本文同步Java知音社區，專注于Java

作者：歪頭兒在帝都

http://cnblogs.com/sword-successful/p/11383723.html

前言

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？

內置tomcat

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>@SpringBootApplication public 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。

發布生產

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。

@SpringBootApplication public 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)+"===");}@Overrideprotected 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logoBanner 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

//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。Java知音公眾號內回復“后端面試”，送你一份Java面試題寶典

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 AbstractServletWebServerFactory JettyServletWebServerFactory TomcatServletWebServerFactory UndertowServletWebServerFactory

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。 @Override public 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===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 @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 @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();}}} }

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。

總結

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。

另

瑣碎時間想看一些技術文章，可以去公眾號菜單欄翻一翻我分類好的內容，應該對部分童鞋有幫助。同時看的過程中發現問題歡迎留言指出，不勝感謝~。另外，有想多了解哪些方面內容的可以留言（什么時候，哪篇文章下留言都行），附菜單欄截圖（PS：很多人不知道公眾號菜單欄是什么）

總結

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