• SpringBoot的啟動主要是通過實例化SpringApplication來啟動的，啟動過程主要做了以下幾件事情：配置屬性、獲取監(jiān)聽器，發(fā)布應(yīng)用開始啟動事件初、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)啟動事件、發(fā)布應(yīng)用啟動完成事件。
• 在SpringBoot中啟動tomcat的工作在刷新上下這一步。而tomcat的啟動主要是實例化兩個組件：Connector、Container，一個tomcat實例就是一個Server，一個Server包含多個Service，也就是多個應(yīng)用程序，每個Service包含多個Connector和一個Container，而一個Container下又包含多個子容器。

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內(nèi)置tomcat

開發(fā)階段對我們來說使用內(nèi)置的tomcat是非常夠用了，當然也可以使用jetty。

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.1.6.RELEASE</version> </dependency> @SpringBootApplication public?class?MySpringbootTomcatStarter{public?static?void?main(String[]?args)?{Long?time=System.currentTimeMillis();SpringApplication.run(MySpringbootTomcatStarter.class);System.out.println("===應(yīng)用啟動耗時："+(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。

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發(fā)布生產(chǎn)

發(fā)布的時候，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configure()方法。

@SpringBootApplication public?class?MySpringbootTomcatStarter?extends?SpringBootServletInitializer?{public?static?void?main(String[]?args)?{Long?time=System.currentTimeMillis();SpringApplication.run(MySpringbootTomcatStarter.class);System.out.println("===應(yīng)用啟動耗時："+(System.currentTimeMillis()-time)+"===");}@Overrideprotected?SpringApplicationBuilder?configure(SpringApplicationBuilder?builder)?{return?builder.sources(this.getClass());} }

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從main函數(shù)說起

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，換成自己項目的logoBanner?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中是怎么啟動的，那么run方法中，重點關(guān)注創(chuàng)建應(yīng)用上下文（createApplicationContext）和刷新上下文（refreshContext）。

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創(chuàng)建上下文

//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); }

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

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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)?{}} }//這里直接調(diào)用最終父類AbstractApplicationContext.refresh()方法 protected?void?refresh(ApplicationContext?applicationContext)?{((AbstractApplicationContext)applicationContext).refresh(); } //AbstractApplicationContext.java public?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)用各個子類的onRefresh()方法，也就說這里要回到子類：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 //在這個方法里看到了熟悉的面孔，this.createWebServer，神秘的面紗就要揭開了。 protected?void?onRefresh()?{super.onRefresh();try?{this.createWebServer();}?catch?(Throwable?var2)?{} }//ServletWebServerApplicationContext.java //這里是創(chuàng)建webServer，但是還沒有啟動tomcat，這里是通過ServletWebServerFactory創(chuàng)建，那么接著看下ServletWebServerFactory private?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); }//實現(xiàn) AbstractServletWebServerFactory JettyServletWebServerFactory TomcatServletWebServerFactory UndertowServletWebServerFactory

這里ServletWebServerFactory接口有4個實現(xiàn)類

而其中我們常用的有兩個：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。 @Override public?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engine不會感到陌生。 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這個方法創(chuàng)建了Tomcat對象，并且做了兩件重要的事情：把Connector對象添加到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)實例化TomcatWebServer public?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?{//在控制臺會看到這句日志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tomcat服務(wù)===this.tomcat.start();rethrowDeferredStartupExceptions();try?{ContextBindings.bindClassLoader(context,?context.getNamingToken(),?getClass().getClassLoader());}catch?(NamingException?ex)?{}//開啟阻塞非守護進程startDaemonAwaitThread();}catch?(Exception?ex)?{stopSilently();destroySilently();throw?new?WebServerException("Unable?to?start?embedded?Tomcat",?ex);}} } //Tomcat.java public?void?start()?throws?LifecycleException?{getServer();server.start(); } //這里server.start又會回到TomcatWebServer的 public?void?stop()?throws?LifecycleException?{getServer();server.stop(); } //TomcatWebServer.java //啟動tomcat服務(wù) @Override public?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;//在控制臺打印這句日志，如果在yml設(shè)置了上下文，這里會打印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) @Override public?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();}}} }

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附：tomcat頂層結(jié)構(gòu)圖

tomcat最頂層容器是Server，代表著整個服務(wù)器，一個Server包含多個Service。從上圖可以看除Service主要包括多個Connector和一個Container。Connector用來處理連接相關(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請求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？我們來看下圖：

綜上所述，一個tomcat只包含一個Server，一個Server可以包含多個Service，一個Service只有一個Container，但有多個Connector，這樣一個服務(wù)可以處理多個連接。

多個Connector和一個Container就形成了一個Service，有了Service就可以對外提供服務(wù)了，但是Service要提供服務(wù)又必須提供一個宿主環(huán)境，那就非Server莫屬了，所以整個tomcat的聲明周期都由Server控制。

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