spring boot 載入web容器tomcat流程原始碼分析

spring boot 載入web容器tomcat流程原始碼分析

我本地的springboot版本是2.5.1，後面的分析都是基於這個版本

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.1</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
  

我們通過在pom檔案中引入

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

來引入web容器，預設的web容器時tomcat。

本文章主要描述spring boot載入web容器 tomcat的部分，為了避免文章知識點過於分散，其他相關的如bean的載入，tomcat內部流程等不做深入討論。

1、在springboot web工程中，全域性上下文是AnnotationConfigServletWebApplicationContext

下面的部分，我們具體來看下

首先，我們的入口程式碼一般都是這樣寫的

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

跳轉到run方法裡面，依次會呼叫下面兩個方法

	public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
		return run(new Class<?>[] { primarySource }, args);
	}

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
   return new SpringApplication(primarySources).run(args);
}

首先會建立SpringApplication例項物件，跳轉到SpringApplication的構造方法去看看，依次會呼叫如下方法

	public SpringApplication(Class<?>... primarySources) {
		this(null, primarySources);
	}

@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
   ......
   //本次不相關的程式碼全部省略掉，只保留相關程式碼
   //這裡的 this.webApplicationType=WebApplicationType.SERVLET， 我們來分析下這個程式碼的具體的執行賦值
   this.webApplicationType = WebApplicationType.deduceFromClasspath();
   ......
}

繼續跳轉到WebApplicationType.deduceFromClasspath()去看看

	//這個方法主要是在當前類路徑下查詢指定的class類是否存在，返回對飲列舉型別
	static WebApplicationType deduceFromClasspath() {
	// WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";
	
	//我們通過pom檔案引入spring-boot-starter-web，會簡介引入spring-webmvc,上面這個類就在這個webmvc中，所以不會進入這個if分支
		if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
				&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
			return WebApplicationType.REACTIVE;
		}
	//SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet",
			"org.springframework.web.context.ConfigurableWebApplicationContext" }
	//javax.servlet.Servlet這個類存在於tomcat-embed-core中
    //org.springframework.web.context.ConfigurableWebApplicationContext這個類存在於spring-web中
    //這兩個jar都是由spring-boot-starter-web間接引入的，所以也不會走這個分支
		for (String className : SERVLET_INDICATOR_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return WebApplicationType.NONE;
			}
		}
		//所以會從這裡返回
		return WebApplicationType.SERVLET;
	}

下面看下jar包的引入

回到new SpringApplication(primarySources).run(args)的呼叫來看run方法的程式碼

public ConfigurableApplicationContext run(String... args) {
		.......
		try {
			......
			//我們來看這個context的建立，context=new AnnotationConfigServletWebServerApplicationContext()下面來具體看這塊的執行
			context = createApplicationContext();
		    ......
		    //後續幾個部分會來說明這個方法
			refreshContext(context);
			......
		}
		catch (Throwable ex) {
			......
		}

		try {
		.......
		}
		catch (Throwable ex) {
		.......
		}
		return context;
	}

createApplicationContext()依次會呼叫如下方法

	protected ConfigurableApplicationContext createApplicationContext() {
		//這裡的this.webApplicationType就是上面的WebApplicationType.SERVLET
		return this.applicationContextFactory.create(this.webApplicationType);
	}

	//最終會呼叫到這個lambda表示式，入參就是上面的WebApplicationType.SERVLET
	ApplicationContextFactory DEFAULT = (webApplicationType) -> {
		try {
			switch (webApplicationType) {
			case SERVLET:
				//會從這裡返回
				return new AnnotationConfigServletWebServerApplicationContext();
			case REACTIVE:
				return new AnnotationConfigReactiveWebServerApplicationContext();
			default:
				return new AnnotationConfigApplicationContext();
			}
		}
		catch (Exception ex) {
			throw new IllegalStateException("Unable create a default ApplicationContext instance, "
					+ "you may need a custom ApplicationContextFactory", ex);
		}
	};

到這裡，我們的上下文context就已經建立出來了，這塊程式碼也比較簡單。就不多說什麼了

2、查詢ServletWebServerFactory

再次回到new SpringApplication(primarySources).run(args)的呼叫來看run方法的程式碼

public ConfigurableApplicationContext run(String... args) {
		.......
		try {
			......
			//上面已經對context做過了講解，context=new AnnotationConfigServletWebServerApplicationContext()
			context = createApplicationContext();
		    ......
		    //下面來看這個方法
			refreshContext(context);
			......
		}
		catch (Throwable ex) {
			......
		}

		try {
		.......
		}
		catch (Throwable ex) {
		.......
		}
		return context;
	}

點到refreshContext(context)

	private void refreshContext(ConfigurableApplicationContext context) {
		if (this.registerShutdownHook) {
			shutdownHook.registerApplicationContext(context);
		}
		refresh(context);
	}

繼續點到refresh(context)

	protected void refresh(ConfigurableApplicationContext applicationContext) {
        //這裡的applicationContext就是AnnotationConfigServletWebServerApplicationContext的物件，由於這個類沒有refresh方法，會跳轉到它的父類ServletWebServerApplicationContext的方法中去,我們繼續點進去
		applicationContext.refresh();
	}

點到ServletWebServerApplicationContext的refresh方法

	public final void refresh() throws BeansException, IllegalStateException {
		try {
            //繼續跳轉到父類AbstractApplicationContext方法
			super.refresh();
		}
		catch (RuntimeException ex) {
			WebServer webServer = this.webServer;
			if (webServer != null) {
				webServer.stop();
			}
			throw ex;
		}
	}

開啟AbstractApplicationContext的refresh方法

//springboot 大部分的初始化工作是在這裡完成的，不過這不是我們本地的重點，不相關的我們統統略過
public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			......
			try {
			......
				//繼續點到這個方法，這裡又會跳轉到ServletWebServerApplicationContext這個類的方法
				onRefresh();
			.....
			}

			catch (BeansException ex) {
			.....
			}

			finally {
			.....
			}
		}
	}

開啟ServletWebServerApplicationContext的onRefresh方法

	protected void onRefresh() {
		super.onRefresh();
		try {
			//這裡就是我們本次的重點，會在這裡建立具體的web容器，我們點進去看看，還是ServletWebServerApplicationContext這個類的方法
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

開啟ServletWebServerApplicationContext的createWebServer方法

	private void createWebServer() {
	
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		//第一次進來webServer servletContext都是null,會進到if分支裡面
		if (webServer  == null && servletContext == null) {
		   //這裡只是做個標記，不用關注，跳過
			StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
			//這裡就會來查詢ServletWebServerFactory，也就是web容器的工廠,具體看下getWebServerFactory()方法，還是ServletWebServerApplicationContext這個類的方法
			ServletWebServerFactory factory = getWebServerFactory();
			createWebServer.tag("factory", factory.getClass().toString());
			this.webServer = factory.getWebServer(getSelfInitializer());
			createWebServer.end();
			getBeanFactory().registerSingleton("webServerGracefulShutdown",
					new WebServerGracefulShutdownLifecycle(this.webServer));
			getBeanFactory().registerSingleton("webServerStartStop",
					new WebServerStartStopLifecycle(this, this.webServer));
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}

開啟ServletWebServerApplicationContext的getWebServerFactory方法

	protected ServletWebServerFactory getWebServerFactory() {
		// Use bean names so that we don't consider the hierarchy
		//從beanFactory中查詢ServletWebServerFactory型別的bean的定義，返回對應bean的名字
		String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
		if (beanNames.length == 0) {
			throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing "
					+ "ServletWebServerFactory bean.");
		}
		if (beanNames.length > 1) {
			throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
					+ "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
		}
        //這裡會從beanFactory中返回bean的名字為beanNames[0],型別為ServletWebServerFactory.class的bean物件，如果當前bean還未建立，則此時就會建立bean物件並返回	
		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
	}

從上面的程式碼並看不出來實際的ServletWebServerFactory物件具體是什麼？下面帶著大家一起簡單過下這部分的載入，這裡具體就是springboot 載入bean的流程了，這部分的邏輯比較多，本次就不具體展開。關於springboot 載入bean的流程計劃後續專門再寫一篇。

springboot在啟動過程中會在當前類路徑下META-INF/spring.factories這個檔案中，key=org.springframework.boot.autoconfigure.EnableAutoConfiguration的屬性的作為bean的定義進行載入，在這過程中還會使用key=org.springframework.boot.autoconfigure.AutoConfigurationImportFilterfilter的屬性作為過濾，配合META-INF/spring-autoconfigure-metadata.properties對這些類做一個過濾，剔除掉不符合的類(後續還會根據類上的註解判斷是否要繼續剔除)。

當前這些主要在spring-boot-autoconfigure-2.5.1.jar這個檔案中

下面擷取上面說的兩部分,可以看到這裡的過濾器就3個，具體不展開討論了，自動匯入的類就是下面的再加過濾去掉的

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
......
#下面這個會在建立servelt中使用，下部分我們再關注它
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
#下面這個就是我們需要用到的
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
......
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

我們看下上面的org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration這個類，這個類在web 場景下，不會被剔除。會被載入。我們看看這個類，我們只看頭部就可以了

這裡我們看到類上有Import註解，會繼續匯入這幾個類，

ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
ServletWebServerFactoryConfiguration.EmbeddedUndertow.class

這三個都是本次相關的，它們都是ServletWebServerFactoryConfiguration的內部類，我們進去看看，類的結構都是一樣的，我們就看下ServletWebServerFactoryConfiguration.EmbeddedTomcat類吧

可以看到EmbeddedTomcat上面有ConditionalOnClass，ConditionalOnMissingBean這兩個註解，

簡單說下，ConditionalOnClass是表示當前類路徑下存在對應類是載入

ConditionalOnMissingBean是表示當前beanFactory中沒有對應型別bean定義的話載入

多個條件都是and的關係，有一個條件不成立，就不會去進行後續處理。

在這裡EmbeddedTomcat類這兩個條件是成立的，這時就會繼續遍歷當前類的所有方法，找到@Bean註解的方法，載入到beanFactory中去

而EmbeddedJetty，EmbeddedUndertow兩個類條件是不成立的，就不會進行後續執行，剔除掉了

這裡就會把EmbeddedTomcat.tomcatServletWebServerFactory這個方法進行載入，返回值是TomcatServletWebServerFactory型別，我們看下TomcatServletWebServerFactory類的繼承圖,可以看到它正好是繼承了ServletWebServerFactory介面。

再次開啟ServletWebServerApplicationContext的getWebServerFactory方法

	protected ServletWebServerFactory getWebServerFactory() {
		.......
        //所以這裡的邏輯實際上會執行ServletWebServerFactoryConfiguration.EmbeddedTomcat類的tomcatServletWebServerFactory方法，返回TomcatServletWebServerFactory物件,相關的屬性注入等等這裡就不講述了            
		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
	}

到這裡，整個ServletWebServerFactory的查詢就完成了

3、建立DispatcherServletRegistrationBean，DispatcherServlet

再看看上面的META-INF/spring.factories檔案

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
......
#現在我們重點來看這個類
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
......
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

我們開啟org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration這個類看看

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
//我們當前只關注這ConditionalOnWebApplication、ConditionalOnClass註解
//ConditionalOnWebApplication是根據type來判斷指定類是否存在
//當前的type是 Type.SERVLET，是來查詢org.springframework.web.context.support.GenericWebApplicationContext類是否存在，這個類存在於spring-web中，所以這個條件是true
@ConditionalOnWebApplication(type = Type.SERVLET)
//這個註解上面說過了 ，就是查詢指定的類是否存在，這個是查詢DispatcherServlet.class是否存在，這裡也會返回true
@ConditionalOnClass(DispatcherServlet.class)
//上面兩個條件都成立，就會執行後續的操作，去遍歷內部類和方法
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {

	/**
	 * The bean name for a DispatcherServlet that will be mapped to the root URL "/".
	 */
	public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

	/**
	 * The bean name for a ServletRegistrationBean for the DispatcherServlet "/".
	 */
	public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

	@Configuration(proxyBeanMethods = false)
    //這裡還是個條件，通過實現Condition介面，通過matches方法來判斷
    //DefaultDispatcherServletCondition這個類就在當前這個檔案裡，matches判斷的結果也是true
	@Conditional(DefaultDispatcherServletCondition.class)
    //ServletRegistration.class這個類存在於tomcat-embed-core裡面，這個結果也是true
	@ConditionalOnClass(ServletRegistration.class)
    //上面兩個條件成立，就會執行後續的操作，去遍歷內部類和方法
	@EnableConfigurationProperties(WebMvcProperties.class)
	protected static class DispatcherServletConfiguration {

        //beanFactory會建立這個DispatcherServletbean的定義，bean的名字就是dispatcherServlet
		@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
		public DispatcherServlet dispatcherServlet(WebMvcProperties webMvcProperties) {
			DispatcherServlet dispatcherServlet = new DispatcherServlet();
			dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
			dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
			dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
			dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
			dispatcherServlet.setEnableLoggingRequestDetails(webMvcProperties.isLogRequestDetails());
			return dispatcherServlet;
		}

		@Bean
		@ConditionalOnBean(MultipartResolver.class)
		@ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
		public MultipartResolver multipartResolver(MultipartResolver resolver) {
			// Detect if the user has created a MultipartResolver but named it incorrectly
			return resolver;
		}

	}

	@Configuration(proxyBeanMethods = false)
    //和上面的一樣，不說了
	@Conditional(DispatcherServletRegistrationCondition.class)
    //和上面的一樣，不說了
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties(WebMvcProperties.class)
    //這裡會要在查詢DispatcherServletConfiguration.class，並執行載入bean定義的流程，這就是上面的類了
	@Import(DispatcherServletConfiguration.class)
	protected static class DispatcherServletRegistrationConfiguration {

		@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        //ConditionalOnBean查詢是否存在指定bean的定義，這個方法要注入引數，需要這個類，當前這裡就是上面的dispatcherServlet方法定義的，這裡也是存在的
		@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        //DispatcherServlet dispatcherServlet這個就是dispatcherServlet這個方法定義的bean，在建立DispatcherServletRegistrationBean這個bean的時候，就會去查詢dispatcherServlet是否存在，如果不存在，先建立dispatcherServlet這個bean,再建立DispatcherServletRegistrationBean
		public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,
				WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {
			DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
					webMvcProperties.getServlet().getPath());
			registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
			registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());
			multipartConfig.ifAvailable(registration::setMultipartConfig);
			return registration;
		}

	}
  ......
}
      

上面就是建立DispatcherServlet，DispatcherServletRegistrationBean的過程了

4、建立tomcat，載入Servlet.class,filter.class，監聽器

再次回到ServletWebServerApplicationContext的createWebServer方法

	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
			//上面我們已經看到了這裡，factory是TomcatServletWebServerFactory類的一個例項物件
			ServletWebServerFactory factory = getWebServerFactory();
            //這裡還是做個標記，不用關注
            createWebServer.tag("factory", factory.getClass().toString());
            //這裡就是具體建立tomcat了，這裡的入參getSelfInitializer()是個lambda表示式，這個後續很重要
			this.webServer = factory.getWebServer(getSelfInitializer());
			createWebServer.end();
			getBeanFactory().registerSingleton("webServerGracefulShutdown",
					new WebServerGracefulShutdownLifecycle(this.webServer));
			getBeanFactory().registerSingleton("webServerStartStop",
					new WebServerStartStopLifecycle(this, this.webServer));
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}

	private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
		return this::selfInitialize;
	}
	//是建立webServer的引數
	private void selfInitialize(ServletContext servletContext) throws ServletException {
		prepareWebApplicationContext(servletContext);
		registerApplicationScope(servletContext);
		WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
		for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
			beans.onStartup(servletContext);
		}
	}

factory.getWebServer(getSelfInitializer())會呼叫到TomcatServletWebServerFactory的getWebServer的方法

	public WebServer getWebServer(ServletContextInitializer... initializers) {
        .......
		//上面的入參會在這裡傳下去
		prepareContext(tomcat.getHost(), initializers);
		return getTomcatWebServer(tomcat);
	}

點進prepareContext(tomcat.getHost(), initializers)去看看

	protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
		......
		ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
		host.addChild(context);
		//繼續傳下去
		configureContext(context, initializersToUse);
		postProcessContext(context);
	}

再點到configureContext(context, initializersToUse)這個呼叫去看看

	protected void configureContext(Context context, ServletContextInitializer[] initializers) {
		//會傳遞給TomcatStarter，作為構造引數，下面我們去這裡看看
		TomcatStarter starter = new TomcatStarter(initializers);
        ......
    }

下面我們去看看TomcatStarter這個類是怎麼使用這個initializers這個構造引數的。

這個類不長
class TomcatStarter implements ServletContainerInitializer {
    ......
	TomcatStarter(ServletContextInitializer[] initializers) {
        //入參會作為它的成員屬性
		this.initializers = initializers;
	}

	@Override
	public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
		try {
			for (ServletContextInitializer initializer : this.initializers) {
                //會在這裡呼叫onStartup方法，這裡的入參就是ApplicationContextFacade的物件，裡面包裝了ApplicationContext，裡面再包裝了TomcatEmbeddedContext，這要就和tomcat聯絡起來了，下面的截圖就是servletContext的物件結構
				initializer.onStartup(servletContext);
			}
		}
		catch (Exception ex) {
			......
	}

上面的initializer.onStartup(servletContext)會呼叫到ServletWebServerApplicationContext的selfInitialize方法

	private void selfInitialize(ServletContext servletContext) throws ServletException {
        //這裡是將ApplicationContextFacade設定到當前的servletContext上
		prepareWebApplicationContext(servletContext);
        //這裡是在beanFactory中註冊application的scope
		registerApplicationScope(servletContext);
        //這裡還是註冊上下文相關的bean
		WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
        //我們重點來看這裡getServletContextInitializerBeans()是定義個一個ServletContextInitializerBeans物件，我們點進去看看
		for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
			beans.onStartup(servletContext);
		}
	}

	protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
		//這裡的getBeanFactory()就是全域性的beanFactory
		return new ServletContextInitializerBeans(getBeanFactory());
	}

	public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		this.initializers = new LinkedMultiValueMap<>();
        //由於我們沒有傳initializerTypes這個引數，所以this.initializerTypes裡面就只有ServletContextInitializer.class這個類
		this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
				: Collections.singletonList(ServletContextInitializer.class);
        //這裡主要是從beanFactory中查詢this.initializerTypes型別，我們進去看看，就是下面這個方法了
		addServletContextInitializerBeans(beanFactory);
		addAdaptableBeans(beanFactory);
		List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
				.flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
				.collect(Collectors.toList());
		this.sortedList = Collections.unmodifiableList(sortedInitializers);
		logMappings(this.initializers);
	}

	private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
		for (Class<? extends ServletContextInitializer> initializerType : this.initializerTypes) {
            //預設情況下這裡找到就只有上面第3部分的DispatcherServletRegistrationBean對應的bean
			for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,
					initializerType)) {
                //這裡的key是bean的名字,value就是bean物件，在進去看看，就是下面這個方法
				addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory);
			}
		}
	}
	private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer,
			ListableBeanFactory beanFactory) {
        //會走到這個分支
		if (initializer instanceof ServletRegistrationBean) {
            //這裡返回的servlet也還是第3部分DispatcherServlet對應的bean
			Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
            //再點進去
			addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source);
		}
		else if (initializer instanceof FilterRegistrationBean) {
			Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();
			addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
		}
		else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {
			String source = ((DelegatingFilterProxyRegistrationBean) initializer).getTargetBeanName();
			addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
		}
		else if (initializer instanceof ServletListenerRegistrationBean) {
			EventListener source = ((ServletListenerRegistrationBean<?>) initializer).getListener();
			addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source);
		}
		else {
			addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory,
					initializer);
		}
	}

	private void addServletContextInitializerBean(Class<?> type, String beanName, ServletContextInitializer initializer,
			ListableBeanFactory beanFactory, Object source) {
        //這裡的initializers是個map,按照型別，bean物件進行載入，這裡的type是javax.servlet.Servlet.class,value是上面的DispatcherServletRegistrationBean
		this.initializers.add(type, initializer);
		if (source != null) {
			// Mark the underlying source as seen in case it wraps an existing bean
            //將DispatcherServlet對應的bean加到這裡
			this.seen.add(source);
		}
		if (logger.isTraceEnabled()) {
			String resourceDescription = getResourceDescription(beanName, beanFactory);
			int order = getOrder(initializer);
			logger.trace("Added existing " + type.getSimpleName() + " initializer bean '" + beanName + "'; order="
					+ order + ", resource=" + resourceDescription);
		}
	}

再回到ServletContextInitializerBeans的構造方法,接著看後面的

	public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		...... //這裡的內容上面已經看過了，我們現在看下面這句，點進去
		addAdaptableBeans(beanFactory);
		List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
				.flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
				.collect(Collectors.toList());
		this.sortedList = Collections.unmodifiableList(sortedInitializers);
		logMappings(this.initializers);
	}

	protected void addAdaptableBeans(ListableBeanFactory beanFactory) {
		//這句不用關注
		MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);
		//這句不用關注
		addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig));
		//點到這裡去看看
		addAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());
        
		for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
			addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,
					new ServletListenerRegistrationBeanAdapter());
		}
	}

	
	private <T, B extends T> void addAsRegistrationBean(ListableBeanFactory beanFactory, Class<T> type,
			Class<B> beanType, RegistrationBeanAdapter<T> adapter) {
		//這裡的beanType是 Filter.class,下面這句就是從beanFactory中獲取所有型別為Filter.class的bean
		List<Map.Entry<String, B>> entries = getOrderedBeansOfType(beanFactory, beanType, this.seen);
		for (Entry<String, B> entry : entries) {
			String beanName = entry.getKey();
			B bean = entry.getValue();
            //將bean放置到this.seen裡面
			if (this.seen.add(bean)) {
				// One that we haven't already seen
                //包裝成RegistrationBean物件
				RegistrationBean registration = adapter.createRegistrationBean(beanName, bean, entries.size());
				int order = getOrder(bean);
				registration.setOrder(order);
                //同樣放置到this.initializers裡面
				this.initializers.add(type, registration);
				if (logger.isTraceEnabled()) {
					logger.trace("Created " + type.getSimpleName() + " initializer for bean '" + beanName + "'; order="
							+ order + ", resource=" + getResourceDescription(beanName, beanFactory));
				}
			}
		}
	}

再回到上面的addAdaptableBeans方法，看後面的

	protected void addAdaptableBeans(ListableBeanFactory beanFactory) {
		......//這裡剛才說過了
         //下面這部分不說了，這裡基本和上面一樣，不過處理的型別變成了ServletContextAttributeListener.class、ServletRequestListener.class、ServletRequestAttributeListener.class、HttpSessionAttributeListener.class、HttpSessionIdListener.class、HttpSessionListener.class、ServletContextListener.class這些型別
		for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
			addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,
					new ServletListenerRegistrationBeanAdapter());
		}
	}

再回到ServletContextInitializerBeans的構造方法,接著看後面的

	public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		......//這裡剛才都說過了，看下面
        //這裡就是把上面所有獲取到的相關的bean放置到this.sortedList中，下面我是我本地this.sortedList的截圖
		List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
				.flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
				.collect(Collectors.toList());
		this.sortedList = Collections.unmodifiableList(sortedInitializers);
		logMappings(this.initializers);
	}

這裡ServletContextInitializerBeans的構造方法就完成了，再回過頭去看看這個類的定義

public class ServletContextInitializerBeans extends AbstractCollection<ServletContextInitializer>

這個類繼承了AbstractCollection類，那它就需要實現下面這個抽象方法

public abstract Iterator<E> iterator();

我們看看ServletContextInitializerBeans的iterator的方法

@Override
public Iterator<ServletContextInitializer> iterator() {
   return this.sortedList.iterator();
}

看到了吧，這就是返回上面的this.sortedList.iterator()

我們再次回到ServletWebServerApplicationContext的selfInitialize方法

private void selfInitialize(ServletContext servletContext) throws ServletException {
	......//這裡上面都說過了
	//getServletContextInitializerBeans()這個方法就是構造了ServletContextInitializerBeans
	//這裡的for迴圈也是呼叫了ServletContextInitializerBeans的iterator的方法，實際上遍歷的也就是上面的this.sortedList
	for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
		//這裡就是把在beanFactory中查詢到的Servlet.class,filter.class，監聽器等等新增到tomcat容器中,我們就只進到servlet裡面去看看
		//進到DispatcherServletRegistrationBean裡面去看看
		beans.onStartup(servletContext);
	}
}

	//這個方法在DispatcherServletRegistrationBean的父類RegistrationBean中
	//所有的Servlet.class,filter.class，監聽器都會走到這裡
	public final void onStartup(ServletContext servletContext) throws ServletException {
		//這裡是返回的表述，不關注
		String description = getDescription();
		if (!isEnabled()) {
			logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
			return;
		}
		//這裡由不同的子類去實現，DispatcherServletRegistrationBean會呼叫到ServletRegistrationBean中
		register(description, servletContext);
	}

	//這個方法是在ServletRegistrationBean這個類中
	@Override
	protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
		String name = getServletName();
		//這裡的servletContext上面的ApplicationContextFacade的物件，這裡就會將DispatcherServlet的bean物件載入到TomcatEmbeddedContext中，後續所有的http請求最後都會流轉到DispatcherServlet去進行具體的分發
		return servletContext.addServlet(name, this.servlet);
	}

到這裡Servlet.class,filter.class，監聽器就全部載入到tomcat中去

5、建立RequestMappingHandlerMapping

再看看上面的META-INF/spring.factories檔案

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
......
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
......
#現在我們重點來看這個類
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

具體的載入和上面部分都類似，就不展開了，直接看我們需要的

		//這裡會建立RequestMappingHandlerMapping的bean
		@Bean
		@Primary
		@Override
		public RequestMappingHandlerMapping requestMappingHandlerMapping(
				@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
				@Qualifier("mvcConversionService") FormattingConversionService conversionService,
				@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {
			// Must be @Primary for MvcUriComponentsBuilder to work
			return super.requestMappingHandlerMapping(contentNegotiationManager, conversionService,
					resourceUrlProvider);

看下這個類的繼承圖

RequestMappingHandlerMapping實現了InitializingBean介面，會在bean物件建立後的invokeInitMethods方法中呼叫afterPropertiesSet方法,最終會呼叫的AbstractHandlerMethodMapping的afterPropertiesSet方法中

	@Override
	public void afterPropertiesSet() {
		initHandlerMethods();
	}

	/**
	 * Scan beans in the ApplicationContext, detect and register handler methods.
	 * @see #getCandidateBeanNames()
	 * @see #processCandidateBean
	 * @see #handlerMethodsInitialized
	 */
	protected void initHandlerMethods() {
        //這裡查詢beanFactory中的所有bean進行遍歷
		for (String beanName : getCandidateBeanNames()) {
			if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                //點到這裡去看看
				processCandidateBean(beanName);
			}
		}
		handlerMethodsInitialized(getHandlerMethods());
	}

	protected void processCandidateBean(String beanName) {
		Class<?> beanType = null;
		try {
			//根據beanName獲取對應bean的Class物件
			beanType = obtainApplicationContext().getType(beanName);
		}
		catch (Throwable ex) {
			// An unresolvable bean type, probably from a lazy bean - let's ignore it.
			if (logger.isTraceEnabled()) {
				logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
			}
		}
		//判斷類上是否有Controller.class，RequestMapping.class註解
		if (beanType != null && isHandler(beanType)) {
			//這裡就會解析beanName上的所有方法進行遍歷，查詢有 RequestMapping.class註解的方法，建立RequestMappingInfo物件，放置到registry屬性中(在AbstractHandlerMethodMapping)中，這樣我們所有定義的controller中的http請求就會全部被掃描到
			detectHandlerMethods(beanName);
		}
	}

6、載入RequestMappingHandlerMapping到DispatcherServlet中

在我們第一次請求的時候，會執行到DispatcherServlet的initStrategies方法，這個方法只會執行一次

	protected void initStrategies(ApplicationContext context) {
		......
		這就會載入之前查詢到的RequestMappingHandlerMapping中的我們定義的controller
		initHandlerMappings(context);
		......
	}

會呼叫到這裡

	private void initHandlerMappings(ApplicationContext context) {
		this.handlerMappings = null;

		if (this.detectAllHandlerMappings) {
			// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
			//這裡會在beanFactory中查詢所有HandlerMapping.class的bean,其中就包含我們第5部分的RequestMappingHandlerMapping
			Map<String, HandlerMapping> matchingBeans =
					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
			if (!matchingBeans.isEmpty()) {
			//將所有查詢到的放置到handlerMappings中
				this.handlerMappings = new ArrayList<>(matchingBeans.values());
				// We keep HandlerMappings in sorted order.
				AnnotationAwareOrderComparator.sort(this.handlerMappings);
			}
		}
		......
	}

當我們瀏覽器請求的時候，最終會走到DispatcherServlet的doDispatch的方法，處理我們的請求並返回,簡單看看

	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		......

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
                //在這裡，就會請求request的請求路徑去查詢實際要執行的controller的方法，點進去看看
				mappedHandler = getHandler(processedRequest);
				......
	}

	@Nullable
	protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		if (this.handlerMappings != null) {
            //這其實就是根據遍歷，查詢對應的路徑，這個this.handlerMappings就是就是在initHandlerMappings方法中賦值的
			for (HandlerMapping mapping : this.handlerMappings) {
				HandlerExecutionChain handler = mapping.getHandler(request);
				if (handler != null) {
					return handler;
				}
			}
		}
		return null;
	}

到這裡，springboot 載入web容器的整個流程基本就算完成了，這塊涉及的東西比較多，所以說的可能比較粗，大家見諒。