背景
我看了一些介绍SpringBoot源码的文章，里面都是从SpringApplication这个外部调用入口开始，重点介绍里面的listener监听器。今天我想想说说SpringBoot的优雅退出机制。
调用入口调用SpringApplication开始启动SpringBoot
@SpringBootApplicationpublic class Application { public static void main(String[] args) throws Exception { SpringApplication.run(Application.class, args); }}SpringApplication实例初始化时有以下操作
public SpringApplication(Object... sources) { this.bannerMode = Mode.CONSOLE; this.logStartupInfo = true; this.addCommandLineProperties = true; this.headless = true; this.registerShutdownHook = true; this.additionalProfiles = new HashSet(); this.initialize(sources);}其中有一步this.registerShutdownHook注册jvm退出前的事件。
概念
jvm有shutdwonHook机制，中文习惯叫优雅退出。相当于在linux系统中执行SIGTERM(kill -15 或者 svc -d)时退出前执行的一些操作。
原理
以linux系统为例说明：
进程在响应kill -15 pid命令进行关闭操作时默认发送SIGTERM信号至指定进程/进程组。如果进程没有捕获该信号的逻辑，则SIGTERM的作用是终止进程。而registerShutdownHook=true说明有注册的事件需要捕获该信号，先执行相应的逻辑再进行终止。
/*** Register a shutdown hook with the JVM runtime, closing this context* on JVM shutdown unless it has already been closed at that time.* <p>Delegates to {@code doClose()} for the actual closing procedure.* @see Runtime#addShutdownHook* @see #close()* @see #doClose()*/@Overridepublic void registerShutdownHook() { if (this.shutdownHook == null) { // No shutdown hook registered yet. this.shutdownHook = new Thread() { @Override public void run() { synchronized (startupShutdownMonitor) { doClose(); } } }; Runtime.getRuntime().addShutdownHook(this.shutdownHook); }}如上面spring里registerShutdownHook的源码所示，就是注册一个jvm的shutdownHook钩子函数。jvm退出前会执行这个钩子函数。这个关闭操作在强制退出的时候不起作用。强制关闭范围详见下图
一个SpringBoot项目在优雅退出时做的事情可以通过log来感受一下
2019/06/20 20:47:20.405 Thread-29 [INFO] NettyHttpServer (NettyHttpServer.java:171) Close NettyHttpServer bind 0.0.0.0/0.0.0.0:5080
2019/06/20 20:47:20.405 Thread-36 [INFO] AnnotationConfigApplicationContext (AbstractApplicationContext.java:984) Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@a37aefe: startup date [Thu Jun 20 20:45:38 CST 2019]; root of context hierarchy
2019/06/20 20:47:20.406 Thread-32 [INFO] NettyServer (NettyServer.java:130) stopping netty server(XXX:9000), sleep 6 seconds!
2019/06/20 20:47:20.407 Thread-35 [INFO] ScheduleManager (ScheduleManager.java:373) Before taskNode quit,check executing task and callback result.
2019/06/20 20:47:20.407 Thread-35 [INFO] ScheduleManager (ScheduleManager.java:228) Destroy task node resources: zk and threadpool.
2019-06-20 20:47:20,430 Thread-35 WARN Unable to register Log4j shutdown hook because JVM is shutting down. Using SimpleLogger总结
分析问题先找抓手
原文链接：https://mp.weixin.qq.com/s/V_sb3vvKTiAiaOyv3yYY8A
