }
}

关于springboot dubbo停服，可以参考
https://blog.csdn.net/j3T9Z7H/article/details/102512557
https://blog.csdn.net/wins22237/article/details/72758644
https://www.jianshu.com/p/69b704279066
https://blog.csdn.net/qq_33220089/article/details/105708331
https://www.jianshu.com/p/555def4ac9b3