JVM钩子
钩子是什么：是一种消息的拦截机制，可以拦截单个进程的消息（线程钩子），可以拦截所有进程的消息（系统钩子），并可以对拦截消息进行自动以处理。
钩子应用的情况：键盘打字时的显示，关闭线程池。
java自带的JVM钩子使用方法：
Runtime.getRuntime().addShutdownHook(Thread）；
Api的解释：
Registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
- The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
- The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.
应用的两种状况：1-程序正常退出，当最新的非守护线程退出或该退出方法被唤醒；
2-JVM被中断时，会进行关闭资源，对象销毁等操作；
A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.
Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
Once the shutdown sequence has begun it is impossible to register a new shutdown hook or de-register a previously-registered hook. Attempting either of these operations will cause an IllegalStateException to be thrown.
Shutdown hooks run at a delicate time in the life cycle of a virtual machine and should therefore be coded defensively. They should, in particular, be written to be thread-safe and to avoid deadlocks insofar as possible. They should also not rely blindly upon services that may have registered their own shutdown hooks and therefore may themselves in the process of shutting down. Attempts to use other thread-based services such as the AWT event-dispatch thread, for example, may lead to deadlocks.
Shutdown hooks should also finish their work quickly. When a program invokes exit the expectation is that the virtual machine will promptly shut down and exit. When the virtual machine is terminated due to user logoff or system shutdown the underlying operating system may only allow a fixed amount of time in which to shut down and exit. It is therefore inadvisable to attempt any user interaction or to perform a long-running computation in a shutdown hook.
Uncaught exceptions are handled in shutdown hooks just as in any other thread, by invoking the uncaughtException method of the thread's ThreadGroup object. The default implementation of this method prints the exception's stack trace to System.err and terminates the thread; it does not cause the virtual machine to exit or halt.
In rare circumstances the virtual machine may abort, that is, stop running without shutting down cleanly. This occurs when the virtual machine is terminated externally, for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows. The virtual machine may also abort if a native method goes awry by, for example, corrupting internal data structures or attempting to access nonexistent memory. If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run.
主要功能是:注册一个JVM钩子，当JVM关闭的时候，会执行系统中已经设置的所有通过方法addShutdownHook添加的钩子，JVM才会关闭。所以这些钩子在JVM关闭的时候运行内存清理，对象销毁等操作。
建议：同一个JVM最好只使用一个关闭钩子，而不是每个服务都使用一个不同的关闭钩子，使用多个关闭钩子可能会出现当前这个钩子所要依赖的服务可能已经被另外一个关闭钩子关闭了。为了避免这种情况，建议关闭操作在单个线程中串行执行，从而避免了再关闭操作之间出现竞态条件或者死锁等问题。
使用的例子：
1-公司会用钩子去关闭连接hbase数据库的线程池。代码以后加上。
2-下面是一段zeppelin的源码：
Runtime.getRuntime().addShutdownHook(new Thread(){ @Override public void run() { LOG.info("Shutting down Zeppelin Server ... "); try { jettyWebServer.stop(); if (!conf.isRecoveryEnabled()) { ZeppelinServer.notebook.getInterpreterSettingManager().close(); } notebook.close(); Thread.sleep(3000); } catch (Exception e) { LOG.error("Error while stopping servlet container", e); } LOG.info("Bye"); } });
参考文献：https://www.cnblogs.com/yanlong300/p/8446261.html
Java8 API
