在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。 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：
程序正常退出使用System.exit()终端使用Ctrl+C触发的中断系统关闭使用Kill pid命令干掉进程Runtime.java中相关方法源码
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); }ApplicationShutdownHooks.java
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) { try { hook.join(); } catch (InterruptedException x) { } } } }很多时候，我们会有这样的一些场景，比如说nginx反向代理若干个负载均衡的web容器，又或者微服务架构中存在的若干个服务节点，需要进行无间断的升级发布。 在重启服务的时候，除非我们去变更nginx的配置，否则重启很可能会导致正在执行的线程突然中断，本来应该要完成的事情只完成了一半，并且调用方出现错误警告。 如果能有一种简单的方式，能够让进程在退出时能执行完当前正在执行的任务，并且让服务的调用方将新的请求定向到其他负载节点，这将会很有意义。 自己注册ShutdownHook可以帮助我们实现java进程的平滑退出。
我们会发现，有一些第三方组件在代码中注册了关闭自身资源的ShutdownHook，这些ShutdownHook对于我们的平滑退出有时候起了反作用。 比如dubbo，在static方法块里面注册了自己的关闭钩子，完全不可控。在进程退出时直接就把长连接给断开了，导致当前的执行线程无法正常完成，源码如下：
static { Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() { public void run() { if (logger.isInfoEnabled()) { logger.info("Run shutdown hook now."); } ProtocolConfig.destroyAll(); } }, "DubboShutdownHook")); }从Runtime.java和ApplicationShutdownHooks.java的源码中，我们看到并没有一个可以遍历操作shutdownHook的方法。 Runtime.java仅有的一个removeShutdownHook的方法，对于未写线程名的匿名类来说，无法获取对象的引用，也无法分辨出彼此。 ApplicationShutdownHooks.java不是public的，类中的hooks也是private的。 只有通过反射的方式才能获取并控制它们。定义ExcludeIdentityHashMap类来帮助我们阻止非自己的ShutdownHook注入
class ExcludeIdentityHashMap<K, V> extends IdentityHashMap<K, V> { public V put(K key, V value) { if (key instanceof Thread) { Thread thread = (Thread) key; if (!thread.getName().startsWith("My-")) { return value; } } return super.put(key, value); } }通过反射的方式注入自己的ShutdownHook并清除其他Thread
String className = "java.lang.ApplicationShutdownHooks"; Class<?> clazz = Class.forName(className); Field field = clazz.getDeclaredField("hooks"); field.setAccessible(true); Thread shutdownThread = new Thread(new Runnable() { @Override public void run() { // TODO } }); shutdownThread.setName("My-WebShutdownThread"); IdentityHashMap<Thread, Thread> excludeIdentityHashMap = new ExcludeIdentityHashMap<>(); excludeIdentityHashMap.put(shutdownThread, shutdownThread); synchronized (clazz) { IdentityHashMap<Thread, Thread> map = (IdentityHashMap<Thread, Thread>) field.get(clazz); for (Thread thread : map.keySet()) { Log.info("found shutdownHook: " + thread.getName()); excludeIdentityHashMap.put(thread, thread); } field.set(clazz, excludeIdentityHashMap); }对于一般的微服务来说，有这几种任务的入口：Http请求、dubbo请求、RabbitMQ消费、Quartz任务
测试发现Jetty容器在stop的时候不能实现平滑退出，springboot默认使用的tomcat容器可以，以下是部分代码示例：
EmbeddedWebApplicationContext embeddedWebApplicationContext = (EmbeddedWebApplicationContext) applicationContext; EmbeddedServletContainer embeddedServletContainer = embeddedWebApplicationContext.getEmbeddedServletContainer(); if (embeddedServletContainer instanceof TomcatEmbeddedServletContainer) { Connector[] connectors = tomcatEmbeddedServletContainer.getTomcat().getService().findConnectors(); for (Connector connector : connectors) { connector.pause(); } for (Connector connector : connectors) { Executor executor = connector.getProtocolHandler().getExecutor(); if (executor instanceof ThreadPoolExecutor) { try { ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) executor; threadPoolExecutor.shutdown(); if (!threadPoolExecutor.awaitTermination(5, TimeUnit.SECONDS)) { log.warn("Tomcat thread pool did not shutdown gracefully within 5 seconds. Proceeding with forceful shutdown"); } } catch (InterruptedException e) { log.warn("TomcatShutdownHook interrupted", e); } } } }尝试了许多次，看了相关的源码，dubbo不支持平滑退出；解决方法只有一个，那就是修改dubbo的源码，以下两个地址有详细介绍：http://frankfan915.iteye.com/blog/2254097https://my.oschina.net/u/1398931/blog/790709
以下是SpringBoot的示例，不使用Spring原理也是一样的
RabbitListenerEndpointRegistry rabbitListenerEndpointRegistry = applicationContext.getBean( RabbitListenerConfigUtils.RABBIT_LISTENER_ENDPOINT_REGISTRY_BEAN_NAME, RabbitListenerEndpointRegistry.class); Collection<MessageListenerContainer> containers = rabbitListenerEndpointRegistry.getListenerContainers(); for (MessageListenerContainer messageListenerContainer : containers) { messageListenerContainer.stop(); }quartz也比较简单
Scheduler scheduler = applicationContext.getBean(Scheduler.class); scheduler.shutdown(true);springboot通过java -jar example.jar的方式启动项目，在使用脚本restart的时候，首先覆盖旧的jar包，然后stop旧线程，启动新线程，这样就可能会出现此问题。因为在stop的时候，ShutdownHook线程被唤醒，在其执行过程中，某些类（尤其是匿名类）还未加载，这时候就会通知ClassLoader去加载；ClassLoader持有的是旧jar包的文件句柄，虽然新旧jar包的名字路径完全一样，但是ClassLoader仍然是使用open着的旧jar包文件，文件已经找不到了，所以类加载不了就ClassNotFound了。
如何解决呢？也许有更优雅的方式，但是我没有找到；但是我们可以简单地把顺序调整一下，先stop、再copy覆盖、最后start，这样就OK了。
