SpringBoot优雅退出

​​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​​

@SpringBootApplication
public 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()
*/
@Override
public 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