SpringBoot中Filter bean是怎麼被添加到Servlet容器中的

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參考資料

對於Spring Boot的IOC容器——ServletWebServerApplicationContext，其中的Filter bean，每個Filter bean都會被獨立的註冊成為Servlet的Filter。大概的註冊過程分成2步：

IOC容器——ServletWebServerApplicationContext將Filter介面的實現類封裝成FilterRegistrationBean，放到ServletContextInitializerBeans實例的成員變數initializers變數(LinkedMultiValueMap)中
Spring 容器（ServletWebServerApplicationContext）從ServletContextInitializerBeans實例的成員變數initializers變數(LinkedMultiValueMap)，中獲取所有的ServletContextInitializer實現類，調用它們的onStartUp函數，FilterRegistrationBean的onStartup函數就是在調用ServletContext的addFilter函數向Servlet添加Filter

1. IOC容器——ServletWebServerApplicationContext將Filter介面的實現類封裝成FilterRegistrationBean，放到ServletContextInitializerBeans實例的成員變數initializers變數(LinkedMultiValueMap)中

Spring容器——ServletWebServerApplicationContext的refresh()函數被調用。

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

ServletWebServerApplicationContext的createWebServer()函數，

在函數中會調用getSelfInitializer()函數獲取所有的ServletContextInitializer實例，
而獲取所有的ServletContextInitializer實例的過程中，會查找所有實現了Filter介面的bean，並註冊成FilterRegistrationBean（實現了ServletContextInitializer介面，所以也屬於ServletContextInitializer實例），
然後把FilterRegistrationBean放到initializers變數(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()函數中獲取所有ServletContextInitializer實例，調用onStartup方法完成註冊
       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);  
    //   獲取所有ServletContextInitializer實例，調用onStartup方法
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
       beans.onStartup(servletContext);   
        }
}
// 傳入BeanFactory，new 一個ServletContextInitializerBeans對象，這個對象實現了Collection介面，是一個集合，
// 集合內的元素是需要內置在Servlet Context上的ServletContextInitializer bean
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory());
    }

ServletContextInitializerBeans的構建

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,       Class<? extends ServletContextInitializer>... initializerTypes) {
    this.initializers = new LinkedMultiValueMap<>();    
    this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
          : Collections.singletonList(ServletContextInitializer.class);    
     // 找直接定義為ServletContextInitializer的bean（比如FilterRegistrationBean），DelegatingFilterProxyRegistrationBean等，
     // 添加到initializers中
    addServletContextInitializerBeans(beanFactory);    
     // 找實現了Filter介面的bean，將他們封裝成ServletContextInitializer bean（對於Filter的實現類也就是封裝成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);    
    // 獲取實現了Servlet介面的bean
    addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig)); 
    // 查找所有實現了Filter介面的bean，並註冊成FilterRegistrationBean，
    // 然後把FilterRegistrationBean放到initializers變數(LinkedMultiValueMap)中
    addAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());    
    for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
       addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,             
                 new ServletListenerRegistrationBeanAdapter());    
       }
}

所以我們可以看到ServletContextInitializerBeans構造函數是怎麼構造一個包含
ServletContextInitializer bean集合的：

找直接定義為ServletContextInitializer的bean（比如FilterRegistrationBean），
DelegatingFilterProxyRegistrationBean等，添加到initializers中
找實現了Filter介面的bean，將他們封裝成ServletContextInitializer bean（對於Filter的實現類也就是封裝成FilterRegistrationBean），添加到initializers中
排序然後一併返回

所以ServletContextInitializerBeans實例表示的是：一個從ListableBeanFactory bean容器中獲得的ServletContextInitializer實例的集合。這個集合中的每個元素來自容器中定義的每個如下類型的bean :

實現了ServletContextInitializer介面的bean
具體可能以ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean的形式存在。這些 bean 設計的目的是用來註冊相應的 Servlet/Filter/EventListener bean 到 ServletContext

- **實現了Servlet/Filter/EventListener介面的 bean**

這些 bean直接以實現裸的Servlet/Filter/EventListener介面的 bean的形式存在，但是Springboot會將它們封裝成相應的 RegistrationBean(也是ServletContextInitializer ),然後也註冊到ServletContext。

也就是說，Spring幫我們將實現裸的Servlet/Filter/EventListener介面的 bean，封裝成ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean。

2. Spring 容器（ServletWebServerApplicationContext）從ServletContextInitializerBeans實例的成員變數initializers變數(LinkedMultiValueMap)，中獲取所有的ServletContextInitializer實現類，調用它們的startUp函數，FilterRegistrationBean的onStartup函數就是在調用ServletContext的addFilter函數向Servlet添加Filter

還是要回到上面1.2中的createWebServer()函數中，這次我們不看getSelfInitializer()，而是看factory.getWebServer(...)函數。

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()函數中獲取所有ServletContextInitializer實例，調用onStartup方法完成註冊
       this.webServer = factory.getWebServer(getSelfInitializer());
       ......
}

TomcatServletWebServerFactory的getWebServer(...)函數

public WebServer getWebServer(ServletContextInitializer... initializers) {
    if (this.disableMBeanRegistry) {
       Registry.disableRegistry();    
    }
    ......
    // 將前面獲取到的所有ServletContextInitializer實例作為參數，傳下去
    prepareContext(tomcat.getHost(), initializers);    
    return getTomcatWebServer(tomcat);
 }

TomcatServletWebServerFactory的prepareContext(...)

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    ......
    // 合併一下
    ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);    
    host.addChild(context);   
    // 將前面獲取到的所有ServletContextInitializer實例作為參數，傳下去 
    configureContext(context, initializersToUse);    
    postProcessContext(context);}

tomcatServletWebServerFactory的configureContext（...），創建了TomcatStarter，並將TomcatStarter綁定到Tomcat上，這樣Tomcat啟動時就會回調TomcatStarter的onStartup函數。

protected void configureContext(Context context, ServletContextInitializer[] initializers) {
    // 將前面獲取到的所有ServletContextInitializer實例作為參數，傳下去 
    TomcatStarter starter = new TomcatStarter(initializers);
    
    // 下麵這一系列操作是將TomcatStarter綁定到Tomcat上，這樣TomcatStarter就會在Tomcat啟動時被回調
    if (context instanceof TomcatEmbeddedContext) {
    TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;    
    embeddedContext.setStarter(starter);    
    embeddedContext.setFailCtxIfServletStartFails(true);
    }
    context.addServletContainerInitializer(starter, NO_CLASSES);
    ......
}

TomcatStarter的startUp函數被調用的時候，就會變數所有的ServletContextInitializer實例的onStartup函數，

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());       
          }
    }
}

回顧第一章：“Filter介面的實現類封裝成FilterRegistrationBean”，而FilterRegistrationBean又是ServletContextInitializer介面的實現類，所以上面第5步，也會回調FilterRegistrationBean的onStartup函數，

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

DynamicRegistrationBean.class

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

AbstractFilterRegistrationBean.class，看到沒，最終還是用ServletContext的addFilter函數，向Servlet容器中添加Filter。

protected Dynamic addRegistration(String description, ServletContext servletContext) {
    // getFilter()在FilterRegistrationBean中被實現，返回的就是那個Filter介面的實現類。
    Filter filter = getFilter();   
    // 將Filter介面的實現類添加到Servlet容器中 
    return servletContext.addFilter(getOrDeduceName(filter), filter);
  }