SpringBoot中Filter bean如何添加到Servlet容器

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

private 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());  
       //  getSelfInitializer()函数中获取所有ServletContextInitializer实例，调用onStartup方法完成注册
       this.webServer = factory.getWebServer(getSelfInitializer());
       ......
}
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
    }
private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);    
    registerApplicationScope(servletContext);    
    WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);  
    //   获取所有ServletContextInitializer实例，调用onStartup方法
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
       beans.onStartup(servletContext);   
        }
}
// 传入BeanFactory，new 一个ServletContextInitializerBeans对象，这个对象实现了Collection接口，是一个集合，
// 集合内的元素是需要内置在Servlet Context上的ServletContextInitializer bean
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory());
    }

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,       Class<? extends ServletContextInitializer>... initializerTypes) {
    this.initializers = new LinkedMultiValueMap<>();    
    this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
          : Collections.singletonList(ServletContextInitializer.class);    
     // 找直接定义为ServletContextInitializer的bean（比如FilterRegistrationBean），DelegatingFilterProxyRegistrationBean等，
     // 添加到initializers中
    addServletContextInitializerBeans(beanFactory);    
     // 找实现了Filter接口的bean，将他们封装成ServletContextInitializer bean（对于Filter的实现类也就是封装成FilterRegistrationBean），
     // 添加到initializers中
    addAdaptableBeans(beanFactory);   
    //排序  
    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) {
    MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);    
    // 获取实现了Servlet接口的bean
    addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig)); 
    // 查找所有实现了Filter接口的bean，并注册成FilterRegistrationBean，
    // 然后把FilterRegistrationBean放到initializers变量(LinkedMultiValueMap)中
    addAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());    
    for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
       addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,             
                 new ServletListenerRegistrationBeanAdapter());    
       }
}

private 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());  
       //  getSelfInitializer()函数中获取所有ServletContextInitializer实例，调用onStartup方法完成注册
       this.webServer = factory.getWebServer(getSelfInitializer());
       ......
}

public WebServer getWebServer(ServletContextInitializer... initializers) {
    if (this.disableMBeanRegistry) {
       Registry.disableRegistry();    
    }
    ......
    // 将前面获取到的所有ServletContextInitializer实例作为参数，传下去
    prepareContext(tomcat.getHost(), initializers);    
    return getTomcatWebServer(tomcat);
 }

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    ......
    // 合并一下
    ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);    
    host.addChild(context);   
    // 将前面获取到的所有ServletContextInitializer实例作为参数，传下去 
    configureContext(context, initializersToUse);    
    postProcessContext(context);}

protected void configureContext(Context context, ServletContextInitializer[] initializers) {
    // 将前面获取到的所有ServletContextInitializer实例作为参数，传下去 
    TomcatStarter starter = new TomcatStarter(initializers);
    // 下面这一系列操作是将TomcatStarter绑定到Tomcat上，这样TomcatStarter就会在Tomcat启动时被回调
    if (context instanceof TomcatEmbeddedContext) {
    TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;    
    embeddedContext.setStarter(starter);    
    embeddedContext.setFailCtxIfServletStartFails(true);
    }
    context.addServletContainerInitializer(starter, NO_CLASSES);
    ......
}

public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
    try {
       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());       
          }
    }
}

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

protected final void register(String description, ServletContext servletContext) {
    //这里
    D registration = addRegistration(description, servletContext);    
    if (registration == null) {
       logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");       
       return;    
       }
    configure(registration);
    }

protected Dynamic addRegistration(String description, ServletContext servletContext) {
    // getFilter()在FilterRegistrationBean中被实现，返回的就是那个Filter接口的实现类。
    Filter filter = getFilter();   
    // 将Filter接口的实现类添加到Servlet容器中 
    return servletContext.addFilter(getOrDeduceName(filter), filter);
  }