SpringBoot中Filter bean是怎么被添加到Servlet容器中的

参考资料

对于Spring Boot的IOC容器——ServletWebServerApplicationContext，其中的Filter bean，每个Filter bean都会被独立的注册成为Servlet的Filter。大概的注册过程分成2步：

1. IOC容器——ServletWebServerApplicationContext将Filter接口的实现类封装成FilterRegistrationBean，放到ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)中
2. Spring 容器（ServletWebServerApplicationContext）从ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)，中获取所有的ServletContextInitializer实现类，调用它们的onStartUp函数，FilterRegistrationBean的onStartup函数就是在调用ServletContext的addFilter函数向Servlet添加Filter

1. IOC容器——ServletWebServerApplicationContext将Filter接口的实现类封装成FilterRegistrationBean，放到ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)中

1. Spring容器——ServletWebServerApplicationContext的refresh()函数被调用。

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

2. ServletWebServerApplicationContext的createWebServer()函数，

• 在函数中会调用getSelfInitializer()函数获取所有的ServletContextInitializer实例，
• 而获取所有的ServletContextInitializer实例的过程中，会查找所有实现了Filter接口的bean，并注册成FilterRegistrationBean（实现了ServletContextInitializer接口，所以也属于ServletContextInitializer实例），
• 然后把FilterRegistrationBean放到initializers变量(LinkedMultiValueMap)中

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());
    }

3. ServletContextInitializerBeans的构建

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());    
       }
}

所以我们可以看到ServletContextInitializerBeans构造函数是怎么构造一个包含
ServletContextInitializer bean集合的：

• 找直接定义为ServletContextInitializer的bean（比如FilterRegistrationBean），
• DelegatingFilterProxyRegistrationBean等，添加到initializers中
  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
• 排序然后一并返回

所以ServletContextInitializerBeans实例表示的是：一个从ListableBeanFactory bean容器中获得的ServletContextInitializer实例的集合。这个集合中的每个元素来自容器中定义的每个如下类型的bean :

• 实现了ServletContextInitializer接口的bean
  具体可能以ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean的形式存在。这些 bean 设计的目的是用来注册相应的 Servlet/Filter/EventListener bean 到 ServletContext

- **实现了Servlet/Filter/EventListener接口的 bean**

这些 bean直接以实现裸的Servlet/Filter/EventListener接口的 bean的形式存在，但是Springboot会将它们封装成相应的 RegistrationBean(也是ServletContextInitializer ),然后也注册到ServletContext。

也就是说，Spring帮我们将实现裸的Servlet/Filter/EventListener接口的 bean，封装成ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean。

2. Spring 容器（ServletWebServerApplicationContext）从ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)，中获取所有的ServletContextInitializer实现类，调用它们的startUp函数，FilterRegistrationBean的onStartup函数就是在调用ServletContext的addFilter函数向Servlet添加Filter

1. 还是要回到上面1.2中的createWebServer()函数中，这次我们不看getSelfInitializer()，而是看factory.getWebServer(...)函数。

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());
       ......
}

2. TomcatServletWebServerFactory的getWebServer(...)函数

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);
 }

3. TomcatServletWebServerFactory的prepareContext(...)

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);}

4. tomcatServletWebServerFactory的configureContext（...），创建了TomcatStarter，并将TomcatStarter绑定到Tomcat上，这样Tomcat启动时就会回调TomcatStarter的onStartup函数。

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);
    ......
}

5. TomcatStarter的startUp函数被调用的时候，就会变量所有的ServletContextInitializer实例的onStartup函数，

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());       
          }
    }
}

6. 回顾第一章：“Filter接口的实现类封装成FilterRegistrationBean”，而FilterRegistrationBean又是ServletContextInitializer接口的实现类，所以上面第5步，也会回调FilterRegistrationBean的onStartup函数，

//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);
}

7. DynamicRegistrationBean.class

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);
    }

8. AbstractFilterRegistrationBean.class， 看到没，最终还是用ServletContext的addFilter函数，向Servlet容器中添加Filter。

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);
  }