spring boot 加载web容器tomcat流程源码分析

我本地的springboot版本是2.5.1，后面的分析都是基于这个版本

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

我们通过在pom文件中引入

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

来引入web容器，默认的web容器时tomcat。

本文章主要描述spring boot加载web容器 tomcat的部分，为了避免文章知识点过于分散，其他相关的如bean的加载，tomcat内部流程等不做深入讨论。

1、在springboot web工程中，全局上下文是AnnotationConfigServletWebApplicationContext

下面的部分，我们具体来看下

首先，我们的入口代码一般都是这样写的

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

跳转到run方法里面，依次会调用下面两个方法

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);
}

首先会创建SpringApplication实例对象，跳转到SpringApplication的构造方法去看看，依次会调用如下方法

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

@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
   ......
   //本次不相关的代码全部省略掉，只保留相关代码
   //这里的 this.webApplicationType=WebApplicationType.SERVLET， 我们来分析下这个代码的具体的执行赋值
   this.webApplicationType = WebApplicationType.deduceFromClasspath();
   ......
}

继续跳转到WebApplicationType.deduceFromClasspath()去看看

//这个方法主要是在当前类路径下查找指定的class类是否存在，返回对饮枚举类型
	static WebApplicationType deduceFromClasspath() {
	// WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";

	//我们通过pom文件引入spring-boot-starter-web，会简介引入spring-webmvc,上面这个类就在这个webmvc中，所以不会进入这个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这个类存在于tomcat-embed-core中
    //org.springframework.web.context.ConfigurableWebApplicationContext这个类存在于spring-web中
    //这两个jar都是由spring-boot-starter-web间接引入的，所以也不会走这个分支
		for (String className : SERVLET_INDICATOR_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return WebApplicationType.NONE;
			}
		}
		//所以会从这里返回
		return WebApplicationType.SERVLET;
	}

下面看下jar包的引入

回到new SpringApplication(primarySources).run(args)的调用来看run方法的代码

public ConfigurableApplicationContext run(String... args) {
		.......
		try {
			......
			//我们来看这个context的创建，context=new AnnotationConfigServletWebServerApplicationContext()下面来具体看这块的执行
			context = createApplicationContext();
		    ......
		    //后续几个部分会来说明这个方法
			refreshContext(context);
			......
		}
		catch (Throwable ex) {
			......
		}

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

createApplicationContext()依次会调用如下方法

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

//最终会调用到这个lambda表达式，入参就是上面的WebApplicationType.SERVLET
	ApplicationContextFactory DEFAULT = (webApplicationType) -> {
		try {
			switch (webApplicationType) {
			case SERVLET:
				//会从这里返回
				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);
		}
	};

到这里，我们的上下文context就已经创建出来了，这块代码也比较简单。就不多说什么了

2、查找ServletWebServerFactory

再次回到new SpringApplication(primarySources).run(args)的调用来看run方法的代码

public ConfigurableApplicationContext run(String... args) {
		.......
		try {
			......
			//上面已经对context做过了讲解，context=new AnnotationConfigServletWebServerApplicationContext()
			context = createApplicationContext();
		    ......
		    //下面来看这个方法
			refreshContext(context);
			......
		}
		catch (Throwable ex) {
			......
		}

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

点到refreshContext(context)

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

继续点到refresh(context)

protected void refresh(ConfigurableApplicationContext applicationContext) {
        //这里的applicationContext就是AnnotationConfigServletWebServerApplicationContext的对象，由于这个类没有refresh方法，会跳转到它的父类ServletWebServerApplicationContext的方法中去,我们继续点进去
		applicationContext.refresh();
	}

点到ServletWebServerApplicationContext的refresh方法

public final void refresh() throws BeansException, IllegalStateException {
		try {
            //继续跳转到父类AbstractApplicationContext方法
			super.refresh();
		}
		catch (RuntimeException ex) {
			WebServer webServer = this.webServer;
			if (webServer != null) {
				webServer.stop();
			}
			throw ex;
		}
	}

打开AbstractApplicationContext的refresh方法

//springboot 大部分的初始化工作是在这里完成的，不过这不是我们本地的重点，不相关的我们统统略过
public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			......
			try {
			......
				//继续点到这个方法，这里又会跳转到ServletWebServerApplicationContext这个类的方法
				onRefresh();
			.....
			}

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

			finally {
			.....
			}
		}
	}

打开ServletWebServerApplicationContext的onRefresh方法

protected void onRefresh() {
		super.onRefresh();
		try {
			//这里就是我们本次的重点，会在这里创建具体的web容器，我们点进去看看，还是ServletWebServerApplicationContext这个类的方法
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

打开ServletWebServerApplicationContext的createWebServer方法

private void createWebServer() {

		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		//第一次进来webServer servletContext都是null,会进到if分支里面
		if (webServer  == null && servletContext == null) {
		   //这里只是做个标记，不用关注，跳过
			StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
			//这里就会来查找ServletWebServerFactory，也就是web容器的工厂,具体看下getWebServerFactory()方法，还是ServletWebServerApplicationContext这个类的方法
			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();
	}

打开ServletWebServerApplicationContext的getWebServerFactory方法

protected ServletWebServerFactory getWebServerFactory() {
		// Use bean names so that we don't consider the hierarchy
		//从beanFactory中查找ServletWebServerFactory类型的bean的定义，返回对应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));
		}
        //这里会从beanFactory中返回bean的名字为beanNames[0],类型为ServletWebServerFactory.class的bean对象，如果当前bean还未创建，则此时就会创建bean对象并返回
		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
	}

从上面的代码并看不出来实际的ServletWebServerFactory对象具体是什么？下面带着大家一起简单过下这部分的加载，这里具体就是springboot 加载bean的流程了，这部分的逻辑比较多，本次就不具体展开。关于springboot 加载bean的流程计划后续专门再写一篇。

springboot在启动过程中会在当前类路径下META-INF/spring.factories这个文件中，key=org.springframework.boot.autoconfigure.EnableAutoConfiguration的属性的作为bean的定义进行加载，在这过程中还会使用key=org.springframework.boot.autoconfigure.AutoConfigurationImportFilterfilter的属性作为过滤，配合META-INF/spring-autoconfigure-metadata.properties对这些类做一个过滤，剔除掉不符合的类(后续还会根据类上的注解判断是否要继续剔除)。

当前这些主要在spring-boot-autoconfigure-2.5.1.jar这个文件中

下面截取上面说的两部分,可以看到这里的过滤器就3个，具体不展开讨论了，自动导入的类就是下面的再加过滤去掉的

# 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=\
......
#下面这个会在创建servelt中使用，下部分我们再关注它
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
#下面这个就是我们需要用到的
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
......
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

我们看下上面的org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration这个类，这个类在web 场景下，不会被剔除。会被加载。我们看看这个类，我们只看头部就可以了

这里我们看到类上有Import注解，会继续导入这几个类，

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

这三个都是本次相关的，它们都是ServletWebServerFactoryConfiguration的内部类，我们进去看看，类的结构都是一样的，我们就看下ServletWebServerFactoryConfiguration.EmbeddedTomcat类吧

可以看到EmbeddedTomcat上面有ConditionalOnClass，ConditionalOnMissingBean这两个注解，

简单说下，ConditionalOnClass是表示当前类路径下存在对应类是加载

ConditionalOnMissingBean是表示当前beanFactory中没有对应类型bean定义的话加载

多个条件都是and的关系，有一个条件不成立，就不会去进行后续处理。

在这里EmbeddedTomcat类这两个条件是成立的，这时就会继续遍历当前类的所有方法，找到@Bean注解的方法，加载到beanFactory中去

而EmbeddedJetty，EmbeddedUndertow两个类条件是不成立的，就不会进行后续执行，剔除掉了

这里就会把EmbeddedTomcat.tomcatServletWebServerFactory这个方法进行加载，返回值是TomcatServletWebServerFactory类型，我们看下TomcatServletWebServerFactory类的继承图,可以看到它正好是继承了ServletWebServerFactory接口。

再次打开ServletWebServerApplicationContext的getWebServerFactory方法

protected ServletWebServerFactory getWebServerFactory() {
		.......
        //所以这里的逻辑实际上会执行ServletWebServerFactoryConfiguration.EmbeddedTomcat类的tomcatServletWebServerFactory方法，返回TomcatServletWebServerFactory对象,相关的属性注入等等这里就不讲述了
		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
	}

到这里，整个ServletWebServerFactory的查找就完成了

3、创建DispatcherServletRegistrationBean，DispatcherServlet

再看看上面的META-INF/spring.factories文件

# 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=\
......
#现在我们重点来看这个类
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
......
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

我们打开org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration这个类看看

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
//我们当前只关注这ConditionalOnWebApplication、ConditionalOnClass注解
//ConditionalOnWebApplication是根据type来判断指定类是否存在
//当前的type是 Type.SERVLET，是来查找org.springframework.web.context.support.GenericWebApplicationContext类是否存在，这个类存在于spring-web中，所以这个条件是true
@ConditionalOnWebApplication(type = Type.SERVLET)
//这个注解上面说过了 ，就是查找指定的类是否存在，这个是查找DispatcherServlet.class是否存在，这里也会返回true
@ConditionalOnClass(DispatcherServlet.class)
//上面两个条件都成立，就会执行后续的操作，去遍历内部类和方法
@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)
    //这里还是个条件，通过实现Condition接口，通过matches方法来判断
    //DefaultDispatcherServletCondition这个类就在当前这个文件里，matches判断的结果也是true
	@Conditional(DefaultDispatcherServletCondition.class)
    //ServletRegistration.class这个类存在于tomcat-embed-core里面，这个结果也是true
	@ConditionalOnClass(ServletRegistration.class)
    //上面两个条件成立，就会执行后续的操作，去遍历内部类和方法
	@EnableConfigurationProperties(WebMvcProperties.class)
	protected static class DispatcherServletConfiguration {

        //beanFactory会创建这个DispatcherServletbean的定义，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)
    //和上面的一样，不说了
	@Conditional(DispatcherServletRegistrationCondition.class)
    //和上面的一样，不说了
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties(WebMvcProperties.class)
    //这里会要在查找DispatcherServletConfiguration.class，并执行加载bean定义的流程，这就是上面的类了
	@Import(DispatcherServletConfiguration.class)
	protected static class DispatcherServletRegistrationConfiguration {

		@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        //ConditionalOnBean查找是否存在指定bean的定义，这个方法要注入参数，需要这个类，当前这里就是上面的dispatcherServlet方法定义的，这里也是存在的
		@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        //DispatcherServlet dispatcherServlet这个就是dispatcherServlet这个方法定义的bean，在创建DispatcherServletRegistrationBean这个bean的时候，就会去查找dispatcherServlet是否存在，如果不存在，先创建dispatcherServlet这个bean,再创建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;
		}

	}
  ......
}

上面就是创建DispatcherServlet，DispatcherServletRegistrationBean的过程了

4、创建tomcat，加载Servlet.class,filter.class，监听器

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");
			//上面我们已经看到了这里，factory是TomcatServletWebServerFactory类的一个实例对象
			ServletWebServerFactory factory = getWebServerFactory();
            //这里还是做个标记，不用关注
            createWebServer.tag("factory", factory.getClass().toString());
            //这里就是具体创建tomcat了，这里的入参getSelfInitializer()是个lambda表达式，这个后续很重要
			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;
	}
	//是创建webServer的参数
	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())会调用到TomcatServletWebServerFactory的getWebServer的方法

public WebServer getWebServer(ServletContextInitializer... initializers) {
        .......
		//上面的入参会在这里传下去
		prepareContext(tomcat.getHost(), initializers);
		return getTomcatWebServer(tomcat);
	}

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

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

再点到configureContext(context, initializersToUse)这个调用去看看

protected void configureContext(Context context, ServletContextInitializer[] initializers) {
		//会传递给TomcatStarter，作为构造参数，下面我们去这里看看
		TomcatStarter starter = new TomcatStarter(initializers);
        ......
    }

下面我们去看看TomcatStarter这个类是怎么使用这个initializers这个构造参数的。

这个类不长
class TomcatStarter implements ServletContainerInitializer {
    ......
	TomcatStarter(ServletContextInitializer[] initializers) {
        //入参会作为它的成员属性
		this.initializers = initializers;
	}

	@Override
	public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
		try {
			for (ServletContextInitializer initializer : this.initializers) {
                //会在这里调用onStartup方法，这里的入参就是ApplicationContextFacade的对象，里面包装了ApplicationContext，里面再包装了TomcatEmbeddedContext，这要就和tomcat联系起来了，下面的截图就是servletContext的对象结构
				initializer.onStartup(servletContext);
			}
		}
		catch (Exception ex) {
			......
	}

上面的initializer.onStartup(servletContext)会调用到ServletWebServerApplicationContext的selfInitialize方法

private void selfInitialize(ServletContext servletContext) throws ServletException {
        //这里是将ApplicationContextFacade设置到当前的servletContext上
		prepareWebApplicationContext(servletContext);
        //这里是在beanFactory中注册application的scope
		registerApplicationScope(servletContext);
        //这里还是注册上下文相关的bean
		WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
        //我们重点来看这里getServletContextInitializerBeans()是定义个一个ServletContextInitializerBeans对象，我们点进去看看
		for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
			beans.onStartup(servletContext);
		}
	}

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
		//这里的getBeanFactory()就是全局的beanFactory
		return new ServletContextInitializerBeans(getBeanFactory());
	}

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		this.initializers = new LinkedMultiValueMap<>();
        //由于我们没有传initializerTypes这个参数，所以this.initializerTypes里面就只有ServletContextInitializer.class这个类
		this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
				: Collections.singletonList(ServletContextInitializer.class);
        //这里主要是从beanFactory中查找this.initializerTypes类型，我们进去看看，就是下面这个方法了
		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) {
            //默认情况下这里找到就只有上面第3部分的DispatcherServletRegistrationBean对应的bean
			for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,
					initializerType)) {
                //这里的key是bean的名字,value就是bean对象，在进去看看，就是下面这个方法
				addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory);
			}
		}
	}
	private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer,
			ListableBeanFactory beanFactory) {
        //会走到这个分支
		if (initializer instanceof ServletRegistrationBean) {
            //这里返回的servlet也还是第3部分DispatcherServlet对应的bean
			Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
            //再点进去
			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) {
        //这里的initializers是个map,按照类型，bean对象进行加载，这里的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
            //将DispatcherServlet对应的bean加到这里
			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的构造方法,接着看后面的

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		...... //这里的内容上面已经看过了，我们现在看下面这句，点进去
		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) {
		//这句不用关注
		MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);
		//这句不用关注
		addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig));
		//点到这里去看看
		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) {
		//这里的beanType是 Filter.class,下面这句就是从beanFactory中获取所有类型为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();
            //将bean放置到this.seen里面
			if (this.seen.add(bean)) {
				// One that we haven't already seen
                //包装成RegistrationBean对象
				RegistrationBean registration = adapter.createRegistrationBean(beanName, bean, entries.size());
				int order = getOrder(bean);
				registration.setOrder(order);
                //同样放置到this.initializers里面
				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，看后面的

protected void addAdaptableBeans(ListableBeanFactory beanFactory) {
		......//这里刚才说过了
         //下面这部分不说了，这里基本和上面一样，不过处理的类型变成了ServletContextAttributeListener.class、ServletRequestListener.class、ServletRequestAttributeListener.class、HttpSessionAttributeListener.class、HttpSessionIdListener.class、HttpSessionListener.class、ServletContextListener.class这些类型
		for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
			addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,
					new ServletListenerRegistrationBeanAdapter());
		}
	}

再回到ServletContextInitializerBeans的构造方法,接着看后面的

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
			Class<? extends ServletContextInitializer>... initializerTypes) {
		......//这里刚才都说过了，看下面
        //这里就是把上面所有获取到的相关的bean放置到this.sortedList中，下面我是我本地this.sortedList的截图
		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);
	}

这里ServletContextInitializerBeans的构造方法就完成了，再回过头去看看这个类的定义

public class ServletContextInitializerBeans extends AbstractCollection<ServletContextInitializer>

这个类继承了AbstractCollection类，那它就需要实现下面这个抽象方法

public abstract Iterator<E> iterator();

我们看看ServletContextInitializerBeans的iterator的方法

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

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

我们再次回到ServletWebServerApplicationContext的selfInitialize方法

private void selfInitialize(ServletContext servletContext) throws ServletException {
	......//这里上面都说过了
	//getServletContextInitializerBeans()这个方法就是构造了ServletContextInitializerBeans
	//这里的for循环也是调用了ServletContextInitializerBeans的iterator的方法，实际上遍历的也就是上面的this.sortedList
	for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
		//这里就是把在beanFactory中查找到的Servlet.class,filter.class，监听器等等添加到tomcat容器中,我们就只进到servlet里面去看看
		//进到DispatcherServletRegistrationBean里面去看看
		beans.onStartup(servletContext);
	}
}

//这个方法在DispatcherServletRegistrationBean的父类RegistrationBean中
	//所有的Servlet.class,filter.class，监听器都会走到这里
	public final void onStartup(ServletContext servletContext) throws ServletException {
		//这里是返回的表述，不关注
		String description = getDescription();
		if (!isEnabled()) {
			logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
			return;
		}
		//这里由不同的子类去实现，DispatcherServletRegistrationBean会调用到ServletRegistrationBean中
		register(description, servletContext);
	}

//这个方法是在ServletRegistrationBean这个类中
	@Override
	protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
		String name = getServletName();
		//这里的servletContext上面的ApplicationContextFacade的对象，这里就会将DispatcherServlet的bean对象加载到TomcatEmbeddedContext中，后续所有的http请求最后都会流转到DispatcherServlet去进行具体的分发
		return servletContext.addServlet(name, this.servlet);
	}

到这里Servlet.class,filter.class，监听器就全部加载到tomcat中去

5、创建RequestMappingHandlerMapping

再看看上面的META-INF/spring.factories文件

# 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,\
......
#现在我们重点来看这个类
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
......

具体的加载和上面部分都类似，就不展开了，直接看我们需要的

//这里会创建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);

看下这个类的继承图

RequestMappingHandlerMapping实现了InitializingBean接口，会在bean对象创建后的invokeInitMethods方法中调用afterPropertiesSet方法,最终会调用的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() {
        //这里查找beanFactory中的所有bean进行遍历
		for (String beanName : getCandidateBeanNames()) {
			if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                //点到这里去看看
				processCandidateBean(beanName);
			}
		}
		handlerMethodsInitialized(getHandlerMethods());
	}

protected void processCandidateBean(String beanName) {
		Class<?> beanType = null;
		try {
			//根据beanName获取对应bean的Class对象
			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);
			}
		}
		//判断类上是否有Controller.class，RequestMapping.class注解
		if (beanType != null && isHandler(beanType)) {
			//这里就会解析beanName上的所有方法进行遍历，查找有 RequestMapping.class注解的方法，创建RequestMappingInfo对象，放置到registry属性中(在AbstractHandlerMethodMapping)中，这样我们所有定义的controller中的http请求就会全部被扫描到
			detectHandlerMethods(beanName);
		}
	}

6、加载RequestMappingHandlerMapping到DispatcherServlet中

在我们第一次请求的时候，会执行到DispatcherServlet的initStrategies方法，这个方法只会执行一次

protected void initStrategies(ApplicationContext context) {
		......
		这就会加载之前查找到的RequestMappingHandlerMapping中的我们定义的controller
		initHandlerMappings(context);
		......
	}

会调用到这里

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

		if (this.detectAllHandlerMappings) {
			// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
			//这里会在beanFactory中查找所有HandlerMapping.class的bean,其中就包含我们第5部分的RequestMappingHandlerMapping
			Map<String, HandlerMapping> matchingBeans =
					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
			if (!matchingBeans.isEmpty()) {
			//将所有查找到的放置到handlerMappings中
				this.handlerMappings = new ArrayList<>(matchingBeans.values());
				// We keep HandlerMappings in sorted order.
				AnnotationAwareOrderComparator.sort(this.handlerMappings);
			}
		}
		......
	}

当我们浏览器请求的时候，最终会走到DispatcherServlet的doDispatch的方法，处理我们的请求并返回,简单看看

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.
                //在这里，就会请求request的请求路径去查找实际要执行的controller的方法，点进去看看
				mappedHandler = getHandler(processedRequest);
				......
	}

@Nullable
	protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		if (this.handlerMappings != null) {
            //这其实就是根据遍历，查找对应的路径，这个this.handlerMappings就是就是在initHandlerMappings方法中赋值的
			for (HandlerMapping mapping : this.handlerMappings) {
				HandlerExecutionChain handler = mapping.getHandler(request);
				if (handler != null) {
					return handler;
				}
			}
		}
		return null;
	}

到这里，springboot 加载web容器的整个流程基本就算完成了，这块涉及的东西比较多，所以说的可能比较粗，大家见谅。