不得不說(shuō)SpringBoot的開(kāi)發(fā)者是在為大眾程序猿謀福利，把大家都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啟動(dòng)系統(tǒng)，那么tomcat在springboot是怎么啟動(dòng)的呢？

開(kāi)發(fā)階段對(duì)我們來(lái)說(shuō)使用內(nèi)置的tomcat是非常夠用了，當(dāng)然也可以使用jetty。

<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> <version>2.1.6.RELEASE</version></dependency>

@SpringBootApplicationpublic class MySpringbootTomcatStarter{ public static void main(String[] args) {Long time=System.currentTimeMillis();SpringApplication.run(MySpringbootTomcatStarter.class);System.out.println('===應(yīng)用啟動(dòng)耗時(shí)：'+(System.currentTimeMillis()-time)+'==='); }}

這里是main函數(shù)入口，兩句代碼最耀眼，分別是SpringBootApplication注解和SpringApplication.run()方法。

發(fā)布的時(shí)候，目前大多數(shù)的做法還是排除內(nèi)置的tomcat，打瓦包（war）然后部署在生產(chǎn)的tomcat中，好吧，那打包的時(shí)候應(yīng)該怎么處理？

<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> <!-- 移除嵌入式tomcat插件 --> <exclusions><exclusion> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-tomcat</artifactId></exclusion> </exclusions></dependency><!--添加servlet-api依賴---><dependency> <groupId>javax.servlet</groupId> <artifactId>javax.servlet-api</artifactId> <version>3.1.0</version> <scope>provided</scope></dependency>

更新main函數(shù)，主要是繼承SpringBootServletInitializer，并重寫(xiě)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('===應(yīng)用啟動(dòng)耗時(shí)：'+(System.currentTimeMillis()-time)+'==='); } @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {return builder.sources(this.getClass()); }}從main函數(shù)說(shuō)起

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) { return run(new Class[]{primarySource}, args);}

--這里run方法返回的是ConfigurableApplicationContext

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { return (new SpringApplication(primarySources)).run(args);}

public ConfigurableApplicationContext run(String... args) { ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> 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，這里你可以自己涂鴉一下，換成自己項(xiàng)目的logo Banner printedBanner = this.printBanner(environment); //創(chuàng)建應(yīng)用上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); //預(yù)處理上下文 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) { }}

既然我們想知道tomcat在SpringBoot中是怎么啟動(dòng)的，那么run方法中，重點(diǎn)關(guān)注創(chuàng)建應(yīng)用上下文（createApplicationContext）和刷新上下文（refreshContext）。

//創(chuàng)建上下文protected ConfigurableApplicationContext createApplicationContext() { Class<?> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch(this.webApplicationType) { case SERVLET: //創(chuàng)建AnnotationConfigServletWebServerApplicationContextcontextClass = 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);}

這里會(huì)創(chuàng)建AnnotationConfigServletWebServerApplicationContext類。而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個(gè)類是最終集成了AbstractApplicationContext。

//SpringApplication.java//刷新上下文private void refreshContext(ConfigurableApplicationContext context) { this.refresh(context); if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch (AccessControlException var3) { } }}

//這里直接調(diào)用最終父類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(); //調(diào)用各個(gè)子類的onRefresh()方法，也就說(shuō)這里要回到子類：ServletWebServerApplicationContext，調(diào)用該類的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//在這個(gè)方法里看到了熟悉的面孔，this.createWebServer，神秘的面紗就要揭開(kāi)了。protected void onRefresh() { super.onRefresh(); try { this.createWebServer(); } catch (Throwable var2) { }}//ServletWebServerApplicationContext.java//這里是創(chuàng)建webServer，但是還沒(méi)有啟動(dòng)tomcat，這里是通過(guò)ServletWebServerFactory創(chuàng)建，那么接著看下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);}//實(shí)現(xiàn)AbstractServletWebServerFactoryJettyServletWebServerFactoryTomcatServletWebServerFactoryUndertowServletWebServerFactory

這里ServletWebServerFactory接口有4個(gè)實(shí)現(xiàn)類

而其中我們常用的有兩個(gè)：TomcatServletWebServerFactory和JettyServletWebServerFactory。

//TomcatServletWebServerFactory.java//這里我們使用的tomcat，所以我們查看TomcatServletWebServerFactory。到這里總算是看到了tomcat的蹤跡。@Overridepublic WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir('tomcat'); tomcat.setBaseDir(baseDir.getAbsolutePath()); //創(chuàng)建Connector對(duì)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，看到這里，如果熟悉tomcat源碼的話，對(duì)engine不會(huì)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是最高級(jí)別容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器

getWebServer這個(gè)方法創(chuàng)建了Tomcat對(duì)象，并且做了兩件重要的事情：把Connector對(duì)象添加到tomcat中，configureEngine(tomcat.getEngine()); getWebServer方法返回的是TomcatWebServer。

//TomcatWebServer.java//這里調(diào)用構(gòu)造函數(shù)實(shí)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 { //在控制臺(tái)會(huì)看到這句日志 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(); } }); //===啟動(dòng)tomcat服務(wù)=== this.tomcat.start(); rethrowDeferredStartupExceptions(); try { ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader()); } catch (NamingException ex) { }//開(kāi)啟阻塞非守護(hù)進(jìn)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();}//這里server.start又會(huì)回到TomcatWebServer的public void stop() throws LifecycleException { getServer(); server.stop();}

//TomcatWebServer.java//啟動(dòng)tomcat服務(wù)@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; //在控制臺(tái)打印這句日志，如果在yml設(shè)置了上下文，這里會(huì)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()); } }}//關(guān)閉tomcat服務(wù)@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頂層結(jié)構(gòu)圖

tomcat最頂層容器是Server，代表著整個(gè)服務(wù)器，一個(gè)Server包含多個(gè)Service。從上圖可以看除Service主要包括多個(gè)Connector和一個(gè)Container。Connector用來(lái)處理連接相關(guān)的事情，并提供Socket到Request和Response相關(guān)轉(zhuǎn)化。Container用于封裝和管理Servlet，以及處理具體的Request請(qǐng)求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？ 我們來(lái)看下圖：

綜上所述，一個(gè)tomcat只包含一個(gè)Server，一個(gè)Server可以包含多個(gè)Service，一個(gè)Service只有一個(gè)Container，但有多個(gè)Connector，這樣一個(gè)服務(wù)可以處理多個(gè)連接。 多個(gè)Connector和一個(gè)Container就形成了一個(gè)Service，有了Service就可以對(duì)外提供服務(wù)了，但是Service要提供服務(wù)又必須提供一個(gè)宿主環(huán)境，那就非Server莫屬了，所以整個(gè)tomcat的聲明周期都由Server控制。

SpringBoot的啟動(dòng)主要是通過(guò)實(shí)例化SpringApplication來(lái)啟動(dòng)的，啟動(dòng)過(guò)程主要做了以下幾件事情：配置屬性、獲取監(jiān)聽(tīng)器，發(fā)布應(yīng)用開(kāi)始啟動(dòng)事件初、始化輸入?yún)?shù)、配置環(huán)境，輸出banner、創(chuàng)建上下文、預(yù)處理上下文、刷新上下文、再刷新上下文、發(fā)布應(yīng)用已經(jīng)啟動(dòng)事件、發(fā)布應(yīng)用啟動(dòng)完成事件。在SpringBoot中啟動(dòng)tomcat的工作在刷新上下這一步。而tomcat的啟動(dòng)主要是實(shí)例化兩個(gè)組件：Connector、Container，一個(gè)tomcat實(shí)例就是一個(gè)Server，一個(gè)Server包含多個(gè)Service，也就是多個(gè)應(yīng)用程序，每個(gè)Service包含多個(gè)Connector和一個(gè)Container，而一個(gè)Container下又包含多個(gè)子容器。

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