SpringBoot內置tomcat啟動原理
- 2019 年 10 月 3 日
- 筆記
前言
不得不說SpringBoot的開發者是在為大眾程式猿謀福利，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，那麼tomcat在springboot是怎麼啟動的呢？
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)+"==="); } }這裡是main函數入口，兩句程式碼最耀眼，分別是SpringBootApplication註解和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）然後部署在生產的tomcat中，好吧，那打包的時候應該怎麼處理？
<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，並重寫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)+"==="); } @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); } --這裡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); //列印banner，這裡你可以自己塗鴉一下，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中是怎麼啟動的，那麼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); }這裡會創建AnnotationConfigServletWebServerApplicationContext類。
而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個類是最終集成了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) { } } } //這裡直接調用最終父類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，也就說這裡要回到子類：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 //這裡是創建webServer，但是還沒有啟動tomcat，這裡是通過ServletWebServerFactory創建，那麼接著看下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(); } //介面 public interface ServletWebServerFactory { WebServer getWebServer(ServletContextInitializer... initializers); } //實現 AbstractServletWebServerFactory JettyServletWebServerFactory TomcatServletWebServerFactory UndertowServletWebServerFactory 這裡ServletWebServerFactory介面有4個實現類
而其中我們常用的有兩個：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java //這裡我們使用的tomcat，所以我們查看TomcatServletWebServerFactory。到這裡總算是看到了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，看到這裡，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，並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 //這裡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 { //在控制台會看到這句日誌 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(); } //這裡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; //在控制台列印這句日誌，如果在yml設置了上下文，這裡會列印 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，代表著整個伺服器，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，並提供Socket到Request和Response相關轉化。Container用於封裝和管理Servlet，以及處理具體的Request請求。那麼上文提到的Engine>Host>Context>Wrapper容器又是怎麼回事呢？ 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，也就是多個應用程式，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。
