SpringMVC源码解析DispatcherServlet#doDispatch方法流程

DispatcherServlet是前端控制器设计模式的实现，提供了Spring Web MVC的集中访问点， 而且负责职责的分派，而且与Spring Ioc容器无缝集成， 从而可以获的Spring的所有好处。

作用

DispatcherServlet主要用作职责调度工作，本身主要用于控制流程，主要职责如下：

• 文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析
• 通过HandlerMapping，将请求映射到处理器（返回一个HandlerExecutionChain，它包括一个处理器、多个HandlerInterceptor拦截器）
• 、通过HandlerAdapter支持多种类型的处理器(HandlerExecutionChain中的处理器)
• 通过ViewResolver解析逻辑视图名到具体视图实现
• 本地化解析
• 渲染具体的视图等
• 如果执行过程中遇到异常将交给HandlerExceptionResolver来解析。

DispatcherServlet的工作流程

DospatcherServlet实际上是一个Servlet（它继承HttpServlet）。DispatcherServlet处理的请求必须在同一个web.xml文件里使用url-mapping定义映射。这是标准的J2EE servlet配置。

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 { processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); mappedHandler = getHandler(processedRequest); if (mappedHandler == null || mappedHandler.getHandler() == null) { noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. String method = request.getMethod(); boolean isGet = "GET".equals(method); if (isGet || "HEAD".equals(method)) { long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } catch (Throwable err) { dispatchException = new NestedServletException("Handler dispatch failed", err); } processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", err)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } } 

下面我们分析一下这些代码的意思，先看这句代码：

processedRequest = checkMultipart(request); 

检查这个请求是不是文件上传的请求的。我们具体的看一下它是怎么判断是否是文件上传的。

checkMultipart

转换请求到multipart请求，使multipart解析器可用。如果没有解析器被设置，只需使用现有的请求

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException { if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) { if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) { } else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) { } else { return this.multipartResolver.resolveMultipart(request); } } return request; } 

这里先是判断multipartResolver这个类是不是为空 multipartResolver是需要我们进行配置的，通常配置如下所示：