SpringBoot內(nèi)置Tomcat啟動方式

更新時間：2024年12月10日 16:27:48 作者：孫振寧1999

Spring Boot通過啟動類上的@EnableAutoConfiguration注解,自動生成并加載ServletWebServerFactoryAutoConfiguration類,該類通過@Import注解導(dǎo)入TomcatServletWebServerFactory類,該類在getWebServer()方法中創(chuàng)建并啟動TomcatServletWebServer對象

一、Tomcat相關(guān)配置類如何加載的？

在springboot項目中，我們只需要引入spring-boot-starter-web依賴，啟動服務(wù)成功，我們一個web服務(wù)就搭建好了，沒有明顯的看到tomcat。

其實打開spring-boot-starter-web依賴，我們可以看到：依賴了tomcat。

1.進入Springboot啟動類

我們加入Springboot最核心的注解@SpringBootApplication，源碼如下圖：重點看注解@EnableAutoConfiguration，

2.進入注解@EnableAutoConfiguration

如下圖：該注解通過@Import注解導(dǎo)入了AutoConfigurationImportSelector類。

其實這個類，就是導(dǎo)入通過加載配置文件，加載了很多工廠方法的配置類。

3.進入AutoConfigurationImportSelector類

首先調(diào)用selectImport()方法，在該方法中調(diào)用了 getAutoConfigurationEntry()方法，在之中又調(diào)用了getCandidateConfigurations()方法， getCandidateConfigurations()方法就去META-INF/spring.factory配置文件中加載相關(guān)配置類。

詳細講解如下：也就是下圖的，方法1調(diào)用方法2，方法2調(diào)用方法3：

到了這里加載了 META-INF/spring.factories文件：

4.我們看到

加載了ServletWebServerFactoryAutoConfiguration這個配置類，web工廠配置類。

@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
		ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
		ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
		ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
  ...
}

從這個配置工廠類，我們看出通過@Import注解加載了tomcat，jetty，undertow三個web服務(wù)器的配置類。

由于沒有導(dǎo)入jetty和undertow的相關(guān)jar包，這兩個類實例的不會真正的加載。

5.進入EmbeddedTomcat類

創(chuàng)建了TomcatServletWebServerFactory類的對象。

@Configuration(proxyBeanMethods = false)
class ServletWebServerFactoryConfiguration {

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	static class EmbeddedTomcat {

		@Bean
		TomcatServletWebServerFactory tomcatServletWebServerFactory(
				ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
				ObjectProvider<TomcatContextCustomizer> contextCustomizers,
				ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
			TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
			factory.getTomcatConnectorCustomizers()
					.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
			factory.getTomcatContextCustomizers()
					.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
			factory.getTomcatProtocolHandlerCustomizers()
					.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
			return factory;
		}
	}

6.進入TomcatServletWebServerFactory類

關(guān)注getWebServer()方法：

	@Override
	public WebServer getWebServer(ServletContextInitializer... initializers) {
		if (this.disableMBeanRegistry) {
			Registry.disableRegistry();
		}
		//實例化一個Tomcat
		Tomcat tomcat = new Tomcat();
		File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
		//設(shè)置Tomcat的工作臨時目錄
		tomcat.setBaseDir(baseDir.getAbsolutePath());
		//默認使用Http11NioProtocal實例化Connector
		Connector connector = new Connector(this.protocol);
		connector.setThrowOnFailure(true);
		//給Service添加Connector
		tomcat.getService().addConnector(connector);
		customizeConnector(connector);
		tomcat.setConnector(connector);
		//關(guān)閉熱部署
		tomcat.getHost().setAutoDeploy(false);
		//配置Engine
		configureEngine(tomcat.getEngine());
		for (Connector additionalConnector : this.additionalTomcatConnectors) {
			tomcat.getService().addConnector(additionalConnector);
		}
		prepareContext(tomcat.getHost(), initializers);
		// 實例化TomcatWebServer時會將DispatcherServlet以及一些Filter添加到Tomcat中
		return getTomcatWebServer(tomcat);
	}

getWebServer()方法在當(dāng)前類，調(diào)用了getTomcatWebServer()方法，其實又new TomcatWebServer()對象：

	protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
		return new TomcatWebServer(tomcat, getPort() >= 0);
	}

7.進入TomcatWebServer類

這個類才是真正的做tomcat啟動的類：

（1）構(gòu)造方法：調(diào)用了initialize()方法

public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
		Assert.notNull(tomcat, "Tomcat Server must not be null");
		this.tomcat = tomcat;
		this.autoStart = autoStart;
		initialize();
	}

（2）進入initialize()方法，這個方法：this.tomcat.start()，啟動tomcat容器了。

private void initialize() throws WebServerException {
		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())) {
						// Remove service connectors so that protocol binding doesn't
						// happen when the service is started.
						removeServiceConnectors();
					}
				});

				// Tomcat在這里啟動了
				this.tomcat.start();

				// We can re-throw failure exception directly in the main thread
				rethrowDeferredStartupExceptions();

				try {
					ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
				}
				catch (NamingException ex) {
					// Naming is not enabled. Continue
				}

				// Unlike Jetty, all Tomcat threads are daemon threads. We create a
				// blocking non-daemon to stop immediate shutdown
				startDaemonAwaitThread();
			}
			catch (Exception ex) {
				stopSilently();
				destroySilently();
				throw new WebServerException("Unable to start embedded Tomcat", ex);
			}
		}
	}

二、getWebServer()的調(diào)用分析，也就是tomcat何時啟動的

上面分析了tomcat的配置到啟動的方法，我們現(xiàn)在來分析，tomcat是何時啟動的。

1.首先進入SpringBoot啟動類的run方法

	public static void main(String[] args) {
		SpringApplication.run(SpringBootMytestApplication.class, args);
	}

最終調(diào)用了本類的一個同名方法：

public ConfigurableApplicationContext run(String... args) {
		//記錄程序運行時間
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		// ConfigurableApplicationContext Spring 的上下文
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		//【1、獲取并啟動監(jiān)聽器】
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			//【2、構(gòu)造應(yīng)用上下文環(huán)境】
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			//處理需要忽略的Bean
			configureIgnoreBeanInfo(environment);
			//打印banner
			Banner printedBanner = printBanner(environment);
			///【3、初始化應(yīng)用上下文】
			context = createApplicationContext();
			//實例化SpringBootExceptionReporter.class，用來支持報告關(guān)于啟動的錯誤
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			//【4、刷新應(yīng)用上下文前的準備階段】
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			//【5、刷新應(yīng)用上下文】
			refreshContext(context);
			//【6、刷新應(yīng)用上下文后的擴展接口】
			afterRefresh(context, applicationArguments);
			//時間記錄停止
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			//發(fā)布容器啟動完成事件
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

這個方法大概做了以下幾件事：

1)獲取并啟動監(jiān)聽器通過加載META-INF/spring.factories 完成了 SpringApplicationRunListener實例化工作
2)構(gòu)造容器環(huán)境，簡而言之就是加載系統(tǒng)變量，環(huán)境變量，配置文件
3)創(chuàng)建容器
4)實例化SpringBootExceptionReporter.class，用來支持報告關(guān)于啟動的錯誤
5)準備容器
6) 刷新容器
7)刷新容器后的擴展接口

2.那么內(nèi)置tomcat啟動源碼

就是隱藏在上面第六步：refreshContext方法里面，該方法最終會調(diào) 用到AbstractApplicationContext類的refresh()方法，進入refreshContext()方法，如圖：

	private void refreshContext(ConfigurableApplicationContext context) {
		refresh(context);
		if (this.registerShutdownHook) {
			try {
				context.registerShutdownHook();
			}
			catch (AccessControlException ex) {
				// Not allowed in some environments.
			}
		}
	}

refreshContext()調(diào)用了refresh()方法：

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();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();
            } catch (BeansException var9) {
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
                }

                this.destroyBeans();
                this.cancelRefresh(var9);
                throw var9;
            } finally {
                this.resetCommonCaches();
            }

        }
    }

refresh()方法調(diào)用了this.onRefresh():

	@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			//核心方法：會獲取嵌入式的Servlet容器工廠，并通過工廠來獲取Servlet容器
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

如下面的代碼：createWebServer() 方法調(diào)用了一個factory.getWebServer()。

	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			//先獲取嵌入式Servlet容器工廠
			ServletWebServerFactory factory = getWebServerFactory();
			this.webServer = factory.getWebServer(getSelfInitializer());
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}

到了這里getWebServer()方法，下一步就是創(chuàng)建TomcatWebServer對象，創(chuàng)建該對象，就在構(gòu)造方法啟動了Tomcat。詳細代碼在第一部分有。