本篇内容主要讲解“Spring之ShutDown Hook死锁现象源码分析”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“Spring之ShutDown Hook死锁现象源码分析”吧!
Spring ShutDown Hook死锁现象
偶然出现一次项目异常spring却没有正常停止的情况，最终发现是Spring Shutdown导致的死锁现象。
某个框架里嵌入了类似这样的一段代码
@Componentpublic class ShutDownHookTest implements ApplicationListener<ContextRefreshedEvent> { @Override public void onApplicationEvent(ContextRefreshedEvent event) { if (onException) { System.out.println("test shutdown hook deadlock");System.exit(0);} }}它的逻辑就是想要在出现异常后，通过System.exit来确保应用程序退出。
而且没有使用异步事件，是在主线程下跑了System.exit，然后就发现springboot server还是正常运行着的。
而且程序看着好像也没问题，由于我们是dubbo服务化系统，在测试环境上服务还是正常的。
这很明显不符常理，正常来说，System.exit这样的指令是spring能够感知到的，并且会执行shutDown处理的，先来看看Spring 注册ShutdownHook
public abstract class AbstractApplicationContext extends DefaultResourceLoaderimplements ConfigurableApplicationContext {@Overridepublic void registerShutdownHook() {if (this.shutdownHook == null) {// No shutdown hook registered yet.this.shutdownHook = new Thread(SHUTDOWN_HOOK_THREAD_NAME) {@Overridepublic void run() { //重点在这里获取startupShutdownMonitor的监视器锁synchronized (startupShutdownMonitor) {doClose();}}};Runtime.getRuntime().addShutdownHook(this.shutdownHook);}} protected void doClose() {// Check whether an actual close attempt is necessary...if (this.active.get() && this.closed.compareAndSet(false, true)) {if (logger.isDebugEnabled()) {logger.debug("Closing " + this);} if (!NativeDetector.inNativeImage()) {LiveBeansView.unregisterApplicationContext(this);} try {// Publish shutdown event.publishEvent(new ContextClosedEvent(this));}catch (Throwable ex) {logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);} // Stop all Lifecycle beans, to avoid delays during individual destruction.if (this.lifecycleProcessor != null) {try {this.lifecycleProcessor.onClose();}catch (Throwable ex) {logger.warn("Exception thrown from LifecycleProcessor on context close", ex);}} // Destroy all cached singletons in the context's BeanFactory.destroyBeans(); // Close the state of this context itself.closeBeanFactory(); // Let subclasses do some final clean-up if they wish...onClose(); // Reset local application listeners to pre-refresh state.if (this.earlyApplicationListeners != null) {this.applicationListeners.clear();this.applicationListeners.addAll(this.earlyApplicationListeners);} // Switch to inactive.this.active.set(false);}}}也就是说spring新起了一个线程，加入了JVM Shutdown钩子函数。
重点是close前要获取startupShutdownMonitor的对象监视器锁，这个锁看着就很眼熟，Spring在refresh时也会获取这把锁。
public abstract class AbstractApplicationContext extends DefaultResourceLoaderimplements ConfigurableApplicationContext { @Overridepublic void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh"); // Prepare this context for refreshing.prepareRefresh(); // Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory); ......}}}这个时候我们猜想，是获取startupShutdownMonitor死锁了。
jstack打下线程栈看看
"SpringContextShutdownHook" #18 prio=5 os_prio=0 tid=0x0000000024e00800 nid=0x407c waiting for monitor entry [0x000000002921f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.springframework.context.support.AbstractApplicationContext$1.run(AbstractApplicationContext.java:991)
- waiting to lock <0x00000006c494f430> (a java.lang.Object)
"main" #1 prio=5 os_prio=0 tid=0x0000000002de4000 nid=0x1ff4 in Object.wait() [0x0000000002dde000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000006c4a43118> (a org.springframework.context.support.AbstractApplicationContext$1)
at java.lang.Thread.join(Thread.java:1252)
- locked <0x00000006c4a43118> (a org.springframework.context.support.AbstractApplicationContext$1)
at java.lang.Thread.join(Thread.java:1326)
at java.lang.ApplicationShutdownHooks.runHooks(ApplicationShutdownHooks.java:107)
at java.lang.ApplicationShutdownHooks$1.run(ApplicationShutdownHooks.java:46)
at java.lang.Shutdown.runHooks(Shutdown.java:123)
at java.lang.Shutdown.sequence(Shutdown.java:167)
at java.lang.Shutdown.exit(Shutdown.java:212)
- locked <0x00000006c4845128> (a java.lang.Class for java.lang.Shutdown)
at java.lang.Runtime.exit(Runtime.java:109)
at java.lang.System.exit(System.java:971)
at io.seata.server.ShutDownHookTest.onApplicationEvent(ShutDownHookTest.java:12)
at io.seata.server.ShutDownHookTest.onApplicationEvent(ShutDownHookTest.java:7)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:176)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:169)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:143)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:421)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:378)
at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:938)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:586)
- locked <0x00000006c494f430> (a java.lang.Object)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:144)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:771)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:763)
乍一看jstack并没有提示线程死锁（jvisualvm、jconsle之类的工具也不行），但是从线程栈来看：
main线程先获取到了startupShutdownMonitor锁 <0x00000006c494f430>
SpringContextShutdownHook线程在等待startupShutdownMonitor锁
main线程掉了Thread.join阻塞在获取<0x00000006c4a43118>这把锁
根本原因是main线程调System.exit阻塞住了，一直往下追踪，会发现阻塞在ApplicationShutdownHooks这里
class ApplicationShutdownHooks { 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 {// 等待shutdow线程结束 hook.join(); break; } catch (InterruptedException ignored) { } } } }}到此，相信大家对“Spring之ShutDown Hook死锁现象源码分析”有了更深的了解，不妨来实际操作一番吧！这里是编程网网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
