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Spring MVC 请求分派器
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--- formatterOff: "@formatter:off" title: Spring MVC 请求分派器 subtitle: dispatcher-servlet summary: Spring MVC 请求分派器 typed: - DispatcherServlet. categories: [spring] tags: [springmvc,ssm] mark: pin: true hot: true banner: https://images.unsplash.com/photo-1507842217343-583bb7270b66 date: 2021-04-30 16:04:20 +800 version: 1.0 formatterOn: "@formatter:on" --- # Spring MVC 源码分析 之 DispatcherServlet ## What `DispatcherServlet`是Spring MVC中用来分派用户请求的`Servlet`。  ## Strategy DispatcherServlet是由一系列实现特定功能的策略对象组成的，这些策略对象相互组合，共同了SpringMVC的核心功能。Spring对这些策略对象进行管理，并提供了了一套默认的配置，同样也提供了灵活的扩展与功能改写。 | Bean类名 | Bean名称 | 功能 | | -------------------------------------------- | -------------------------- | ------------------------------------------------------------ | | `MultipartResolver` | `multipartResolver` | 在相应的解析策略支持下，对`multi-part`请求（比如浏览器的表单文件上传）进行解析。 | | `LocaleResolver`<br/>`LocaleContextResolver` | `localeResolver` | 解析客户端当前请求的区域信息，从而能够返回该地区的国际化视图。 | | `ThemeResolver` | `themeResolver` | 解析当前应用的主题，从而提供个性化布局 | | `HandlerMapping` | `handlerMapping` | 将请求映射到处理程序以及用于预处理和后续处理的一系列拦截器。 这种映射有着一套标准，具体的功能因 HandlerMapping 实现而异。 HandlerMapping 的两个最主要实现是 RequestMappingHandlerMapping 和 SimpleUrlHandlerMapping ,前者支持 @RequestMapping 注释方法，它为请求的处理进行 URI 映射的注册。 | | `HandlerAdapter` | `handlerAdapter` | 协助 DispatcherServlet 调用匹配请求映射的处理程序，且不需要关心如何调用处理程序以及处理程序的任何细节。 例如，调用带注释的控制器中的方法需要先对 @RequestMapping 等注释进行解析。 HandlerAdapter 的主要功能是屏蔽 DispatcherServlet 的实现细节。 | | `HandlerExceptionResolver` | `handlerExceptionResolver` | 异常处理策略，可以处理HandlerMethod中抛出的异常，但不能处理视图渲染过程中的异常。 | | `RequestToViewNameTranslator` | `viewNameTranslator` | | | `ViewResolver` | `viewResolver` | 将处理程序返回的逻辑视图名称（String）解析为真实视图并返回给客户。 | | `FlashMapManager` | `flashMapManager` | 存储和检索可将参数从一个请求传递到另一个请求的“输入”和“输出”的 FlashMap，通常通过重定向来实现。 | ## Init `DispatcherServlet`本质是一个`Servlet`，因此，其初始化的入口为`init()`方法： ### HttpServletBean.init() ```java @Override public final void init() throws ServletException { // Set bean properties from init parameters. PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties); if (!pvs.isEmpty()) { try { BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this); ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext()); bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment())); initBeanWrapper(bw); bw.setPropertyValues(pvs, true); } catch (BeansException ex) { if (logger.isErrorEnabled()) { logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex); } throw ex; } } // Let subclasses do whatever initialization they like. initServletBean(); } ``` ### FrameworkServlet.initServletBean() ```java @Override protected final void initServletBean() throws ServletException { getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'"); if (logger.isInfoEnabled()) { logger.info("Initializing Servlet '" + getServletName() + "'"); } long startTime = System.currentTimeMillis(); try { this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet(); } catch (ServletException | RuntimeException ex) { logger.error("Context initialization failed", ex); throw ex; } if (logger.isDebugEnabled()) { String value = this.enableLoggingRequestDetails ? "shown which may lead to unsafe logging of potentially sensitive data" : "masked to prevent unsafe logging of potentially sensitive data"; logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails + "': request parameters and headers will be " + value); } if (logger.isInfoEnabled()) { logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms"); } } ``` ## How ### Dispatch ```java protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); try { ModelAndView mv = null; Exception dispatchException = null; try { /** * 1、调用 {@link MultipartResolver#isMultipart(HttpServletRequest)}方法检测请求是否是文件上传， * 如果是，则调用 {@link MultipartResolver#resolveMultipart(HttpServletRequest)} 对请求进行解析 */ processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // Determine handler for the current request. /** * 2、逐个调用 {@link HandlerMapping#getHandler(HttpServletRequest)}方法，以获取 {@link HandlerExecutionChain}对象实例 */ mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. /** * 3、逐个调用 {@link HandlerAdapter#supports(Object)}}方法，找到第一个支持处理 Handler 的 {@link HandlerAdapter} */ HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. String method = request.getMethod(); boolean isGet = "GET".equals(method); /** * 4、处理 GET｜HEAD 请求中的 last-modified 请求头 */ if (isGet || "HEAD".equals(method)) { long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } /** * 5、触发拦截器 {@link HandlerInterceptor#preHandle(HttpServletRequest, HttpServletResponse, Object) }方法 */ if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. /** * 6、调用 {@link HandlerAdapter#handle(HttpServletRequest, HttpServletResponse, Object)}方法真实处理 {@code handler} */ mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } /** * 7、从 Request 翻译视图名称 {@link RequestToViewNameTranslator#getViewName(HttpServletRequest)} */ applyDefaultViewName(processedRequest, mv); /** * 8、触发拦截器的 {@link HandlerInterceptor#postHandle(HttpServletRequest, HttpServletResponse, Object, ModelAndView)} 方法 */ mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } catch (Throwable err) { // As of 4.3, we're processing Errors thrown from handler methods as well, // making them available for @ExceptionHandler methods and other scenarios. dispatchException = new NestedServletException("Handler dispatch failed", err); } /** * 9、处理 DispatchResult */ processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { /** * 10、触发 {@link HandlerInterceptor#afterCompletion(HttpServletRequest, HttpServletResponse, Object, Exception)}方法 */ triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", err)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { /** * 11、触发 {@link AsyncHandlerInterceptor#afterConcurrentHandlingStarted(HttpServletRequest, HttpServletResponse, Object)}方法 */ mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { // Clean up any resources used by a multipart request. /** * 12、如果是文件上传请求，则清理 {@link MultipartResolver#cleanupMultipart(MultipartHttpServletRequest)} */ if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } } ``` 1. 通过`checkMultipart()`方法中调用`MultipartResolver`的`isMultipart()`方法检测当前请求`HttpServletRequest`是否是文件上传，如果是，则用`resolveMultipart()`来解析当前请求。 2. 通过`getHandler()`方法逐个调个`HandlerMapping`的`getHandler()`方法，以获取`HandlerExecutionChain`实例，如果未获取到，则抛出异常`NoHandlerFoundException`或直接返回`404`。 3. 逐个调用`HandlerAdapter`的`supports()`方法，以找到能够处理当前`Handler` 的`HandlerAdapter`实例； 4. 处理`Get`或`HEAD`请求中的`last-modified`请求头； 5. 通过`HandlerExecutionChain`触发拦截器`HandlerInterceptor`的`preHandle()`方法； 6. 通过`HandlerAdapter`的`handle()`方法真实处理`handler`； 7. 调用`RequestToViewNameTranslator`的`getViewName()`方法，从请求中获取视图名； 8. 通过`HandlerExecutionChain`的`applyPostHandle()`方法触发拦截器`HandlerInterceptor`的`postHandle()`方法； 9. 处理结果 10. 通过`HandlerExecutionChain`的`triggerAfterCompletion()`方法触发拦截器的`afterCompletion()`方法； 11. 通过`HandlerExecutionChain`的`applyAfterConcurrentHandlingStarted()`方法触发拦截器的`afterConcurrentHandlingStarted()`方法； 12. 如果当前请求是文件上传，则调用`MultipartResolver`的`cleanupMultipart()`方法清理现场。  ## See * [Spring MVC 解析之 DispatcherServlet](https://juejin.cn/post/6844903577077415950)
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