Spring之ShutDown?Hook死鎖現(xiàn)象解讀
Spring ShutDown Hook死鎖現(xiàn)象
偶然出現(xiàn)一次項目異常spring卻沒有正常停止的情況,最終發(fā)現(xiàn)是Spring Shutdown導(dǎo)致的死鎖現(xiàn)象。
某個框架里嵌入了類似這樣的一段代碼
@Component public class ShutDownHookTest implements ApplicationListener<ContextRefreshedEvent> { @Override public void onApplicationEvent(ContextRefreshedEvent event) { if (onException) { System.out.println("test shutdown hook deadlock"); System.exit(0); } } }
它的邏輯就是想要在出現(xiàn)異常后,通過System.exit來確保應(yīng)用程序退出。
而且沒有使用異步事件,是在主線程下跑了System.exit,然后就發(fā)現(xiàn)springboot server還是正常運行著的。
而且程序看著好像也沒問題,由于我們是dubbo服務(wù)化系統(tǒng),在測試環(huán)境上服務(wù)還是正常的。
這很明顯不符常理,正常來說,System.exit這樣的指令是spring能夠感知到的,并且會執(zhí)行shutDown處理的,先來看看Spring 注冊ShutdownHook
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext { @Override public void registerShutdownHook() { if (this.shutdownHook == null) { // No shutdown hook registered yet. this.shutdownHook = new Thread(SHUTDOWN_HOOK_THREAD_NAME) { @Override public void run() { //重點在這里獲取startupShutdownMonitor的監(jiān)視器鎖 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); } } }
也就是說spring新起了一個線程,加入了JVM Shutdown鉤子函數(shù)。
重點是close前要獲取startupShutdownMonitor的對象監(jiān)視器鎖,這個鎖看著就很眼熟,Spring在refresh時也會獲取這把鎖。
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext { @Override public 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); ...... } } }
這個時候我們猜想,是獲取startupShutdownMonitor死鎖了。
jstack打下線程??纯?/h2>
"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并沒有提示線程死鎖(jvisualvm、jconsle之類的工具也不行),但是從線程棧來看:
- main線程先獲取到了startupShutdownMonitor鎖 <0x00000006c494f430>
- SpringContextShutdownHook線程在等待startupShutdownMonitor鎖
- main線程掉了Thread.join阻塞在獲取<0x00000006c4a43118>這把鎖
根本原因是main線程調(diào)System.exit阻塞住了,一直往下追蹤,會發(fā)現(xiàn)阻塞在ApplicationShutdownHooks這里
class ApplicationShutdownHooks { /* 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 { // 等待shutdow線程結(jié)束 hook.join(); break; } catch (InterruptedException ignored) { } } } } }
總結(jié)
整個死鎖的流程:
- main線程-spring refresh開始時會獲取startupShutdownMonitor對象監(jiān)視器鎖
- main線程-在spring refresh還未完成的時候,觸發(fā)了System.exit指令
- SpringContextShutdownHook線程-SpringContextShutdownHook線程開始工作,等待獲取startupShutdownMonitor對象監(jiān)視器鎖
- main線程調(diào)用Thread.join等待SpringContextShutdownHook線程結(jié)束
所以在Spring未完成refresh時,是不能夠觸發(fā)System.exit指令的
以上為個人經(jīng)驗,希望能給大家一個參考,也希望大家多多支持腳本之家。
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