不得不說SpringBoot的開發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，典型的一鍵啟動系統，那么tomcat在springboot是怎么啟動的呢？

開發階段對我們來說使用內置的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>

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

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

發布的時候，目前大多數的做法還是排除內置的tomcat，打瓦包（war）然后部署在生產的tomcat中，好吧，那打包的時候應該怎么處理？

<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，主要是繼承SpringBootServletInitializer，并重寫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('===應用啟動耗時：'+(System.currentTimeMillis()-time)+'==='); } @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {return builder.sources(this.getClass()); }}從main函數說起

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logo Banner printedBanner = this.printBanner(environment); //創建應用上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); //預處理上下文 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，重點關注創建應用上下文（createApplicationContext）和刷新上下文（refreshContext）。

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

這里會創建AnnotationConfigServletWebServerApplicationContext類。而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個類是最終集成了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(); //調用各個子類的onRefresh()方法，也就說這里要回到子類：ServletWebServerApplicationContext，調用該類的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//這里是創建webServer，但是還沒有啟動tomcat，這里是通過ServletWebServerFactory創建，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);}//實現AbstractServletWebServerFactoryJettyServletWebServerFactoryTomcatServletWebServerFactoryUndertowServletWebServerFactory

這里ServletWebServerFactory接口有4個實現類

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。@Overridepublic WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir('tomcat'); tomcat.setBaseDir(baseDir.getAbsolutePath()); //創建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//這里調用構造函數實例化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=== 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.javapublic 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@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; //在控制臺打印這句日志，如果在yml設置了上下文，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()); } }}//關閉tomcat服務@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頂層結構圖

tomcat最頂層容器是Server，代表著整個服務器，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用來處理連接相關的事情，并提供Socket到Request和Response相關轉化。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了，但是Service要提供服務又必須提供一個宿主環境，那就非Server莫屬了，所以整個tomcat的聲明周期都由Server控制。

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、獲取監聽器，發布應用開始啟動事件初、始化輸入參數、配置環境，輸出banner、創建上下文、預處理上下文、刷新上下文、再刷新上下文、發布應用已經啟動事件、發布應用啟動完成事件。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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