解析源码---结合@ServletComponentScan与@WebServlet，@WebFilter，@WebListener注册servlet，filter，listener

比眉伴天荒 2021-10-24 02:32 541阅读 0赞

## 概述 ##

由于@ServletComponentScan所在类被加载BeanDefinition时，会加载其Registar。所以会导入ServletComponentScanRegistrar，由此向beanfactory添加了一个ServletComponentRegisteringPostProcessor。当最后调用各种nonOrderedPostProcessors时，会调用ServletComponentRegisteringPostProcessor的postProcessBeanFactory，最终扫描路径获取由@WebServlet，@WebFilter，@WebListener注册的servlet，filter，listener。

### 前言 ###

我们可以看见@ServletComponentScan import 了ServletComponentScanRegistrar

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Import(ServletComponentScanRegistrar.class)
    public @interface ServletComponentScan {
    ......
    }

而ServletComponentScanRegistrar向beanfactory添加了ServletComponentRegisteringPostProcessor

class ServletComponentScanRegistrar implements ImportBeanDefinitionRegistrar {
    	......
       private void addPostProcessor(BeanDefinitionRegistry registry,
             Set<String> packagesToScan) {
          GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
          beanDefinition.setBeanClass(ServletComponentRegisteringPostProcessor.class);
          beanDefinition.getConstructorArgumentValues()
                .addGenericArgumentValue(packagesToScan);
          beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
          registry.registerBeanDefinition(BEAN_NAME, beanDefinition);
       }
       ......  
       
    }

### 入口 ###

通过@ServletComponentScan所在组件类加载BeanDefinition时，加载ServletComponentScanRegistrar

private void loadBeanDefinitionsForConfigurationClass(
          ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {
    ......
    
       loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
        //加载ServletComponentScanRegistrar
       loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
    }

然后调用ServletComponentScanRegistrar的registerBeanDefinitions：向BeanDefinitionRegistry添加ServletComponentRegisteringPostProcessor

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
          BeanDefinitionRegistry registry) {
        //根据注解所在类的元信息获取扫描包路径，如果没有定义注解扫描的包路径，就以注解所在类的包路径为默认路径
       Set<String> packagesToScan = getPackagesToScan(importingClassMetadata);
        //是否已有ServletComponentRegisteringPostProcessor，无则添加，有则更新
       if (registry.containsBeanDefinition(BEAN_NAME)) {
          updatePostProcessor(registry, packagesToScan);
       }
       else {
          addPostProcessor(registry, packagesToScan);
       }
    }

### 执行 ###

AbstractApplicationContext：其中refresh方法会调用各个后处理器

public void refresh() throws BeansException, IllegalStateException {
       synchronized (this.startupShutdownMonitor) {
          ......
             // 调用各个后处理器，添加各种beandefinition
             invokeBeanFactoryPostProcessors(beanFactory);
          ......
    }

PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors:

统一处理各种BeanFactoryPostProcessor，最后一种BeanFactoryPostProcessor处理就是有关ServletComponentRegisteringPostProcessor处理servlet，filter，listener

public static void invokeBeanFactoryPostProcessors(
          ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
    
       // Invoke BeanDefinitionRegistryPostProcessors first, if any.
       Set<String> processedBeans = new HashSet<>();
    
       if (beanFactory instanceof BeanDefinitionRegistry) {
        ......
            //添加各类bean
       }
    ......
    
       // Finally, invoke all other BeanFactoryPostProcessors.
       List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
       for (String postProcessorName : nonOrderedPostProcessorNames) {
          nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
       }
        //调用ServletComponentRegisteringPostProcessor处理servlet，filter，listener
       invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
    ......
    }

ServletComponentRegisteringPostProcessor

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)
          throws BeansException {
        //必须是嵌入式的tomcat并且此时ServletContext为null
       if (isRunningInEmbeddedWebServer()) {
           //获取环境和ResourceLoader，及确定需要扫描@WebServlet，@WebFilter，@WebListener
          ClassPathScanningCandidateComponentProvider componentProvider = createComponentProvider();
          //packagesToScan是@ServletComponentScan的扫描路径
           for (String packageToScan : this.packagesToScan) {
             scanPackage(componentProvider, packageToScan);
          }
       }
    }
    
    private void scanPackage(
    			ClassPathScanningCandidateComponentProvider componentProvider,
    			String packageToScan) {
             //获取包路径下有@WebServlet，@WebFilter，@WebListener的类
    		for (BeanDefinition candidate : componentProvider
    				.findCandidateComponents(packageToScan)) {
    			if (candidate instanceof ScannedGenericBeanDefinition) {
    				for (ServletComponentHandler handler : HANDLERS) {
    					handler.handle(((ScannedGenericBeanDefinition) candidate),
    							(BeanDefinitionRegistry) this.applicationContext);
    				}
    			}
    		}
    	}

WebServletHandler.doHandle：最终添加Servlet的BeanDefinition

@Override
    public void doHandle(Map<String, Object> attributes,
          ScannedGenericBeanDefinition 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);
       builder.addPropertyValue("servlet", beanDefinition);
       builder.addPropertyValue("urlMappings", extractUrlPatterns(attributes));
       builder.addPropertyValue("multipartConfig",
             determineMultipartConfig(beanDefinition));
        //注册BeanDefinition
       registry.registerBeanDefinition(name, builder.getBeanDefinition());
    }

WebFilterHandler：最终添加合法的filter的BeanDefinition

@Override
    public void doHandle(Map<String, Object> attributes,
          ScannedGenericBeanDefinition 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));
        //注册BeanDefinition
       registry.registerBeanDefinition(name, builder.getBeanDefinition());
    }

对于Listener处理差不多，通过WebListenerHandler.doHandle()

@Override
       protected void doHandle(Map<String, Object> attributes,
             ScannedGenericBeanDefinition beanDefinition,
             BeanDefinitionRegistry registry) {
          BeanDefinitionBuilder builder = BeanDefinitionBuilder
                .rootBeanDefinition(ServletListenerRegistrationBean.class);
          builder.addPropertyValue("listener", beanDefinition);
           //注册BeanDefinition
          registry.registerBeanDefinition(beanDefinition.getBeanClassName(),
                builder.getBeanDefinition());
       }