我本地的springboot版本是2.5.1，后面的分析都是基于這個版本

<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文件中引入

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

來引入web容器，默認的web容器時tomcat。

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

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

下面的部分，我們具體來看下

首先，我們的入口代碼一般都是這樣寫的

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

跳轉到run方法里面，依次會調用下面兩個方法

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);}

首先會創建SpringApplication實例對象，跳轉到SpringApplication的構造方法去看看，依次會調用如下方法

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

@SuppressWarnings({ 'unchecked', 'rawtypes' })public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { ...... //本次不相關的代碼全部省略掉，只保留相關代碼 //這里的 this.webApplicationType=WebApplicationType.SERVLET， 我們來分析下這個代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找指定的class類是否存在，返回對飲枚舉類型static WebApplicationType deduceFromClasspath() {// WEBMVC_INDICATOR_CLASS = 'org.springframework.web.servlet.DispatcherServlet';//我們通過pom文件引入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這個類存在于tomcat-embed-core中 //org.springframework.web.context.ConfigurableWebApplicationContext這個類存在于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;}}//所以會從這里返回return WebApplicationType.SERVLET;}

下面看下jar包的引入

回到new SpringApplication(primarySources).run(args)的調用來看run方法的代碼

public ConfigurableApplicationContext run(String... args) {.......try {......//我們來看這個context的創建，context=new AnnotationConfigServletWebServerApplicationContext()下面來具體看這塊的執行context = createApplicationContext(); ...... //后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()依次會調用如下方法

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

//最終會調用到這個lambda表達式，入參就是上面的WebApplicationType.SERVLETApplicationContextFactory DEFAULT = (webApplicationType) -> {try {switch (webApplicationType) {case SERVLET://會從這里返回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);}};

到這里，我們的上下文context就已經創建出來了，這塊代碼也比較簡單。就不多說什么了

2、查找ServletWebServerFactory

再次回到new SpringApplication(primarySources).run(args)的調用來看run方法的代碼

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) {//這里的applicationContext就是AnnotationConfigServletWebServerApplicationContext的對象，由于這個類沒有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;}}

打開AbstractApplicationContext的refresh方法

//springboot 大部分的初始化工作是在這里完成的，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，這里又會跳轉到ServletWebServerApplicationContext這個類的方法onRefresh();.....}catch (BeansException ex) {.....}finally {.....}}}

打開ServletWebServerApplicationContext的onRefresh方法

protected void onRefresh() {super.onRefresh();try {//這里就是我們本次的重點，會在這里創建具體的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);}}

打開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分支里面if (webServer == null && servletContext == null) { //這里只是做個標記，不用關注，跳過StartupStep createWebServer = this.getApplicationStartup().start('spring.boot.webserver.create');//這里就會來查找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();}

打開ServletWebServerApplicationContext的getWebServerFactory方法

protected ServletWebServerFactory getWebServerFactory() {// Use bean names so that we don’t consider the hierarchy//從beanFactory中查找ServletWebServerFactory類型的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));}//這里會從beanFactory中返回bean的名字為beanNames[0],類型為ServletWebServerFactory.class的bean對象，如果當前bean還未創建，則此時就會創建bean對象并返回return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);}

從上面的代碼并看不出來實際的ServletWebServerFactory對象具體是什么？下面帶著大家一起簡單過下這部分的加載，這里具體就是springboot 加載bean的流程了，這部分的邏輯比較多，本次就不具體展開。關于springboot 加載bean的流程計劃后續專門再寫一篇。

springboot在啟動過程中會在當前類路徑下META-INF/spring.factories這個文件中，key=org.springframework.boot.autoconfigure.EnableAutoConfiguration的屬性的作為bean的定義進行加載，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(后續還會根據類上的注解判斷是否要繼續剔除)。

當前這些主要在spring-boot-autoconfigure-2.5.1.jar這個文件中

下面截取上面說的兩部分,可以看到這里的過濾器就3個，具體不展開討論了，自動導入的類就是下面的再加過濾去掉的

# Auto Configuration Import Filtersorg.springframework.boot.autoconfigure.AutoConfigurationImportFilter=org.springframework.boot.autoconfigure.condition.OnBeanCondition,org.springframework.boot.autoconfigure.condition.OnClassCondition,org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition# Auto Configureorg.springframework.boot.autoconfigure.EnableAutoConfiguration=......#下面這個會在創建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。會被加載。我們看看這個類，我們只看頭部就可以了

這里我們看到類上有Import注解，會繼續導入這幾個類，

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這兩個注解，

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

ConditionalOnMissingBean是表示當前beanFactory中沒有對應類型bean定義的話加載

多個條件都是and的關系，有一個條件不成立，就不會去進行后續處理。

在這里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注解的方法，加載到beanFactory中去

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

這里就會把EmbeddedTomcat.tomcatServletWebServerFactory這個方法進行加載，返回值是TomcatServletWebServerFactory類型，我們看下TomcatServletWebServerFactory類的繼承圖,可以看到它正好是繼承了ServletWebServerFactory接口。

再次打開ServletWebServerApplicationContext的getWebServerFactory方法

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

到這里，整個ServletWebServerFactory的查找就完成了

3、創建DispatcherServletRegistrationBean，DispatcherServlet

再看看上面的META-INF/spring.factories文件

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

我們打開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注解//ConditionalOnWebApplication是根據type來判斷指定類是否存在//當前的type是 Type.SERVLET，是來查找org.springframework.web.context.support.GenericWebApplicationContext類是否存在，這個類存在于spring-web中，所以這個條件是true@ConditionalOnWebApplication(type = Type.SERVLET)//這個注解上面說過了 ，就是查找指定的類是否存在，這個是查找DispatcherServlet.class是否存在，這里也會返回true@ConditionalOnClass(DispatcherServlet.class)//上面兩個條件都成立，就會執行后續的操作，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) //這里還是個條件，通過實現Condition接口，通過matches方法來判斷 //DefaultDispatcherServletCondition這個類就在當前這個文件里，matches判斷的結果也是true@Conditional(DefaultDispatcherServletCondition.class) //ServletRegistration.class這個類存在于tomcat-embed-core里面，這個結果也是true@ConditionalOnClass(ServletRegistration.class) //上面兩個條件成立，就會執行后續的操作，去遍歷內部類和方法@EnableConfigurationProperties(WebMvcProperties.class)protected static class DispatcherServletConfiguration {//beanFactory會創建這個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 incorrectlyreturn 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) //這里會要在查找DispatcherServletConfiguration.class，并執行加載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查找是否存在指定bean的定義，這個方法要注入參數，需要這個類，當前這里就是上面的dispatcherServlet方法定義的，這里也是存在的@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，在創建DispatcherServletRegistrationBean這個bean的時候，就會去查找dispatcherServlet是否存在，如果不存在，先創建dispatcherServlet這個bean,再創建DispatcherServletRegistrationBeanpublic 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;}} ......}

上面就是創建DispatcherServlet，DispatcherServletRegistrationBean的過程了

4、創建tomcat，加載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里，factory是TomcatServletWebServerFactory類的一個實例對象ServletWebServerFactory factory = getWebServerFactory(); //這里還是做個標記，不用關注 createWebServer.tag('factory', factory.getClass().toString()); //這里就是具體創建tomcat了，這里的入參getSelfInitializer()是個lambda表達式，這個后續很重要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;}//是創建webServer的參數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())會調用到TomcatServletWebServerFactory的getWebServer的方法

public WebServer getWebServer(ServletContextInitializer... initializers) {.......//上面的入參會在這里傳下去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)這個調用去看看

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

下面我們去看看TomcatStarter這個類是怎么使用這個initializers這個構造參數的。

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;}@Overridepublic void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {try {for (ServletContextInitializer initializer : this.initializers) {//會在這里調用onStartup方法，這里的入參就是ApplicationContextFacade的對象，里面包裝了ApplicationContext，里面再包裝了TomcatEmbeddedContext，這要就和tomcat聯系起來了，下面的截圖就是servletContext的對象結構initializer.onStartup(servletContext);}}catch (Exception ex) {......}

上面的initializer.onStartup(servletContext)會調用到ServletWebServerApplicationContext的selfInitialize方法

private void selfInitialize(ServletContext servletContext) throws ServletException {//這里是將ApplicationContextFacade設置到當前的servletContext上prepareWebApplicationContext(servletContext);//這里是在beanFactory中注冊application的scoperegisterApplicationScope(servletContext);//這里還是注冊上下文相關的beanWebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);//我們重點來看這里getServletContextInitializerBeans()是定義個一個ServletContextInitializerBeans對象，我們點進去看看for (ServletContextInitializer beans : getServletContextInitializerBeans()) {beans.onStartup(servletContext);}}

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

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,Class<? extends ServletContextInitializer>... initializerTypes) {this.initializers = new LinkedMultiValueMap<>();//由于我們沒有傳initializerTypes這個參數，所以this.initializerTypes里面就只有ServletContextInitializer.class這個類this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes): Collections.singletonList(ServletContextInitializer.class);//這里主要是從beanFactory中查找this.initializerTypes類型，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) { //默認情況下這里找到就只有上面第3部分的DispatcherServletRegistrationBean對應的beanfor (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,initializerType)) {//這里的key是bean的名字,value就是bean對象，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) { //這里返回的servlet也還是第3部分DispatcherServlet對應的beanServlet 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) {//這里的initializers是個map,按照類型，bean對象進行加載，這里的type是javax.servlet.Servlet.class,value是上面的DispatcherServletRegistrationBeanthis.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的構造方法,接著看后面的

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,Class<? extends ServletContextInitializer>... initializerTypes) {...... //這里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) {//這里的beanType是 Filter.class,下面這句就是從beanFactory中獲取所有類型為Filter.class的beanList<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里面if (this.seen.add(bean)) {// One that we haven’t already seen//包裝成RegistrationBean對象RegistrationBean registration = adapter.createRegistrationBean(beanName, bean, entries.size());int order = getOrder(bean);registration.setOrder(order);//同樣放置到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) {......//這里剛才說過了 //下面這部分不說了，這里基本和上面一樣，不過處理的類型變成了ServletContextAttributeListener.class、ServletRequestListener.class、ServletRequestAttributeListener.class、HttpSessionAttributeListener.class、HttpSessionIdListener.class、HttpSessionListener.class、ServletContextListener.class這些類型for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,new ServletListenerRegistrationBeanAdapter());}}

再回到ServletContextInitializerBeans的構造方法,接著看后面的

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,Class<? extends ServletContextInitializer>... initializerTypes) {......//這里剛才都說過了，看下面//這里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);}

這里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

@Overridepublic 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 {......//這里上面都說過了//getServletContextInitializerBeans()這個方法就是構造了ServletContextInitializerBeans//這里的for循環也是調用了ServletContextInitializerBeans的iterator的方法，實際上遍歷的也就是上面的this.sortedListfor (ServletContextInitializer beans : getServletContextInitializerBeans()) {//這里就是把在beanFactory中查找到的Servlet.class,filter.class，監聽器等等添加到tomcat容器中,我們就只進到servlet里面去看看//進到DispatcherServletRegistrationBean里面去看看beans.onStartup(servletContext);}}

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

//這個方法是在ServletRegistrationBean這個類中@Overrideprotected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {String name = getServletName();//這里的servletContext上面的ApplicationContextFacade的對象，這里就會將DispatcherServlet的bean對象加載到TomcatEmbeddedContext中，后續所有的http請求最后都會流轉到DispatcherServlet去進行具體的分發return servletContext.addServlet(name, this.servlet);}

到這里Servlet.class,filter.class，監聽器就全部加載到tomcat中去

5、創建RequestMappingHandlerMapping

再看看上面的META-INF/spring.factories文件

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

具體的加載和上面部分都類似，就不展開了，直接看我們需要的

//這里會創建RequestMappingHandlerMapping的bean@Bean@Primary@Overridepublic RequestMappingHandlerMapping requestMappingHandlerMapping(@Qualifier('mvcContentNegotiationManager') ContentNegotiationManager contentNegotiationManager,@Qualifier('mvcConversionService') FormattingConversionService conversionService,@Qualifier('mvcResourceUrlProvider') ResourceUrlProvider resourceUrlProvider) {// Must be @Primary for MvcUriComponentsBuilder to workreturn super.requestMappingHandlerMapping(contentNegotiationManager, conversionService,resourceUrlProvider);

看下這個類的繼承圖

RequestMappingHandlerMapping實現了InitializingBean接口，會在bean對象創建后的invokeInitMethods方法中調用afterPropertiesSet方法,最終會調用的AbstractHandlerMethodMapping的afterPropertiesSet方法中

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

protected void processCandidateBean(String beanName) {Class<?> beanType = null;try {//根據beanName獲取對應bean的Class對象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);}}//判斷類上是否有Controller.class，RequestMapping.class注解if (beanType != null && isHandler(beanType)) {//這里就會解析beanName上的所有方法進行遍歷，查找有 RequestMapping.class注解的方法，創建RequestMappingInfo對象，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、加載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) {......這就會加載之前查找到的RequestMappingHandlerMapping中的我們定義的controllerinitHandlerMappings(context);......}

會調用到這里

private void initHandlerMappings(ApplicationContext context) {this.handlerMappings = null;if (this.detectAllHandlerMappings) {// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.//這里會在beanFactory中查找所有HandlerMapping.class的bean,其中就包含我們第5部分的RequestMappingHandlerMappingMap<String, HandlerMapping> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);if (!matchingBeans.isEmpty()) {//將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);}}......}

當我們瀏覽器請求的時候，最終會走到DispatcherServlet的doDispatch的方法，處理我們的請求并返回,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.//在這里，就會請求request的請求路徑去查找實際要執行的controller的方法，點進去看看mappedHandler = getHandler(processedRequest);......}

@Nullableprotected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) { //這其實就是根據遍歷，查找對應的路徑，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;}

到這里，springboot 加載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。