1. 前言

????spring boot打包成fat jar的形式啟動時，這時tomcat作為內嵌容器，由spring boot帶動起來，并注冊servlet/filter等，這個過程是如何實現的呢？本文將從源碼級別帶你探索spring boot帶起tomcat的實現原理。

????以spring boot demo這個模塊為例，一步一步深入挖掘，著重看內嵌tomcat如何啟動，并講下外部tomcat啟動spring boot的原理。

2. 在bean definition加載完畢后，啟動并初始化tomcat

????一般情況下，我們是使用的是servlet，這時spring boot使用的是AnnotationConfigServletWebServerApplicationContext，繼承自ServletWebServerApplicationContext，這個父類，對AbstractApplicationContext的onRefresh方法進行了覆蓋，在這個階段加載了tomcat容器。

@Overrideprotected void onRefresh() {super.onRefresh();try {createWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}}

????那么，選擇在這個擴展點創建的好處是什么呢？在onRefresh之前，已經使用BeanFactoryPostProcessor加載了所有的BeanDefinition，這個時候ApplicationContext可以getBean，獲取到ServletContext的初始化器，例如servlet/filter等spring bean初始化器，對tomcat進行配置。

????如下為創建tomcat的代碼，可以發現代碼有兩個分支，剛好分為內外tomcat兩種情況，內部tomcat的話，會進行創建并初始化；而外部tomcat僅僅會進行初始化，配置serlvet/filter等。

3. 外部tomcat加載SpringApplication

????以war包的形式打包時，這個時候需要單獨部署外部tomcat，由tomcat帶動spring boot容器。
????從代碼可以發現，外部tomcat的話，servletContext必然是賦值的，這里是在哪賦值呢？外部tomcat留了什么鉤子，可以讓spring來編程初始化servelt/filter呢？
????對于外部tomcat部署spring boot應用，我們會讓main類繼承SpringBootServletInitializer，這是一個WebApplicationInitializer，是spring對于servlet的擴展點。在tomcat啟動時，會調用其onStartup方法，創建SpringApplication，設置初始化器，最終會設置servletContext，完成整個SpringApplication的構建。

public abstract class SpringBootServletInitializer implements WebApplicationInitializer {protected Log logger; // Don't initialize earlyprivate boolean registerErrorPageFilter = true;/*** Set if the {@link ErrorPageFilter} should be registered. Set to {@code false} if* error page mappings should be handled via the server and not Spring Boot.* @param registerErrorPageFilter if the {@link ErrorPageFilter} should be registered.*/protected final void setRegisterErrorPageFilter(boolean registerErrorPageFilter) {this.registerErrorPageFilter = registerErrorPageFilter;}@Overridepublic void onStartup(ServletContext servletContext) throws ServletException {servletContext.setAttribute(LoggingApplicationListener.REGISTER_SHUTDOWN_HOOK_PROPERTY, false);// Logger initialization is deferred in case an ordered// LogServletContextInitializer is being usedthis.logger = LogFactory.getLog(getClass());WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext);if (rootApplicationContext != null) {servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext));}else {this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "+ "return an application context");}}protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {SpringApplicationBuilder builder = createSpringApplicationBuilder();builder.main(getClass());ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);if (parent != null) {this.logger.info("Root context already created (using as parent).");servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);builder.initializers(new ParentContextApplicationContextInitializer(parent));}// 設置ApplicationContextInitializer，在ApplicationContext構建后，立即設置ServletContextbuilder.initializers(new ServletContextApplicationContextInitializer(servletContext));builder.contextFactory((webApplicationType) -> new AnnotationConfigServletWebServerApplicationContext());// 在這里配置sourcebuilder = configure(builder);builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));SpringApplication application = builder.build();if (application.getAllSources().isEmpty()&& MergedAnnotations.from(getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) {application.addPrimarySources(Collections.singleton(getClass()));}Assert.state(!application.getAllSources().isEmpty(),"No SpringApplication sources have been defined. Either override the "+ "configure method or add an @Configuration annotation");// Ensure error pages are registeredif (this.registerErrorPageFilter) {application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));}application.setRegisterShutdownHook(false);return run(application);} }/*** {@link ApplicationContextInitializer} for setting the servlet context.** @author Dave Syer* @author Phillip Webb* @since 2.0.0*/ public class ServletContextApplicationContextInitializerimplements ApplicationContextInitializer<ConfigurableWebApplicationContext>, Ordered {private int order = Ordered.HIGHEST_PRECEDENCE;private final ServletContext servletContext;private final boolean addApplicationContextAttribute;/*** Create a new {@link ServletContextApplicationContextInitializer} instance.* @param servletContext the servlet that should be ultimately set.*/public ServletContextApplicationContextInitializer(ServletContext servletContext) {this(servletContext, false);}/*** Create a new {@link ServletContextApplicationContextInitializer} instance.* @param servletContext the servlet that should be ultimately set.* @param addApplicationContextAttribute if the {@link ApplicationContext} should be* stored as an attribute in the {@link ServletContext}* @since 1.3.4*/public ServletContextApplicationContextInitializer(ServletContext servletContext,boolean addApplicationContextAttribute) {this.servletContext = servletContext;this.addApplicationContextAttribute = addApplicationContextAttribute;}public void setOrder(int order) {this.order = order;}@Overridepublic int getOrder() {return this.order;}@Overridepublic void initialize(ConfigurableWebApplicationContext applicationContext) {applicationContext.setServletContext(this.servletContext);if (this.addApplicationContextAttribute) {this.servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE,applicationContext);}}}

????那么SpringBootServletInitializer#onStartup這個方法，是在哪調用的呢？是SpringServletContainerInitializer，通過注釋可以看到，因為這個類上有@HandlesTypes注解，指定了類接口WebApplicationInitializer，servlet 3.0容器會自動掃描類路徑下的WebApplicationInitializer實現類，并把掃描到的實現類集合作為第一個參數傳遞給SpringServletContainerInitializer。

@HandlesTypes(WebApplicationInitializer.class) public class SpringServletContainerInitializer implements ServletContainerInitializer {/*** Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}* implementations present on the application classpath.* <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},* Servlet 3.0+ containers will automatically scan the classpath for implementations* of Spring's {@code WebApplicationInitializer} interface and provide the set of all* such types to the {@code webAppInitializerClasses} parameter of this method.* <p>If no {@code WebApplicationInitializer} implementations are found on the classpath,* this method is effectively a no-op. An INFO-level log message will be issued notifying* the user that the {@code ServletContainerInitializer} has indeed been invoked but that* no {@code WebApplicationInitializer} implementations were found.* <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,* they will be instantiated (and <em>sorted</em> if the @{@link* org.springframework.core.annotation.Order @Order} annotation is present or* the {@link org.springframework.core.Ordered Ordered} interface has been* implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}* method will be invoked on each instance, delegating the {@code ServletContext} such* that each instance may register and configure servlets such as Spring's* {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},* or any other Servlet API componentry such as filters.* @param webAppInitializerClasses all implementations of* {@link WebApplicationInitializer} found on the application classpath* @param servletContext the servlet context to be initialized* @see WebApplicationInitializer#onStartup(ServletContext)* @see AnnotationAwareOrderComparator*/@Overridepublic void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)throws ServletException {List<WebApplicationInitializer> initializers = Collections.emptyList();if (webAppInitializerClasses != null) {initializers = new ArrayList<>(webAppInitializerClasses.size());for (Class<?> waiClass : webAppInitializerClasses) {// Be defensive: Some servlet containers provide us with invalid classes,// no matter what @HandlesTypes says...if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&WebApplicationInitializer.class.isAssignableFrom(waiClass)) {try {initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass).newInstance());}catch (Throwable ex) {throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);}}}}if (initializers.isEmpty()) {servletContext.log("No Spring WebApplicationInitializer types detected on classpath");return;}servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");AnnotationAwareOrderComparator.sort(initializers);for (WebApplicationInitializer initializer : initializers) {// servlet會找到所有的WebApplicationInitializer SPI，然后調用，其中就包括SpringBootServletInitializer的實現類initializer.onStartup(servletContext);}}}package javax.servlet;import java.util.Set;/*** ServletContainerInitializers (SCIs) are registered via an entry in the* file META-INF/services/javax.servlet.ServletContainerInitializer that must be* included in the JAR file that contains the SCI implementation.* <p>* SCI processing is performed regardless of the setting of metadata-complete.* SCI processing can be controlled per JAR file via fragment ordering. If* absolute ordering is defined, then only the JARs included in the ordering* will be processed for SCIs. To disable SCI processing completely, an empty* absolute ordering may be defined.* <p>* SCIs register an interest in annotations (class, method or field) and/or* types via the {@link javax.servlet.annotation.HandlesTypes} annotation which* is added to the class.** @since Servlet 3.0*/ public interface ServletContainerInitializer {/*** Receives notification during startup of a web application of the classes* within the web application that matched the criteria defined via the* {@link javax.servlet.annotation.HandlesTypes} annotation.** @param c The (possibly null) set of classes that met the specified* criteria* @param ctx The ServletContext of the web application in which the* classes were discovered** @throws ServletException If an error occurs*/void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException; }

????對應的java spi如下，在spring web中實現了servlet標準的擴展點ServletContainerInitializer，該擴展點從3.0開始，且是為了替換web.xml而生的。

????一張圖總結外部tomcat啟動時，加載SpringApplication的整個過程：

4. fat jar啟動內嵌tomcat

????如下，會從spring容器查找ServletWebServerFactory，然后調用其getWebServer方法，并把ServletContextInitializer初始化器傳遞進去。

//org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext#createWebServerprivate void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");ServletWebServerFactory factory = getWebServerFactory();createWebServer.tag("factory", factory.getClass().toString());this.webServer = factory.getWebServer(getSelfInitializer());createWebServer.end();// （3）注冊Lifecycle，啟動后注冊到consul/nacos，以及優雅銷毀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();}protected ServletWebServerFactory getWebServerFactory() {// Use bean names so that we don't consider the hierarchyString[] 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));}return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);}

????在ServletWebServerFactoryConfiguration這個配置類里，如果有tomcat這個jar，會自動注入tomcat factory，也就是TomcatServletWebServerFactory。ServletWebServerFactoryConfiguration類由ServletWebServerFactoryAutoConfiguration這個自動配置類導入。

@Configuration(proxyBeanMethods = false) class ServletWebServerFactoryConfiguration {@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)static class EmbeddedTomcat {// 注冊tomcat ，可以自定義配置，這里會加載進來@BeanTomcatServletWebServerFactory tomcatServletWebServerFactory(ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,ObjectProvider<TomcatContextCustomizer> contextCustomizers,ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();factory.getTomcatConnectorCustomizers().addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));factory.getTomcatContextCustomizers().addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));factory.getTomcatProtocolHandlerCustomizers().addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));return factory;}}/*** Nested configuration if Jetty is being used.*/@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)static class EmbeddedJetty {@BeanJettyServletWebServerFactory JettyServletWebServerFactory(ObjectProvider<JettyServerCustomizer> serverCustomizers) {JettyServletWebServerFactory factory = new JettyServletWebServerFactory();factory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));return factory;}}/*** Nested configuration if Undertow is being used.*/@Configuration(proxyBeanMethods = false)@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)static class EmbeddedUndertow {@BeanUndertowServletWebServerFactory undertowServletWebServerFactory(ObjectProvider<UndertowDeploymentInfoCustomizer> deploymentInfoCustomizers,ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();factory.getDeploymentInfoCustomizers().addAll(deploymentInfoCustomizers.orderedStream().collect(Collectors.toList()));factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));return factory;}@BeanUndertowServletWebServerFactoryCustomizer undertowServletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new UndertowServletWebServerFactoryCustomizer(serverProperties);}}}@Configuration(proxyBeanMethods = false) @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE) @ConditionalOnClass(ServletRequest.class) @ConditionalOnWebApplication(type = Type.SERVLET) @EnableConfigurationProperties(ServerProperties.class) @Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,ServletWebServerFactoryConfiguration.EmbeddedJetty.class,ServletWebServerFactoryConfiguration.EmbeddedUndertow.class }) public class ServletWebServerFactoryAutoConfiguration {// 注冊bean，可以自定義容器設置，例如端口號@Beanpublic ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,ObjectProvider<WebListenerRegistrar> webListenerRegistrars,ObjectProvider<CookieSameSiteSupplier> cookieSameSiteSuppliers) {return new ServletWebServerFactoryCustomizer(serverProperties,webListenerRegistrars.orderedStream().collect(Collectors.toList()),cookieSameSiteSuppliers.orderedStream().collect(Collectors.toList()));}@Bean@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(ServerProperties serverProperties) {return new TomcatServletWebServerFactoryCustomizer(serverProperties);}@Configuration(proxyBeanMethods = false)@ConditionalOnProperty(value = "server.forward-headers-strategy", havingValue = "framework")@ConditionalOnMissingFilterBean(ForwardedHeaderFilter.class)static class ForwardedHeaderFilterConfiguration {@Bean@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")@ConditionalOnMissingFilterBean(ForwardedHeaderFilter.class)ForwardedHeaderFilterCustomizer tomcatForwardedHeaderFilterCustomizer(ServerProperties serverProperties) {return (filter) -> filter.setRelativeRedirects(serverProperties.getTomcat().isUseRelativeRedirects());}@BeanFilterRegistrationBean<ForwardedHeaderFilter> forwardedHeaderFilter(ObjectProvider<ForwardedHeaderFilterCustomizer> customizerProvider) {ForwardedHeaderFilter filter = new ForwardedHeaderFilter();customizerProvider.ifAvailable((customizer) -> customizer.customize(filter));FilterRegistrationBean<ForwardedHeaderFilter> registration = new FilterRegistrationBean<>(filter);registration.setDispatcherTypes(DispatcherType.REQUEST, DispatcherType.ASYNC, DispatcherType.ERROR);registration.setOrder(Ordered.HIGHEST_PRECEDENCE);return registration;}}interface ForwardedHeaderFilterCustomizer {void customize(ForwardedHeaderFilter filter);}/*** Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via* {@link ImportBeanDefinitionRegistrar} for early registration.*/public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {private ConfigurableListableBeanFactory beanFactory;@Overridepublic void setBeanFactory(BeanFactory beanFactory) throws BeansException {if (beanFactory instanceof ConfigurableListableBeanFactory) {this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;}}@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {if (this.beanFactory == null) {return;}// 注冊WebServerFactoryCustomizerBeanPostProcessor，對webServerFactory進行初始化registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor",WebServerFactoryCustomizerBeanPostProcessor.class,WebServerFactoryCustomizerBeanPostProcessor::new);registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor",ErrorPageRegistrarBeanPostProcessor.class, ErrorPageRegistrarBeanPostProcessor::new);}private <T> void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry, String name,Class<T> beanClass, Supplier<T> instanceSupplier) {if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(beanClass, true, false))) {RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass, instanceSupplier);beanDefinition.setSynthetic(true);registry.registerBeanDefinition(name, beanDefinition);}}}}

4.1 初始化WebServerFactory，添加connector等初始化器

????在WebServerFactory構造時，借助bean后處理器WebServerFactoryCustomizerBeanPostProcessor，會利用WebServerFactoryCustomizer對應類型的bean對其進行初始化。內嵌tomcat時，可以通過application.yaml對tomcat容器的參數進行配置。外部tomcat時application.yaml關于tomcat的配置不生效

• TomcatWebSocketServletWebServerCustomizer，添加websocket ServerEndpoint相關處理器
• ServletWebServerFactoryCustomizer，主要是將yaml中配置的端口，設置到factory里。
• TomcatServletWebServerFactoryCustomizer，設置重定向相關。
• TomcatWebServerFactoryCustomizer，tomcat特定配置，核心配置，如線程池大小，訪問日志，連接大小，請求頭大小，錯誤路徑等。

/** Copyright 2012-2019 the original author or authors.** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at** https://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/package org.springframework.boot.web.server;import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.List;import org.springframework.beans.BeansException; import org.springframework.beans.factory.BeanFactory; import org.springframework.beans.factory.BeanFactoryAware; import org.springframework.beans.factory.ListableBeanFactory; import org.springframework.beans.factory.config.BeanPostProcessor; import org.springframework.boot.util.LambdaSafe; import org.springframework.core.annotation.AnnotationAwareOrderComparator; import org.springframework.util.Assert;/*** {@link BeanPostProcessor} that applies all {@link WebServerFactoryCustomizer} beans* from the bean factory to {@link WebServerFactory} beans.** @author Dave Syer* @author Phillip Webb* @author Stephane Nicoll* @since 2.0.0*/ public class WebServerFactoryCustomizerBeanPostProcessor implements BeanPostProcessor, BeanFactoryAware {private ListableBeanFactory beanFactory;private List<WebServerFactoryCustomizer<?>> customizers;@Overridepublic void setBeanFactory(BeanFactory beanFactory) {Assert.isInstanceOf(ListableBeanFactory.class, beanFactory,"WebServerCustomizerBeanPostProcessor can only be used with a ListableBeanFactory");this.beanFactory = (ListableBeanFactory) beanFactory;}@Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {if (bean instanceof WebServerFactory) {postProcessBeforeInitialization((WebServerFactory) bean);}return bean;}@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {return bean;}@SuppressWarnings("unchecked")private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {LambdaSafe.callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory).withLogger(WebServerFactoryCustomizerBeanPostProcessor.class).invoke((customizer) -> customizer.customize(webServerFactory));}private Collection<WebServerFactoryCustomizer<?>> getCustomizers() {if (this.customizers == null) {// Look up does not include the parent contextthis.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans());this.customizers.sort(AnnotationAwareOrderComparator.INSTANCE);this.customizers = Collections.unmodifiableList(this.customizers);}return this.customizers;}@SuppressWarnings({ "unchecked", "rawtypes" })private Collection<WebServerFactoryCustomizer<?>> getWebServerFactoryCustomizerBeans() {return (Collection) this.beanFactory.getBeansOfType(WebServerFactoryCustomizer.class, false, false).values();}}

4.2 創建tomcat

????主要是配置tomcat，類似server.xml，構建Connector，配置引擎。將4.1里設置到factory的初始化器，真正應用到tomcat配置上，例如最大線程數。

// org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory#getWebServer@Overridepublic WebServer getWebServer(ServletContextInitializer... initializers) {if (this.disableMBeanRegistry) {Registry.disableRegistry();}Tomcat tomcat = new Tomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());for (LifecycleListener listener : this.serverLifecycleListeners) {tomcat.getServer().addLifecycleListener(listener);}Connector connector = new Connector(this.protocol);connector.setThrowOnFailure(true);tomcat.getService().addConnector(connector);customizeConnector(connector);tomcat.setConnector(connector);tomcat.getHost().setAutoDeploy(false);configureEngine(tomcat.getEngine());for (Connector additionalConnector : this.additionalTomcatConnectors) {tomcat.getService().addConnector(additionalConnector);}prepareContext(tomcat.getHost(), initializers);return getTomcatWebServer(tomcat);}// Needs to be protected so it can be used by subclassesprotected void customizeConnector(Connector connector) {int port = Math.max(getPort(), 0);// 設置端口到connectorconnector.setPort(port);if (StringUtils.hasText(getServerHeader())) {connector.setProperty("server", getServerHeader());}if (connector.getProtocolHandler() instanceof AbstractProtocol) {customizeProtocol((AbstractProtocol<?>) connector.getProtocolHandler());}invokeProtocolHandlerCustomizers(connector.getProtocolHandler());if (getUriEncoding() != null) {connector.setURIEncoding(getUriEncoding().name());}// Don't bind to the socket prematurely if ApplicationContext is slow to startconnector.setProperty("bindOnInit", "false");if (getHttp2() != null && getHttp2().isEnabled()) {connector.addUpgradeProtocol(new Http2Protocol());}if (getSsl() != null && getSsl().isEnabled()) {customizeSsl(connector);}TomcatConnectorCustomizer compression = new CompressionConnectorCustomizer(getCompression());compression.customize(connector);for (TomcatConnectorCustomizer customizer : this.tomcatConnectorCustomizers) {// 設置線程池，請求頭大小等customizer.customize(connector);}}protected void configureContext(Context context, ServletContextInitializer[] initializers) {// 添加初始化器TomcatStarter starter = new TomcatStarter(initializers);if (context instanceof TomcatEmbeddedContext) {TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;embeddedContext.setStarter(starter);embeddedContext.setFailCtxIfServletStartFails(true);}// TomcatStarter實現了ServletContainerInitializer，啟動后會觸發context.addServletContainerInitializer(starter, NO_CLASSES);for (LifecycleListener lifecycleListener : this.contextLifecycleListeners) {context.addLifecycleListener(lifecycleListener);}for (Valve valve : this.contextValves) {context.getPipeline().addValve(valve);}for (ErrorPage errorPage : getErrorPages()) {org.apache.tomcat.util.descriptor.web.ErrorPage tomcatErrorPage = new org.apache.tomcat.util.descriptor.web.ErrorPage();tomcatErrorPage.setLocation(errorPage.getPath());tomcatErrorPage.setErrorCode(errorPage.getStatusCode());tomcatErrorPage.setExceptionType(errorPage.getExceptionName());context.addErrorPage(tomcatErrorPage);}for (MimeMappings.Mapping mapping : getMimeMappings()) {context.addMimeMapping(mapping.getExtension(), mapping.getMimeType());}configureSession(context);configureCookieProcessor(context);new DisableReferenceClearingContextCustomizer().customize(context);for (String webListenerClassName : getWebListenerClassNames()) {context.addApplicationListener(webListenerClassName);}for (TomcatContextCustomizer customizer : this.tomcatContextCustomizers) {customizer.customize(context);}}/*** Factory method called to create the {@link TomcatWebServer}. Subclasses can* override this method to return a different {@link TomcatWebServer} or apply* additional processing to the Tomcat server.* @param tomcat the Tomcat server.* @return a new {@link TomcatWebServer} instance*/protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());}

4.3 ServletContextInitializer配置tomcat

????在TomcatWebServer的實例化過程的最后一步，會調用tomcat.start()方法，觸發initializer初始化servlet，內嵌tomcat由TomcatStarter觸發，因為TomcatStarter實現了ServletContainerInitializer，且也添加到了Tomcat上。外部tomcat時ApplicationContext#onRefresh直接調用這里，這里統一講解。

// TomcatWebServerpublic TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {Assert.notNull(tomcat, "Tomcat Server must not be null");this.tomcat = tomcat;this.autoStart = autoStart;this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;initialize();}private void initialize() throws WebServerException {logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));synchronized (this.monitor) {try {addInstanceIdToEngineName();Context context = findContext();context.addLifecycleListener((event) -> {if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {// Remove service connectors so that protocol binding doesn't// happen when the service is started.removeServiceConnectors();}});// Start the server to trigger initialization listenersthis.tomcat.start();// We can re-throw failure exception directly in the main threadrethrowDeferredStartupExceptions();try {ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());}catch (NamingException ex) {// Naming is not enabled. Continue}// Unlike Jetty, all Tomcat threads are daemon threads. We create a// blocking non-daemon to stop immediate shutdownstartDaemonAwaitThread();}catch (Exception ex) {stopSilently();destroySilently();throw new WebServerException("Unable to start embedded Tomcat", ex);}}}

????其中，有個initializer，在TomcatServletWebServerFactory創建tomcat server時傳入，是個lambda表達式適配的，也就是：

// ServletWebServerApplicationContextprivate org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {return this::selfInitialize;}private void selfInitialize(ServletContext servletContext) throws ServletException {prepareWebApplicationContext(servletContext);// 設置servletContext的root app context屬性registerApplicationScope(servletContext);WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);// 核心，從beanfactory，找到所有的ServletContextInitializer，并注冊到ServletContext中。for (ServletContextInitializer beans : getServletContextInitializerBeans()) {beans.onStartup(servletContext);}}/*** Returns {@link ServletContextInitializer}s that should be used with the embedded* web server. By default this method will first attempt to find* {@link ServletContextInitializer}, {@link Servlet}, {@link Filter} and certain* {@link EventListener} beans.* @return the servlet initializer beans*/protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {return new ServletContextInitializerBeans(getBeanFactory());}

????ServletContextInitializerBeans會加載ServletContextInitializer類型的bean。在spring boot中，實現了一個基類，也就是RegistrationBean，實現了ServletContextInitializer，底層抽象了配置一個對象到servletContext的過程，大部分servlet/filter都可以使用RegistrationBean相應子類實現注冊。

@SuppressWarnings("varargs")public ServletContextInitializerBeans(ListableBeanFactory beanFactory,Class<? extends ServletContextInitializer>... initializerTypes) {this.initializers = new LinkedMultiValueMap<>();this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes): Collections.singletonList(ServletContextInitializer.class);// (1) 加載所有ServletContextInitializer類型的bean，并設置到initializers中addServletContextInitializerBeans(beanFactory);// (2) 添加適配類addAdaptableBeans(beanFactory);// 對initializers進行排序，外層會全部應用到initializer.onStartUp(serverltContext)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);}

????RegistrationBean繼承體系，onStartUp分為兩部，先添加到serveltContext，然后使用返回的registration對添加的對象做進一步配置，例如配置映射路徑

public abstract class RegistrationBean implements ServletContextInitializer, Ordered {private static final Log logger = LogFactory.getLog(RegistrationBean.class);private int order = Ordered.LOWEST_PRECEDENCE;private boolean enabled = true;@Overridepublic final void onStartup(ServletContext servletContext) throws ServletException {String description = getDescription();if (!isEnabled()) {logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");return;}register(description, servletContext);}/*** Return a description of the registration. For example "Servlet resourceServlet"* @return a description of the registration*/protected abstract String getDescription();/*** Register this bean with the servlet context.* @param description a description of the item being registered* @param servletContext the servlet context*/protected abstract void register(String description, ServletContext servletContext);/*** Flag to indicate that the registration is enabled.* @param enabled the enabled to set*/public void setEnabled(boolean enabled) {this.enabled = enabled;}/*** Return if the registration is enabled.* @return if enabled (default {@code true})*/public boolean isEnabled() {return this.enabled;}/*** Set the order of the registration bean.* @param order the order*/public void setOrder(int order) {this.order = order;}/*** Get the order of the registration bean.* @return the order*/@Overridepublic int getOrder() {return this.order;}}public abstract class DynamicRegistrationBean<D extends Registration.Dynamic> extends RegistrationBean {private static final Log logger = LogFactory.getLog(RegistrationBean.class);private String name;private boolean asyncSupported = true;private Map<String, String> initParameters = new LinkedHashMap<>();/*** Set the name of this registration. If not specified the bean name will be used.* @param name the name of the registration*/public void setName(String name) {Assert.hasLength(name, "Name must not be empty");this.name = name;}/*** Sets if asynchronous operations are supported for this registration. If not* specified defaults to {@code true}.* @param asyncSupported if async is supported*/public void setAsyncSupported(boolean asyncSupported) {this.asyncSupported = asyncSupported;}/*** Returns if asynchronous operations are supported for this registration.* @return if async is supported*/public boolean isAsyncSupported() {return this.asyncSupported;}/*** Set init-parameters for this registration. Calling this method will replace any* existing init-parameters.* @param initParameters the init parameters* @see #getInitParameters* @see #addInitParameter*/public void setInitParameters(Map<String, String> initParameters) {Assert.notNull(initParameters, "InitParameters must not be null");this.initParameters = new LinkedHashMap<>(initParameters);}/*** Returns a mutable Map of the registration init-parameters.* @return the init parameters*/public Map<String, String> getInitParameters() {return this.initParameters;}/*** Add a single init-parameter, replacing any existing parameter with the same name.* @param name the init-parameter name* @param value the init-parameter value*/public void addInitParameter(String name, String value) {Assert.notNull(name, "Name must not be null");this.initParameters.put(name, value);}@Overrideprotected final void register(String description, ServletContext servletContext) {D registration = addRegistration(description, servletContext);if (registration == null) {logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");return;}configure(registration);}protected abstract D addRegistration(String description, ServletContext servletContext);protected void configure(D registration) {// 添加到serveltContext后，可以使用返回的registration對添加的對象做進一步配置registration.setAsyncSupported(this.asyncSupported);if (!this.initParameters.isEmpty()) {registration.setInitParameters(this.initParameters);}}/*** Deduces the name for this registration. Will return user specified name or fallback* to convention based naming.* @param value the object used for convention based names* @return the deduced name*/protected final String getOrDeduceName(Object value) {return (this.name != null) ? this.name : Conventions.getVariableName(value);}}

????按照序號(1)，如果用戶沒有配置RegistrationBean的話，默認會添加三個bean：

4.3.1 添加默認的DispatcherServlet

????其中DispatcherServletAutoConfiguration在用戶未定義DispatcherSerlvet時自動注冊DispatcherServletRegistrationBean

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE) @Configuration(proxyBeanMethods = false) @ConditionalOnWebApplication(type = Type.SERVLET) @ConditionalOnClass(DispatcherServlet.class) @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)@Conditional(DefaultDispatcherServletCondition.class)@ConditionalOnClass(ServletRegistration.class)@EnableConfigurationProperties(WebMvcProperties.class)protected static class DispatcherServletConfiguration {@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)@Conditional(DispatcherServletRegistrationCondition.class)@ConditionalOnClass(ServletRegistration.class)@EnableConfigurationProperties(WebMvcProperties.class)@Import(DispatcherServletConfiguration.class)protected static class DispatcherServletRegistrationConfiguration {@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {// 使用spring.mvc.servlet.path作為servlet映射路徑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;}}@Order(Ordered.LOWEST_PRECEDENCE - 10)private static class DefaultDispatcherServletCondition extends SpringBootCondition {@Overridepublic ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {ConditionMessage.Builder message = ConditionMessage.forCondition("Default DispatcherServlet");ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();List<String> dispatchServletBeans = Arrays.asList(beanFactory.getBeanNamesForType(DispatcherServlet.class, false, false));if (dispatchServletBeans.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {return ConditionOutcome.noMatch(message.found("dispatcher servlet bean").items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));}if (beanFactory.containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {return ConditionOutcome.noMatch(message.found("non dispatcher servlet bean").items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));}if (dispatchServletBeans.isEmpty()) {return ConditionOutcome.match(message.didNotFind("dispatcher servlet beans").atAll());}return ConditionOutcome.match(message.found("dispatcher servlet bean", "dispatcher servlet beans").items(Style.QUOTE, dispatchServletBeans).append("and none is named " + DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));}}@Order(Ordered.LOWEST_PRECEDENCE - 10)private static class DispatcherServletRegistrationCondition extends SpringBootCondition {@Overridepublic ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();ConditionOutcome outcome = checkDefaultDispatcherName(beanFactory);if (!outcome.isMatch()) {return outcome;}return checkServletRegistration(beanFactory);}private ConditionOutcome checkDefaultDispatcherName(ConfigurableListableBeanFactory beanFactory) {boolean containsDispatcherBean = beanFactory.containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);if (!containsDispatcherBean) {return ConditionOutcome.match();}List<String> servlets = Arrays.asList(beanFactory.getBeanNamesForType(DispatcherServlet.class, false, false));if (!servlets.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {return ConditionOutcome.noMatch(startMessage().found("non dispatcher servlet").items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));}return ConditionOutcome.match();}private ConditionOutcome checkServletRegistration(ConfigurableListableBeanFactory beanFactory) {ConditionMessage.Builder message = startMessage();List<String> registrations = Arrays.asList(beanFactory.getBeanNamesForType(ServletRegistrationBean.class, false, false));boolean containsDispatcherRegistrationBean = beanFactory.containsBean(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME);if (registrations.isEmpty()) {if (containsDispatcherRegistrationBean) {return ConditionOutcome.noMatch(message.found("non servlet registration bean").items(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));}return ConditionOutcome.match(message.didNotFind("servlet registration bean").atAll());}if (registrations.contains(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)) {return ConditionOutcome.noMatch(message.found("servlet registration bean").items(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));}if (containsDispatcherRegistrationBean) {return ConditionOutcome.noMatch(message.found("non servlet registration bean").items(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));}return ConditionOutcome.match(message.found("servlet registration beans").items(Style.QUOTE, registrations).append("and none is named " + DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));}private ConditionMessage.Builder startMessage() {return ConditionMessage.forCondition("DispatcherServlet Registration");}}}

????spring-boot-actuator-autoconfigure包里的WebMvcMetricsAutoConfiguration引進WebMvcMetricsFilter registration

@Configuration(proxyBeanMethods = false) @AutoConfigureAfter({ MetricsAutoConfiguration.class, CompositeMeterRegistryAutoConfiguration.class,SimpleMetricsExportAutoConfiguration.class }) @ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET) @ConditionalOnClass(DispatcherServlet.class) @ConditionalOnBean(MeterRegistry.class) @EnableConfigurationProperties(MetricsProperties.class) public class WebMvcMetricsAutoConfiguration {private final MetricsProperties properties;public WebMvcMetricsAutoConfiguration(MetricsProperties properties) {this.properties = properties;}@Bean@ConditionalOnMissingBean(WebMvcTagsProvider.class)public DefaultWebMvcTagsProvider webMvcTagsProvider(ObjectProvider<WebMvcTagsContributor> contributors) {return new DefaultWebMvcTagsProvider(this.properties.getWeb().getServer().getRequest().isIgnoreTrailingSlash(),contributors.orderedStream().collect(Collectors.toList()));}@Beanpublic FilterRegistrationBean<WebMvcMetricsFilter> webMvcMetricsFilter(MeterRegistry registry,WebMvcTagsProvider tagsProvider) {ServerRequest request = this.properties.getWeb().getServer().getRequest();WebMvcMetricsFilter filter = new WebMvcMetricsFilter(registry, tagsProvider, request.getMetricName(),request.getAutotime());FilterRegistrationBean<WebMvcMetricsFilter> registration = new FilterRegistrationBean<>(filter);registration.setOrder(Ordered.HIGHEST_PRECEDENCE + 1);registration.setDispatcherTypes(DispatcherType.REQUEST, DispatcherType.ASYNC);return registration;}

4.3.2 添加actuator endpoint servlet

????ServletEndpointRegistrar是actuator的servelt注冊器，其不走上面RegistrationBean，而是直接繼承了ServletContextInitializer。默認每個endpoint會單獨注冊成一個servlet。ServletEndpointRegistrar實際由ServletEndpointManagementContextConfiguration引進注冊成bean，涉及到endpoint的實現原理，另外講解。

// ServletEndpointRegistrarprivate void register(ServletContext servletContext, ExposableServletEndpoint endpoint) {String name = endpoint.getEndpointId().toLowerCaseString() + "-actuator-endpoint";String path = this.basePath + "/" + endpoint.getRootPath();String urlMapping = path.endsWith("/") ? path + "*" : path + "/*";EndpointServlet endpointServlet = endpoint.getEndpointServlet();Dynamic registration = servletContext.addServlet(name, endpointServlet.getServlet());registration.addMapping(urlMapping);registration.setInitParameters(endpointServlet.getInitParameters());registration.setLoadOnStartup(endpointServlet.getLoadOnStartup());logger.info("Registered '" + path + "' to " + name);}

4.3.3 適配servlet/filter/listener類型的bean為RegistrationBean，并添加到servletContext

????在上面第(2)步中，還會添加servlet/filter類型的bean，包裝為RegistrationBean。

//org.springframework.boot.web.servlet.ServletContextInitializerBeans#addAdaptableBeansprotected void addAdaptableBeans(ListableBeanFactory beanFactory) {// multiconfig，上傳附件，MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);// 這里會在beanFactory里找Servlet的bean，排除已經被使用的DispatcherServlet，也就是source，包裝為Registration，并帶上multipartConfig配置addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig));// 將所有Filter類型的bean適配成FilterRegistrationBeanaddAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());// 將所有支持的listener適配成beanfor (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,new ServletListenerRegistrationBeanAdapter());}}

????其中附件配置時默認自動配置，

public class MultipartAutoConfiguration {private final MultipartProperties multipartProperties;public MultipartAutoConfiguration(MultipartProperties multipartProperties) {this.multipartProperties = multipartProperties;}@Bean@ConditionalOnMissingBean({ MultipartConfigElement.class, CommonsMultipartResolver.class })public MultipartConfigElement multipartConfigElement() {return this.multipartProperties.createMultipartConfig();}

????filter有三個，主要是字符編碼過濾器OrderedCharacterEncodingFilter，由HttpEncodingAutoConfiguration注入。RequestContextFilter主要是暴露ServletRequestAttributes到線程變量，也就是httpServletRequest和httpServletResponse，由WebMvcAutoConfiguration注入。

????將下面默認有7個 listener bean適配成ServletListenerRegistrationBean，默認沒有。

4.3.4 注冊到servletContext

????對于DispatcherServlet，DispatcherServletRegistrationBean會將其urlmapping/loadStartUp順序/multipartConfig配置到servletContext上去。

5. spring容器啟動后，注冊服務實例到consul

????在上面序號(3)中，創建嵌入tomcat成功（注意是內嵌tomcat），會手動注冊兩個bean，兩個bean均實現了SmartLifecycle接口，該接口屬于spring實現的生命周期接口，當前spring容器單例加載完成后和關閉前，會分別觸發對應的start和stop方法。如果有多個SmartLifecycle接口，按照phase從小到大調用start方法，按照phase從大到調用stop方法。注意不同版本的spring boot實現不同，但是整體是在spring容器單例加載完成后和關閉前觸發調用。

class WebServerStartStopLifecycle implements SmartLifecycle {private final ServletWebServerApplicationContext applicationContext;private final WebServer webServer;private volatile boolean running;WebServerStartStopLifecycle(ServletWebServerApplicationContext applicationContext, WebServer webServer) {this.applicationContext = applicationContext;this.webServer = webServer;}@Overridepublic void start() {this.webServer.start();this.running = true;this.applicationContext.publishEvent(new ServletWebServerInitializedEvent(this.webServer, this.applicationContext));}@Overridepublic void stop() {this.webServer.stop();}@Overridepublic boolean isRunning() {return this.running;}@Overridepublic int getPhase() {return Integer.MAX_VALUE - 1;}}/*** {@link SmartLifecycle} to trigger {@link WebServer} graceful shutdown.** @author Andy Wilkinson* @since 2.5.0*/ public final class WebServerGracefulShutdownLifecycle implements SmartLifecycle {/*** {@link SmartLifecycle#getPhase() SmartLifecycle phase} in which graceful shutdown* of the web server is performed.*/public static final int SMART_LIFECYCLE_PHASE = SmartLifecycle.DEFAULT_PHASE;private final WebServer webServer;private volatile boolean running;/*** Creates a new {@code WebServerGracefulShutdownLifecycle} that will gracefully shut* down the given {@code webServer}.* @param webServer web server to shut down gracefully*/public WebServerGracefulShutdownLifecycle(WebServer webServer) {this.webServer = webServer;}@Overridepublic void start() {this.running = true;}@Overridepublic void stop() {throw new UnsupportedOperationException("Stop must not be invoked directly");}@Overridepublic void stop(Runnable callback) {this.running = false;this.webServer.shutDownGracefully((result) -> callback.run());}@Overridepublic boolean isRunning() {return this.running;}@Overridepublic int getPhase() {return SMART_LIFECYCLE_PHASE;}}

????WebServerStartStopLifecycle在spring單例刷新完畢后，會發送ServletWebServerInitializedEvent事件，spring cloud利用這個事件，實現了注冊當前服務實例到注冊中心的功能。具體可以看spring-cloud-common的AbstractAutoServiceRegistration。具體可見spring cloud demo，里面的consumer模塊。

// org.springframework.cloud.client.serviceregistry.AbstractAutoServiceRegistrationpublic void onApplicationEvent(WebServerInitializedEvent event) {this.bind(event);}/** @deprecated */@Deprecatedpublic void bind(WebServerInitializedEvent event) {ApplicationContext context = event.getApplicationContext();if (!(context instanceof ConfigurableWebServerApplicationContext) || !"management".equals(((ConfigurableWebServerApplicationContext)context).getServerNamespace())) {// 設置端口，注冊的端口從這里獲取的this.port.compareAndSet(0, event.getWebServer().getPort());this.start();}}

????serviceId為spring.application.name，端口為tomcat端口。如果有management端口，一般也會進行注冊。

6. spring容器銷毀，停止tomcat和解除服務注冊

????在spring容器停止后，也就是收到信號，觸發shutdownhook，調用close時，按照不同spring boot版本，有兩種關閉順序，高版本spring boot：

• 先調用WebServerGracefulShutdownLifecycle（order大）優雅關閉，再調用WebServerStartStopLifecycle，停止tomcat。
• 之后才銷毀單例disposable bean。AbstractAutoServiceRegistration在銷毀時，會調用stop方法，解除注冊。

????低版本spring boot：

• 銷毀單例disposable bean。AbstractAutoServiceRegistration在銷毀時，會調用stop方法，解除注冊。
• 也就是所有單例都銷毀后，在AbstractApplicationContext#onClose進行，停止tomcat

????個人認為解除注冊放到第一步，在收到onClosedEvent時進行合理些，然后銷毀tomcat，最后再銷毀單例。

7. 內外tomcat與spring boot整體交互

????一張圖總結：

總結

以上是生活随笔為你收集整理的spring boot启动加载tomcat原理深度剖析的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。