org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration.ConditionalOnClass=javax.servlet.ServletRequest
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration.ConditionalOnWebApplication=SERVLET
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration.AutoConfigureOrder=-2147483648

@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
// 判斷是否有 ServletRequest 類
@ConditionalOnClass(ServletRequest.class)
// 判斷 WebApplication 是 SERVLET 容器
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
    <version>2.2.5.RELEASE</version>
    <scope>compile</scope>
</dependency>

// 導(dǎo)入具體服務(wù)器類
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
         ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
         ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
         ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })

@Configuration(proxyBeanMethods = false)
#當(dāng)前工程下需要有Servlet、Tomcat UpgradeProtocol類
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
#當(dāng)前的Spring容器類不能有ServletWebServerFactory類型的Bean，搜索的是當(dāng)前容器。
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
    @Bean
    TomcatServletWebServerFactory tomcatServletWebServerFactory(
        ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
        ObjectProvider<TomcatContextCustomizer> contextCustomizers,
        ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
        // 創(chuàng)建 TomcatServletWebServerFactory ，是產(chǎn)生TomcatServletWebServer的工廠類
        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;
    }
}

<servlet>
    <servlet-name>dispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:springmvc.xml</param-value>
            </init-param>
            <!--Web服務(wù)器一旦啟動(dòng)，Servlet就會(huì)實(shí)例化創(chuàng)建對象，然后初始化(預(yù)備創(chuàng)建對象)-->
            <load-on-startup>1</load-on-startup>
</servlet>

#WebApplication是SERVLET容器
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration.ConditionalOnWebApplication=SERVLET
#工程中需要引入 DispatcherServlet 類
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration.ConditionalOnClass=org.springframework.web.servlet.DispatcherServlet
#容器注入在 ServletWebServerFactoryAutoConfiguration（服務(wù)器自動(dòng)配置類） 類之后，
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration.AutoConfigureAfter=org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration.AutoConfigureOrder=-2147483648

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)

@Configuration(proxyBeanMethods = false)
@Conditional(DefaultDispatcherServletCondition.class)
@ConditionalOnClass(ServletRegistration.class)
@EnableConfigurationProperties({ HttpProperties.class, WebMvcProperties.class })
protected static class DispatcherServletConfiguration {
    @Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
    public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {
        DispatcherServlet dispatcherServlet = new DispatcherServlet();
        dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
        dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
        dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
        dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
        dispatcherServlet.setEnableLoggingRequestDetails(httpProperties.isLogRequestDetails());
        return dispatcherServlet;
    }
}

@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) {
        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;
    }
}

#工程中需要引入Servlet、WebMvcConfigurer、 DispatcherServlet類 org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.ConditionalOnClass=javax.servlet.Servlet,org.springframework.web.servlet.config.annotation.WebMvcConfigurer,org.springframework.web.servlet.DispatcherServlet
# 容器注入在 DispatcherServletAutoConfiguration、TaskExecutionAutoConfiguration、ValidationAutoConfiguration之
# 后
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.AutoConfigureAfter=org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.AutoConfigureOrder=-2147483638

@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
		ValidationAutoConfiguration.class })

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.2.4.RELEASE</version>
    <scope>compile</scope>
</dependency>

protected void onRefresh() {
    super.onRefresh();
    try {
        // 創(chuàng)建 web 服務(wù)器
        createWebServer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    // 如果為 null，則創(chuàng)建
    if (webServer == null && servletContext == null) {
        // 獲取WebServerFactory
        ServletWebServerFactory factory = getWebServerFactory();
       // 獲取 webServer ，getSelfInitializer() 返回的是 函數(shù)式接口
	   // 是一個(gè) 實(shí)現(xiàn)了@FunctionalInterface 接口的列表
        this.webServer = factory.getWebServer(getSelfInitializer());
    }
    else if (servletContext != null) {
        try {
            getSelfInitializer().onStartup(servletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context", ex);
        }
    }
    initPropertySources();
}

protected ServletWebServerFactory getWebServerFactory() {
    // 先獲 BeanFactory ，再從 BeanFactory取 ServletWebServerFactory 類型的 BeanNames
    String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
    // 如果長度為 0，則拋異常
    if (beanNames.length == 0) {
        throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing "
                                              + "ServletWebServerFactory bean.");
    }
    // 如果長度大于 1，則拋異常
    if (beanNames.length > 1) {
        throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
                                              + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
    }
    // 返回根據(jù)第一個(gè)beanName和ServletWebServerFactory類型獲取容器中的Bean
    return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
}

@FunctionalInterface
public interface ServletContextInitializer {
	/**
	 * Configure the given {@link ServletContext} with any servlets, filters, listeners
	 * context-params and attributes necessary for initialization.
	 * @param servletContext the {@code ServletContext} to initialize
	 * @throws ServletException if any call against the given {@code ServletContext}
	 * throws a {@code ServletException}
	 */
	void onStartup(ServletContext servletContext) throws ServletException;
}

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
}

private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);
    registerApplicationScope(servletContext);
    // 注入 servletContext 類型的 bean
    WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
    // 注入實(shí)現(xiàn)了ServletContextInitializer 接口的類 ，并進(jìn)行循環(huán)調(diào)用onStartup
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

//getServletContextInitializerBeans具體代碼
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    // 構(gòu)造ServletContextInitializerBeans對象，該對象實(shí)現(xiàn)了 AbstractCollection<ServletContextInitializer> 接口
    // 是 ServletContextInitializer的 一個(gè)集合類。
    return new ServletContextInitializerBeans(getBeanFactory());
}

// ServletContextInitializerBeans 構(gòu)造方法
public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
                                      Class<? extends ServletContextInitializer>... initializerTypes) {
    // 初始化 initializers
    this.initializers = new LinkedMultiValueMap<>();
    this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
        : Collections.singletonList(ServletContextInitializer.class);
    // 在beanFactory中查找實(shí)現(xiàn)了 ServletContextInitializer 接口的類
    // 這里包括了  ServletRegistrationBean 、FilterRegistrationBean
    // DelegatingFilterProxyRegistrationBean、ServletListenerRegistrationBean 和其他。
    // 添加到 initializers 集合中
    addServletContextInitializerBeans(beanFactory);
    // 添加適配的 Bean
    addAdaptableBeans(beanFactory);
    // initializers 排序
    List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
        .flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
        .collect(Collectors.toList());
    // 給該集合可遍歷對象賦值
    this.sortedList = Collections.unmodifiableList(sortedInitializers);
    // 返回 initializers
    logMappings(this.initializers);
}

public final void onStartup(ServletContext servletContext) throws ServletException {
    String description = getDescription();
    if (!isEnabled()) {
        logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
        return;
    }
    // 注冊 Servlet 到 tomcat的 servletContext中
    register(description, servletContext);
}

@Override
protected final void register(String description, ServletContext servletContext) {
    // 添加當(dāng)前的 servlet 到 tomcat 的 servletContext 中
    D registration = addRegistration(description, servletContext);
    if (registration == null) {
        logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");
        return;
    }
    configure(registration);
}

@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
    // 獲取 當(dāng)前對象的 servlet 名稱
    String name = getServletName();
    //添加當(dāng)前的 servlet 到 tomcat 的 servletContext 中
    return servletContext.addServlet(name, this.servlet);
}

public WebServer getWebServer(ServletContextInitializer... initializers) {
    if (this.disableMBeanRegistry) {
        Registry.disableRegistry();
    }
    // 創(chuàng)建具體的Tomcat類
    Tomcat tomcat = new Tomcat();
    // 獲取主路徑，并且設(shè)置主路徑
    File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    // 創(chuàng)建連接器和協(xié)議
    Connector connector = new Connector(this.protocol);
    connector.setThrowOnFailure(true);
    // 添加連接器
    tomcat.getService().addConnector(connector);
    customizeConnector(connector);
    tomcat.setConnector(connector);
    // 設(shè)置 host 的自動(dòng)部署屬性為 false
    tomcat.getHost().setAutoDeploy(false);
    // 配置 engine
    configureEngine(tomcat.getEngine());
    for (Connector additionalConnector : this.additionalTomcatConnectors) {
        tomcat.getService().addConnector(additionalConnector);
    }
    // 初始化 TomcatEmbeddedContext ，并把 TomcatEmbeddedContext 添加至 host 中
	// 用 initializersToUse 初始化 TomcatStarter ，并設(shè)置 TomcatEmbeddedContext 的Starter屬性為  TomcatStarter 
    // initializersToUse 為函數(shù)式接口 ，即 實(shí)現(xiàn)了ServletContextInitializer接口 的匿名類
    prepareContext(tomcat.getHost(), initializers);
    // 創(chuàng)建具體的 TomcatwebServer
    return getTomcatWebServer(tomcat);
}

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

public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    // 自動(dòng)開啟
    this.autoStart = autoStart;
    // 初始化方法
    initialize();
}

// Start the server to trigger initialization listeners
// tomcat 服務(wù)器啟動(dòng)
this.tomcat.start();

public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
    try {
        // 遍歷已經(jīng)注冊的 initializers， 
        for (ServletContextInitializer initializer : this.initializers) {
            initializer.onStartup(servletContext);
        }
    }
    catch (Exception ex) {
        this.startUpException = ex;
        // Prevent Tomcat from logging and re-throwing when we know we can
        // deal with it in the main thread, but log for information here.
        if (logger.isErrorEnabled()) {
            logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: "
                         + ex.getMessage());
        }
    }
}