SpringBoot注册Servlet、Filter和Listener的方式和原理

1 实战

在Spring Boot项目中，如果使用内嵌Web服务器，可以很方便地注册Servlet、Filter和Listener等组件。

总的来说，包括以下方式：

创建实现ServletContextInitializer接口的bean，自定义注册逻辑。
开启@ServletCompnentScan功能，扫描标注@WebServlet、@WebFilter或WebListener的bean。
注册实现Servlet、Filter或Listener接口的bean。

1.1 实现`ServletContextInitializer`接口

Spring提供了ServletRegistrationBean、FilterRegistrationBean和ServletListenerRegistrationBean，只需要创建对应bean即可：

@Bean  
public ServletRegistrationBean servletRegistrationBean() {  
    ServletRegistrationBean servletRegistrationBean = new ServletRegistrationBean();  
    // servletRegistrationBean.setServlet(); 设置Servlet  
    // servletRegistrationBean.addUrlMappings(""); 设置映射地址  
    return servletRegistrationBean;  
}  
  
@Bean  
public FilterRegistrationBean filterRegistrationBean() {  
    FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();  
    // filterRegistrationBean.setFilter(); 设置Filter  
    return filterRegistrationBean;  
}  
  
@Bean  
public ServletListenerRegistrationBean servletListenerRegistrationBean() {  
    ServletListenerRegistrationBean servletListenerRegistrationBean = new ServletListenerRegistrationBean();  
    // servletListenerRegistrationBean.setListener(); 设置Listener  
    return servletListenerRegistrationBean;  
}

1.2 开启`@ServletCompnentScan`功能

在配置类中开启功能：

@Configuration  
@ServletComponentScan  
public class ServletContextConfig {
}

在对应包下添加对应bean：

@Component  
@WebServlet  
public class MyServlet implements Servlet {  
}

@Component  
@WebFilter  
public class MyFilter implements Filter {  
}

@Component  
@WebListener  
public class MyListener implements EventListener {  
}

1.3 实现`Servlet`、`Filter`或`Listener`接口

实现Servlet、Filter或Listener接口:

@Component  
public class MyServlet implements Servlet {  
}

@Component  
public class MyFilter implements Filter {  
}

@Component  
public class MyListener implements EventListener {  
}

2 源码

2.1 ServletContextInitializer

web服务器在初始化时，会调用ServletContextInitializer#onStartup()方法进行自定义注册。

在ServletWebServerApplicationContext的onRefresh()阶段，会创建Web服务器：

protected void onRefresh() {  
   // 调用父类的onRefresh()方法
   super.onRefresh();  
   try {  
      // 创建web服务器
      createWebServer();  
   }  
   catch (Throwable ex) {  
      // 抛异常
   }  
}

在ServletWebServerApplicationContext#createWebServer()方法中，会创建并初始化web服务器：

private void createWebServer() {  
   WebServer webServer = this.webServer;  
   ServletContext servletContext = getServletContext();  
   if (webServer == null && servletContext == null) {  
      // 获取factory
      ServletWebServerFactory factory = getWebServerFactory();  
      // 传入getSelfInitializer()，创建web服务器
      this.webServer = factory.getWebServer(getSelfInitializer());  
   }  
   else if (servletContext != null) {  
      // 初始化servletContext
      getSelfInitializer().onStartup(servletContext);  
   }  
}

ServletWebServerApplicationContext#getSelfInitializer()是一个方法引用（匿名对象）：

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

会在后续服务器初始化调用ServletContextInitializer#onStartup()方法时，触发ServletWebServerApplicationContext#selfInitialize()方法：

private void selfInitialize(ServletContext servletContext) throws ServletException {  
   // 遍历调用org.springframework.boot.web.servlet.ServletContextInitializer#onStartup()
   for (ServletContextInitializer beans : getServletContextInitializerBeans()) {  
      beans.onStartup(servletContext);  
   }  
}

ServletWebServerApplicationContext#getServletContextInitializerBeans()方法会返回ServletContextInitializerBeans对象：

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {  
   return new ServletContextInitializerBeans(getBeanFactory());  
}

ServletContextInitializerBeans#ServletContextInitializerBeans()方法中定义了获取ServletContextInitializer的逻辑：

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,  
      Class<? extends ServletContextInitializer>... initializerTypes) {  
   this.initializers = new LinkedMultiValueMap<>();  
   this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)  
         : Collections.singletonList(ServletContextInitializer.class);  
   // 添加ServletContextInitializer
   addServletContextInitializerBeans(beanFactory);  
   addAdaptableBeans(beanFactory);  
   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);  
}

ServletContextInitializerBeans#addServletContextInitializerBeans()方法会从容器中获取ServletContextInitializer实现类，

private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {  
   for (Class<? extends ServletContextInitializer> initializerType : this.initializerTypes) {  
      // 从容器中获取ServletContextInitializer实现类
      for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,  
            initializerType)) {  
         addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory);  
      }  
   }  
}

ServletContextInitializerBeans#addServletContextInitializerBean()会注册ServletContextInitializer实现类：

private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer,  
      ListableBeanFactory beanFactory) {  
   if (initializer instanceof ServletRegistrationBean) {  
      Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();  
      addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source);  
   }  
   else if (initializer instanceof FilterRegistrationBean) {  
      Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();  
      addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);  
   }  
   else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {  
      String source = ((DelegatingFilterProxyRegistrationBean) initializer).getTargetBeanName();  
      addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);  
   }  
   else if (initializer instanceof ServletListenerRegistrationBean) {  
      EventListener source = ((ServletListenerRegistrationBean<?>) initializer).getListener();  
      addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source);  
   }  
   else {  
      addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory,  
            initializer);  
   }  
}

三种内嵌容器的初始化流程都很相似。对于内嵌Tomcat，它的初始化流程如下：

TomcatServletWebServerFactory#getWebServer()和TomcatWebServer#TomcatWebServer()方法创建服务器。
在创建服务器过程中，将selfInitialize添加到TomcatStarter中。
TomcatWebServer#initialize()方法初始化服务器。
Tomcat#start()和LifecycleBase#start()启动服务器。
StandardContext#startInternal()启动上下文。
javax.servlet.ServletContainerInitializer#onStartup()初始化容器。

org.springframework.boot.web.embedded.tomcat.TomcatStarter实现了javax.servlet.ServletContainerInitializer，所以会直接触发前者的onStartup()方法。需要注意全限定类名的不同，TomcatStarter在这里起着适配器的作用：

public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {  
   try {  
      // 遍历所有org.springframework.boot.web.servlet.ServletContextInitializer
      for (ServletContextInitializer initializer : this.initializers) {  
         initializer.onStartup(servletContext);  
      }  
   }  
   catch (Exception ex) {   
   }  
}

理论上，我们只需要注册实现org.springframework.boot.web.servlet.ServletContextInitializer的bean，就可以在服务器初始化时进行额外操作，例如注册Servlet、Filter和Listener。

实际上，Spring提供了专门用来注册Servlet、Filter和Listener的实现类，它们定义好了注册三大组件的模板方法，更加方便开发人员使用：

org.springframework.boot.web.servlet.ServletRegistrationBean：注册Servlet。
org.springframework.boot.web.servlet.FilterRegistrationBean：注册Filter。
org.springframework.boot.web.servlet.ServletListenerRegistrationBean：注册Listener。

这些实现的使用方式都十分简单，只需要设置对应成员变量即可。

org.springframework.boot.web.servlet.ServletRegistrationBean：

org.springframework.boot.web.servlet.FilterRegistrationBean：

org.springframework.boot.web.servlet.ServletListenerRegistrationBean

2.2 @ServletComponentScan

@ServletComponentScan会将指定路径下标注@WebServlet、@WebFilter或@WebListener的bean注册成三大组件。

将@ServletComponentScan标注到配置类上，会将指定路径下标注@WebServlet、@WebFilter或@WebListener的bean注册到web服务器中。

@ServletComponentScan源码如下，它会导入ServletComponentScanRegistrar：

@Import(ServletComponentScanRegistrar.class)  
public @interface ServletComponentScan {  
   @AliasFor("basePackages")  
   String[] value() default {};  
  
   @AliasFor("value")  
   String[] basePackages() default {};  
   
   Class<?>[] basePackageClasses() default {};  
}

ServletComponentScanRegistrar会注册servletComponentRegisteringPostProcessor作为BeanFactoryPostProcessor。ServletComponentScanRegistrar#registerBeanDefinitions()：

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {  
   Set<String> packagesToScan = getPackagesToScan(importingClassMetadata);  
   if (registry.containsBeanDefinition(BEAN_NAME)) {  
      updatePostProcessor(registry, packagesToScan);  
   }  
   else {  
      addPostProcessor(registry, packagesToScan);  
   }  
}

ServletComponentRegisteringPostProcessor#postProcessBeanFactory()方法会从容器中获取指定路径下、标注@WebServlet、@WebFilter或@WebListener的BeanDefinition，然后使用ServletComponentHandler进行处理：

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {  
   if (isRunningInEmbeddedWebServer()) {  
      ClassPathScanningCandidateComponentProvider componentProvider = createComponentProvider();  
      for (String packageToScan : this.packagesToScan) {  
         // 扫描指定路径
         scanPackage(componentProvider, packageToScan);  
      }  
   }  
}

ServletComponentRegisteringPostProcessor#scanPackage()方法中定义了实际扫描逻辑：

private void scanPackage(ClassPathScanningCandidateComponentProvider componentProvider, String packageToScan) {  
   // 获取指定路径下的BeanDefinition
   for (BeanDefinition candidate : componentProvider.findCandidateComponents(packageToScan)) {  
      // 过滤注解标注的BeanDefinition
      if (candidate instanceof AnnotatedBeanDefinition) {  
         // 使用ServletComponentHandler进行分别处理
         for (ServletComponentHandler handler : HANDLERS) {  
            handler.handle(((AnnotatedBeanDefinition) candidate),  
                  (BeanDefinitionRegistry) this.applicationContext);  
         }  
      }  
   }  
}

org.springframework.boot.web.servlet.ServletComponentHandler#handle()方法会对@WebServlet、@WebFilter或@WebListener进行过滤：

void handle(AnnotatedBeanDefinition beanDefinition, BeanDefinitionRegistry registry) {  
   Map<String, Object> attributes = beanDefinition.getMetadata()  
         .getAnnotationAttributes(this.annotationType.getName());  
   if (attributes != null) {  
      doHandle(attributes, beanDefinition, registry);  
   }  
}

WebServletHandler#doHandle()方法会注册一个ServletRegistrationBean：

public void doHandle(Map<String, Object> attributes, AnnotatedBeanDefinition beanDefinition,  
      BeanDefinitionRegistry registry) {  
   BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ServletRegistrationBean.class);  
   builder.addPropertyValue("asyncSupported", attributes.get("asyncSupported"));  
   builder.addPropertyValue("initParameters", extractInitParameters(attributes));  
   builder.addPropertyValue("loadOnStartup", attributes.get("loadOnStartup"));  
   String name = determineName(attributes, beanDefinition);  
   builder.addPropertyValue("name", name);  
   // 设置servlet
   builder.addPropertyValue("servlet", beanDefinition);  
   builder.addPropertyValue("urlMappings", extractUrlPatterns(attributes));  
   builder.addPropertyValue("multipartConfig", determineMultipartConfig(beanDefinition));  
   registry.registerBeanDefinition(name, builder.getBeanDefinition());  
}

WebFilterHandler#doHandle()方法会注册一个FilterRegistrationBean：

public void doHandle(Map<String, Object> attributes, AnnotatedBeanDefinition beanDefinition,  
      BeanDefinitionRegistry registry) {  
   BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(FilterRegistrationBean.class);  
   builder.addPropertyValue("asyncSupported", attributes.get("asyncSupported"));  
   builder.addPropertyValue("dispatcherTypes", extractDispatcherTypes(attributes));  
   builder.addPropertyValue("filter", beanDefinition);  
   builder.addPropertyValue("initParameters", extractInitParameters(attributes));  
   String name = determineName(attributes, beanDefinition);  
   builder.addPropertyValue("name", name);  
   builder.addPropertyValue("servletNames", attributes.get("servletNames"));  
   builder.addPropertyValue("urlPatterns", extractUrlPatterns(attributes));  
   registry.registerBeanDefinition(name, builder.getBeanDefinition());  
}

WebListenerHandler#doHandle()方法会注册一个ServletComponentWebListenerRegistrar：

protected void doHandle(Map<String, Object> attributes, AnnotatedBeanDefinition beanDefinition,  
      BeanDefinitionRegistry registry) {  
   BeanDefinitionBuilder builder = BeanDefinitionBuilder  
         .rootBeanDefinition(ServletComponentWebListenerRegistrar.class);  
   builder.addConstructorArgValue(beanDefinition.getBeanClassName());  
   registry.registerBeanDefinition(beanDefinition.getBeanClassName() + "Registrar", builder.getBeanDefinition());  
}

不同于其他两个，ServletComponentWebListenerRegistrar实现了WebListenerRegistrar接口，会在org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryCustomizer#customize()方法被触发：

public void customize(ConfigurableServletWebServerFactory factory) {  
   for (WebListenerRegistrar registrar : this.webListenerRegistrars) {  
      registrar.register(factory);  
   }  
}

ServletWebServerFactoryCustomizer会被Spring Boot自动配置机制注入，并添加上述WebListenerRegistrar实现类。同时，还会注册ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar：

// 注册ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class})
public class ServletWebServerFactoryAutoConfiguration {  
   // 注册ServletWebServerFactoryCustomizer
   @Bean  
   public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,  
         ObjectProvider<WebListenerRegistrar> webListenerRegistrars,  
         ObjectProvider<CookieSameSiteSupplier> cookieSameSiteSuppliers) {  
      return new ServletWebServerFactoryCustomizer(serverProperties,  
            webListenerRegistrars.orderedStream().collect(Collectors.toList()),  
            cookieSameSiteSuppliers.orderedStream().collect(Collectors.toList()));  
   }
}

WebServerFactoryCustomizerBeanPostProcessor#postProcessBeforeInitialization()方法会触发ServletWebServerFactoryCustomizer#customize()的执行：

private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {  
   LambdaSafe.callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory)  
         .withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)  
         .invoke((customizer) -> customizer.customize(webServerFactory));  
}

2.3 ServletContextInitializerBeans#addAdaptableBeans

ServletContextInitializerBeans#addAdaptableBeans()方法会注册实现Servlet、Filter或Listener接口的bean作为三大组件。

之前说过，会在服务器初始化调用ServletContextInitializer#onStartup()方法时，触发ServletWebServerApplicationContext#selfInitialize()方法：

private void selfInitialize(ServletContext servletContext) throws ServletException {  
   // 遍历调用org.springframework.boot.web.servlet.ServletContextInitializer#onStartup()
   for (ServletContextInitializer beans : getServletContextInitializerBeans()) {  
      beans.onStartup(servletContext);  
   }  
}

getServletContextInitializerBeans()方法会创建ServletContextInitializerBeans对象。

ServletContextInitializerBeans#ServletContextInitializerBeans()方法中定义了获取ServletContextInitializer的逻辑：

public ServletContextInitializerBeans(ListableBeanFactory beanFactory,  
      Class<? extends ServletContextInitializer>... initializerTypes) {  
   this.initializers = new LinkedMultiValueMap<>();  
   this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)  
         : Collections.singletonList(ServletContextInitializer.class);  
   addServletContextInitializerBeans(beanFactory);  
   // 扫描实现Servlet、Filter或EventListener的bean
   addAdaptableBeans(beanFactory);  
   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);  
}

ServletContextInitializerBeans#addAdaptableBeans()方法会从容器获取所有实现javax.servlet.Servlet、javax.servlet.Filter或java.util.EventListener的bean，为它们创建对应的RegistrationBean实现类：

protected void addAdaptableBeans(ListableBeanFactory beanFactory) {  
   MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);  
   // 扫描&注册Servlet实现类
   addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig));  
   // 扫描&注册Filter实现类
   addAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());  
   // 扫描&注册EventListener实现类
   for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {  
      addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,  
            new ServletListenerRegistrationBeanAdapter());  
   }  
}

ServletContextInitializerBeans#addAsRegistrationBean()定义了具体扫描&注册逻辑：

private <T, B extends T> void addAsRegistrationBean(ListableBeanFactory beanFactory, Class<T> type,  
      Class<B> beanType, RegistrationBeanAdapter<T> adapter) {  
   // 从容器中获取指定类型的bean
   List<Map.Entry<String, B>> entries = getOrderedBeansOfType(beanFactory, beanType, this.seen);  
   for (Entry<String, B> entry : entries) {  
      String beanName = entry.getKey();  
      B bean = entry.getValue();  
      // 添加缓存
      if (this.seen.add(bean)) {  
         // 创建对应RegistrationBean实现类
         RegistrationBean registration = adapter.createRegistrationBean(beanName, bean, entries.size());  
         // 根据@Order注解设置执行顺序
         int order = getOrder(bean);  
         registration.setOrder(order);  
         this.initializers.add(type, registration);
      }  
   }  
}

创建RegistrationBean实现类的逻辑位于对应的RegistrationBeanAdapter实现类中。

ServletContextInitializerBeans.ServletRegistrationBeanAdapter#createRegistrationBean()方法创建ServletRegistrationBean：

public RegistrationBean createRegistrationBean(String name, Servlet source, int totalNumberOfSourceBeans) {  
   String url = (totalNumberOfSourceBeans != 1) ? "/" + name + "/" : "/";  
   if (name.equals(DISPATCHER_SERVLET_NAME)) {  
      url = "/"; // always map the main dispatcherServlet to "/"  
   }  
   ServletRegistrationBean<Servlet> bean = new ServletRegistrationBean<>(source, url);  
   bean.setName(name);  
   bean.setMultipartConfig(this.multipartConfig);  
   return bean;  
}

ServletContextInitializerBeans.FilterRegistrationBeanAdapter#createRegistrationBean()方法创建FilterRegistrationBean：

public RegistrationBean createRegistrationBean(String name, Filter source, int totalNumberOfSourceBeans) {  
   FilterRegistrationBean<Filter> bean = new FilterRegistrationBean<>(source);  
   bean.setName(name);  
   return bean;  
}

ServletContextInitializerBeans.ServletListenerRegistrationBeanAdapter#createRegistrationBean()方法创建ServletListenerRegistrationBean：

public RegistrationBean createRegistrationBean(String name, EventListener source,  
      int totalNumberOfSourceBeans) {  
   return new ServletListenerRegistrationBean<>(source);  
}

posted @ 2023-02-15 17:35 Xianuii 阅读(615) 评论(0) 编辑收藏举报

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最通俗易懂的技术文章！

SpringBoot注册Servlet、Filter和Listener的方式和原理

1 实战

1.1 实现`ServletContextInitializer`接口

1.2 开启`@ServletCompnentScan`功能

1.3 实现`Servlet`、`Filter`或`Listener`接口

2 源码

2.1 ServletContextInitializer

2.2 @ServletComponentScan

2.3 ServletContextInitializerBeans#addAdaptableBeans

Loading

Xianhuii

最通俗易懂的技术文章！

SpringBoot注册Servlet、Filter和Listener的方式和原理

1 实战

1.1 实现ServletContextInitializer接口

1.2 开启@ServletCompnentScan功能

1.3 实现Servlet、Filter或Listener接口

2 源码

2.1 ServletContextInitializer

2.2 @ServletComponentScan

2.3 ServletContextInitializerBeans#addAdaptableBeans

1.1 实现`ServletContextInitializer`接口

1.2 开启`@ServletCompnentScan`功能

1.3 实现`Servlet`、`Filter`或`Listener`接口