一个有效的收拾程序运行残局的方法--ShutdownHook

发现addShutdownHook

在阅读QMQ的源码的时候，在Server端启动的时候，注册了一个shutdown的代码,具体的代码如下：

Runtime.getRuntime().addShutdownHook(new Thread(wrapper::destroy));

addShutdownHook作用

addShutdownHook方法可以添加一个指定的线程来在Java程序退出的时候做一些事情，在以下几个场景会被调用：

1. 程序运行完成后退出
2. 使用Ctrl+C时终端退出
3. 调用系统退出的方法， System.exit(0)

具体的使用Demo如下：

public class AppMain {
  public static void main(String[] args) {
    Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit01));
    Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit02));
    Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit03));
    while (true){
      System.out.println(".....");
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }

  }
  public static void exit01(){
    System.out.println("exit01");
  }
  public static void exit02(){
    System.out.println("exit02");
  }
  public static void exit03(){
    System.out.println("exit03");
  }
}

执行当执行退出的时候：

addShutdownHook的使用场景

addShutdownHook的使用场景很多， 尤其是在通信的模块功能，比如Netty为例，当服务端或者客户端异常断开的时候，需要告诉对方释放资源，从而保证不会触发异常机制。

例如分布式队列，RPC框架都需要在异常后，释放相关资源。

addShutdownHook使用注意几点

1. 在多次使用addShutdownHook增加退出的方法是，由于是开一个线程，所以不能保证方法的执行是按顺序的。 所以尽量保证方法在一个方法里面完成。

2. 在释放的时候不应该增加阻塞的方法，否者会导致推出时阻塞，导致无法退出

   Runtime.getRuntime().addShutdownHook(new Thread(() -> {
       while (true) {
         //
       }
     }));

hooks相关源码

public void addShutdownHook(Thread hook) {
       SecurityManager sm = System.getSecurityManager();
       if (sm != null) {
           sm.checkPermission(new RuntimePermission("shutdownHooks"));
       }
       ApplicationShutdownHooks.add(hook);
   }
 
    public boolean removeShutdownHook(Thread hook) {
       SecurityManager sm = System.getSecurityManager();
       if (sm != null) {
           sm.checkPermission(new RuntimePermission("shutdownHooks"));
       }
       return ApplicationShutdownHooks.remove(hook);
   }

class ApplicationShutdownHooks {
    /* The set of registered hooks */
    private static IdentityHashMap<Thread, Thread> hooks;
    static {
        try {
            Shutdown.add(1 /* shutdown hook invocation order */,
                false /* not registered if shutdown in progress */,
                new Runnable() {
                    public void run() {
                        runHooks();
                    }
                }
            );
            hooks = new IdentityHashMap<>();
        } catch (IllegalStateException e) {
            // application shutdown hooks cannot be added if
            // shutdown is in progress.
            hooks = null;
        }
    }


    private ApplicationShutdownHooks() {}

    /* Add a new shutdown hook.  Checks the shutdown state and the hook itself,
     * but does not do any security checks.
     */
    static synchronized void add(Thread hook) {
        if(hooks == null)
            throw new IllegalStateException("Shutdown in progress");

        if (hook.isAlive())
            throw new IllegalArgumentException("Hook already running");

        if (hooks.containsKey(hook))
            throw new IllegalArgumentException("Hook previously registered");

        hooks.put(hook, hook);
    }

    /* Remove a previously-registered hook.  Like the add method, this method
     * does not do any security checks.
     */
    static synchronized boolean remove(Thread hook) {
        if(hooks == null)
            throw new IllegalStateException("Shutdown in progress");

        if (hook == null)
            throw new NullPointerException();

        return hooks.remove(hook) != null;
    }

    /* Iterates over all application hooks creating a new thread for each
     * to run in. Hooks are run concurrently and this method waits for
     * them to finish.
     */
    static void runHooks() {
        Collection<Thread> threads;
        synchronized(ApplicationShutdownHooks.class) {
            threads = hooks.keySet();
            hooks = null;
        }

        for (Thread hook : threads) {
            hook.start();
        }
        for (Thread hook : threads) {
            while (true) {
                try {
                    hook.join();
                    break;
                } catch (InterruptedException ignored) {
                }
            }
        }
    }
}

Hooks管理问题

在很多开源的框架中，有的Hooks反而会起到反作用，我们怎么样能够移除我们不需要的Hooks呢？

在看了上面的代码，我们发现ApplicationShutdownHooks 不是一个public的类，JDK也没有提供对外管理的方法，只有add和remove方法。

通过分析ApplicationShutdownHooks类源码的结构，我们可以发现，用来存放hooks的文件的是一个IdentityHashMap容器，而且是一个静态的属性。

从反射入手

既然是一个变量，可以通过反射来获得对应的Hooks的值，代码如下：

String className = "java.lang.ApplicationShutdownHooks";
Class<?> clazz = Class.forName(className);
Field field = clazz.getDeclaredField("hooks");
field.setAccessible(true);
IdentityHashMap<Thread, Thread> map = (IdentityHashMap<Thread, Thread>) field.get(clazz);
for (Thread thread : map.keySet()) {
    RzLogger.info("found shutdownHook: " + thread.getName());
}

增加三个Hook验证：

Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit01));
Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit02));
Runtime.getRuntime().addShutdownHook(new Thread(AppMain::exit03));

23:33:11,102  INFO [main] (RzLogger.java:12) - found shutdownHook: Thread-2
23:33:11,106  INFO [main] (RzLogger.java:12) - found shutdownHook: Thread-0
23:33:11,106  INFO [main] (RzLogger.java:12) - found shutdownHook: Thread-1
exit03
exit01
exit02   

收获

1. ShutDown的使用，在程序退出后清理现场
2. 一步一步去理解代码的原理和特性，细节很重要