今天折腾了半天才回到家，写篇简单的，然后趁早洗洗睡吧。
在Java程序退出时——尤其是非正常退出时，我们可能需要先执行一些善后工作，如关闭线程池、连接池、文件句柄等，即所谓“优雅停机”（graceful shutdown）。如何保证善后工作的代码能够被执行到呢？Java为用户提供了关闭钩子（shutdown hook），它在以下情景都会被调用：
- 程序正常退出，即最后一个非守护线程结束时；
- 程序中执行到了System.exit()方法；
- 终端接收到了CTRL-C中断，或者注销登录；
- 通过
kill命令杀死进程（但是kill -9除外）。
关闭钩子在以下情景不会被调用：
- 通过
kill -9命令杀死进程——所以kill -9一定要慎用； - 程序中执行到了Runtime.getRuntime().halt()方法；
- 操作系统突然崩溃，或机器掉电。
用户通过Runtime.getRuntime().addShutdownHook()方法来注册关闭钩子，示例：
Runtime.getRuntime().addShutdownHook(new Thread(() -> {someThreadPool.shutdown();someConnectionPool.close();LOGGER.info("Shutdown hook called");
}));可见，关闭钩子的本质就是已经初始化但在JVM关闭之前最后一刻才会执行的线程。当然，Java也提供了removeShutdownHook()方法来删除关闭钩子。
JDK内是通过ApplicationShutdownHooks类来维护关闭钩子的，该类中有一个IdentityHashMap容器。
private static IdentityHashMap<Thread, Thread> hooks;addShutdownHook()方法实际上是代理了ApplicationShutdownHooks.add()方法。在注册关闭钩子之前，会先判断是否符合以下三个条件，如果是，则钩子无法注册：
- JVM正在关闭，即钩子已经被触发（此时IdentityHashMap为null）；
- 当前关闭钩子正在执行；
- IdentityHashMap中已经存在了要注册的钩子。
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);
}启动钩子的方法则是ApplicationShutdownHooks.runHooks()。
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) {}}}
}这里要注意Thread.join()方法的语义，即挂起调用线程，等待被调用线程（在这里就是钩子线程）执行完毕之后，调用线程才继续执行。也就是说，这里必须保证关闭钩子在主线程真正关闭之前执行完毕。
那么runHooks()方法又是在哪里被调用的呢？由ApplicationShutdownHooks方法的static代码块可知是在Shutdown类，但是仍然包装成了Runnable，没有立即执行。
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;}
}Shutdown类内用Runnable的数组hooks维护关闭钩子的执行，并且该数组同时表示关闭钩子的优先级，排在前面slot的会先执行。虽然该数组的长度为10，但是目前只用了3个slot，用户注册的应用关闭钩子的优先级夹在两种系统钩子的中间（即固定占用slot 1）。
private static final int RUNNING = 0;
private static final int HOOKS = 1;
private static final int FINALIZERS = 2;
private static int state = RUNNING;// The system shutdown hooks are registered with a predefined slot.
// The list of shutdown hooks is as follows:
// (0) Console restore hook
// (1) Application hooks
// (2) DeleteOnExit hook
private static final int MAX_SYSTEM_HOOKS = 10;
private static final Runnable[] hooks = new Runnable[MAX_SYSTEM_HOOKS];static void add(int slot, boolean registerShutdownInProgress, Runnable hook) {synchronized (lock) {if (hooks[slot] != null)throw new InternalError("Shutdown hook at slot " + slot + " already registered");if (!registerShutdownInProgress) {if (state > RUNNING)throw new IllegalStateException("Shutdown in progress");} else {if (state > HOOKS || (state == HOOKS && slot <= currentRunningHook))throw new IllegalStateException("Shutdown in progress");}hooks[slot] = hook;}
}registerShutdownInProgress表示是否允许在关闭过程中注册钩子，前面传入的是false。如果为true的话，则可以在当前运行的钩子后面注册优先级更低的钩子。
加入到hooks数组的钩子最终会在sequence()方法触发执行，最后还会根据runFinalizersOnExit标志位来判断是否需要执行finalizer。runAllFinalizers()是一个native方法。
private static void sequence() {synchronized (lock) {/* Guard against the possibility of a daemon thread invoking exit* after DestroyJavaVM initiates the shutdown sequence*/if (state != HOOKS) return;}runHooks();boolean rfoe;synchronized (lock) {state = FINALIZERS;rfoe = runFinalizersOnExit;}if (rfoe) runAllFinalizers();
}private static void runHooks() {for (int i=0; i < MAX_SYSTEM_HOOKS; i++) {try {Runnable hook;synchronized (lock) {// acquire the lock to make sure the hook registered during// shutdown is visible here.currentRunningHook = i;hook = hooks[i];}if (hook != null) hook.run();} catch(Throwable t) {if (t instanceof ThreadDeath) {ThreadDeath td = (ThreadDeath)t;throw td;}}}
}就酱吧，民那晚安晚安。