前言
不得不说SpringBoot的开发者是在为大众程序猿谋福利，把大家都惯成了懒汉，xml不配置了，连tomcat也懒的配置了，典型的一键启动系统，那么tomcat在springboot是怎么启动的呢？
内置tomcat
开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。
org.springframework.boot spring-boot-starter-web 2.1.6.RELEASE@SpringBootApplicationpublic class MySpringbootTomcatStarter{ public static void main(String[] args) { Long time=System.currentTimeMillis(); SpringApplication.run(MySpringbootTomcatStarter.class); System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); }}这里是main函数入口，两句代码最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。
发布生产
发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包(war)然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？
org.springframework.boot spring-boot-starter-web org.springframework.boot spring-boot-starter-tomcat javax.servlet javax.servlet-api 3.1.0 provided更新main函数，主要是继承SpringBootServletInitializer，并重写configure()方法。
@SpringBootApplicationpublic class MySpringbootTomcatStarter extends SpringBootServletInitializer { public static void main(String[] args) { Long time=System.currentTimeMillis(); SpringApplication.run(MySpringbootTomcatStarter.class); System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); } @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) { return builder.sources(this.getClass()); }}从main函数说起
public static ConfigurableApplicationContext run(Class> primarySource, String... args) { return run(new Class[]{primarySource}, args);}--这里run方法返回的是ConfigurableApplicationContextpublic static ConfigurableApplicationContext run(Class>[] primarySources, String[] args) { return (new SpringApplication(primarySources)).run(args);}public ConfigurableApplicationContext run(String... args) { ConfigurableApplicationContext context = null; Collection exceptionReporters = new ArrayList(); this.configureHeadlessProperty(); SpringApplicationRunListeners listeners = this.getRunListeners(args); listeners.starting(); Collection exceptionReporters; try { ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments); this.configureIgnoreBeanInfo(environment); //打印banner，这里你可以自己涂鸦一下，换成自己项目的logo Banner printedBanner = this.printBanner(environment); //创建应用上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); //预处理上下文 this.prepareContext(context, environment, listeners, applicationArguments, printedBanner); //刷新上下文 this.refreshContext(context); //再刷新上下文 this.afterRefresh(context, applicationArguments); listeners.started(context); this.callRunners(context, applicationArguments); } catch (Throwable var10) { } try { listeners.running(context); return context; } catch (Throwable var9) { }}既然我们想知道tomcat在SpringBoot中是怎么启动的，那么run方法中，重点关注创建应用上下文(createApplicationContext)和刷新上下文(refreshContext)。
创建上下文
//创建上下文protected ConfigurableApplicationContext createApplicationContext() { Class> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch(this.webApplicationType) { case SERVLET: //创建AnnotationConfigServletWebServerApplicationContext contextClass = Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext"); break; case REACTIVE: contextClass = Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext"); break; default: contextClass = Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext"); } } catch (ClassNotFoundException var3) { throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3); } } return (ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass);}这里会创建AnnotationConfigServletWebServerApplicationContext类。而AnnotationConfigServletWebServerApplicationContext类继承了ServletWebServerApplicationContext，而这个类是最终集成了AbstractApplicationContext。
刷新上下文
//SpringApplication.java//刷新上下文private void refreshContext(ConfigurableApplicationContext context) { this.refresh(context); if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch (AccessControlException var3) { } }}//这里直接调用最终父类AbstractApplicationContext.refresh()方法protected void refresh(ApplicationContext applicationContext) { ((AbstractApplicationContext)applicationContext).refresh();}//AbstractApplicationContext.javapublic void refresh() throws BeansException, IllegalStateException { synchronized(this.startupShutdownMonitor) { this.prepareRefresh(); ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory(); this.prepareBeanFactory(beanFactory); try { this.postProcessBeanFactory(beanFactory); this.invokeBeanFactoryPostProcessors(beanFactory); this.registerBeanPostProcessors(beanFactory); this.initMessageSource(); this.initApplicationEventMulticaster(); //调用各个子类的onRefresh()方法，也就说这里要回到子类：ServletWebServerApplicationContext，调用该类的onRefresh()方法 this.onRefresh(); this.registerListeners(); this.finishBeanFactoryInitialization(beanFactory); this.finishRefresh(); } catch (BeansException var9) { this.destroyBeans(); this.cancelRefresh(var9); throw var9; } finally { this.resetCommonCaches(); } }}//ServletWebServerApplicationContext.java//在这个方法里看到了熟悉的面孔，this.createWebServer，神秘的面纱就要揭开了。protected void onRefresh() { super.onRefresh(); try { this.createWebServer(); } catch (Throwable var2) { }}//ServletWebServerApplicationContext.java//这里是创建webServer，但是还没有启动tomcat，这里是通过ServletWebServerFactory创建，那么接着看下ServletWebServerFactoryprivate void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = this.getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = this.getWebServerFactory(); this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()}); } else if (servletContext != null) { try { this.getSelfInitializer().onStartup(servletContext); } catch (ServletException var4) { } } this.initPropertySources();}//接口public interface ServletWebServerFactory { WebServer getWebServer(ServletContextInitializer... initializers);}//实现AbstractServletWebServerFactoryJettyServletWebServerFactoryTomcatServletWebServerFactoryUndertowServletWebServerFactory这里ServletWebServerFactory接口有4个实现类
而其中我们常用的有两个：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java//这里我们使用的tomcat，所以我们查看TomcatServletWebServerFactory。到这里总算是看到了tomcat的踪迹。@Overridepublic WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); //创建Connector对象 Connector connector = new Connector(this.protocol); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat);}protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) { return new TomcatWebServer(tomcat, getPort() >= 0);}//Tomcat.java//返回Engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。public Engine getEngine() { Service service = getServer().findServices()[0]; if (service.getContainer() != null) { return service.getContainer(); } Engine engine = new StandardEngine(); engine.setName( "Tomcat" ); engine.setDefaultHost(hostname); engine.setRealm(createDefaultRealm()); service.setContainer(engine); return engine;}//Engine是最高级别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器getWebServer这个方法创建了Tomcat对象，并且做了两件重要的事情：把Connector对象添加到tomcat中，configureEngine(tomcat.getEngine());
getWebServer方法返回的是TomcatWebServer。
//TomcatWebServer.java//这里调用构造函数实例化TomcatWebServerpublic TomcatWebServer(Tomcat tomcat, boolean autoStart) { Assert.notNull(tomcat, "Tomcat Server must not be null"); this.tomcat = tomcat; this.autoStart = autoStart; initialize();}private void initialize() throws WebServerException { //在控制台会看到这句日志 logger.info("Tomcat initialized with port(s): " + getPortsDescription(false)); synchronized (this.monitor) { try { addInstanceIdToEngineName(); Context context = findContext(); context.addLifecycleListener((event) -> { if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) { removeServiceConnectors(); } }); //===启动tomcat服务=== this.tomcat.start(); rethrowDeferredStartupExceptions(); try { ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader()); } catch (NamingException ex) { } //开启阻塞非守护进程 startDaemonAwaitThread(); } catch (Exception ex) { stopSilently(); destroySilently(); throw new WebServerException("Unable to start embedded Tomcat", ex); } }}//Tomcat.javapublic void start() throws LifecycleException { getServer(); server.start();}//这里server.start又会回到TomcatWebServer的public void stop() throws LifecycleException { getServer(); server.stop();}//TomcatWebServer.java//启动tomcat服务@Overridepublic void start() throws WebServerException { synchronized (this.monitor) { if (this.started) { return; } try { addPreviouslyRemovedConnectors(); Connector connector = this.tomcat.getConnector(); if (connector != null && this.autoStart) { performDeferredLoadOnStartup(); } checkThatConnectorsHaveStarted(); this.started = true; //在控制台打印这句日志，如果在yml设置了上下文，这里会打印 logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '" + getContextPath() + "'"); } catch (ConnectorStartFailedException ex) { stopSilently(); throw ex; } catch (Exception ex) { throw new WebServerException("Unable to start embedded Tomcat server", ex); } finally { Context context = findContext(); ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader()); } }}//关闭tomcat服务@Overridepublic void stop() throws WebServerException { synchronized (this.monitor) { boolean wasStarted = this.started; try { this.started = false; try { stopTomcat(); this.tomcat.destroy(); } catch (LifecycleException ex) { } } catch (Exception ex) { throw new WebServerException("Unable to stop embedded Tomcat", ex); } finally { if (wasStarted) { containerCounter.decrementAndGet(); } } }}附：tomcat顶层结构图
tomcat最顶层容器是Server，代表着整个服务器，一个Server包含多个Service。从上图可以看除Service主要包括多个Connector和一个Container。Connector用来处理连接相关的事情，并提供Socket到Request和Response相关转化。
Container用于封装和管理Servlet，以及处理具体的Request请求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？我们来看下图：
综上所述，一个tomcat只包含一个Server，一个Server可以包含多个Service，一个Service只有一个Container，但有多个Connector，这样一个服务可以处理多个连接。
多个Connector和一个Container就形成了一个Service，有了Service就可以对外提供服务了，但是Service要提供服务又必须提供一个宿主环境，那就非Server莫属了，所以整个tomcat的声明周期都由Server控制。
总结
SpringBoot的启动主要是通过实例化SpringApplication来启动的，启动过程主要做了以下几件事情：配置属性、获取监听器，发布应用开始启动事件初、始化输入参数、配置环境，输出banner、创建上下文、预处理上下文、刷新上下文、再刷新上下文、发布应用已经启动事件、发布应用启动完成事件。
在SpringBoot中启动tomcat的工作在刷新上下这一步。而tomcat的启动主要是实例化两个组件：Connector、Container，一个tomcat实例就是一个Server，一个Server包含多个Service，也就是多个应用程序，每个Service包含多个Connector和一个Container，而一个Container下又包含多个子容器。
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