从源码角度看spring mvc的请求处理过程

写过java后端的好像没有不会spring mvc的，至于spring mvc的介绍优点，自行google，在这里就不balabala的介绍了。

在分析spring mvc源码之前，先看一张图：

spring mvc.png

请求处理的过程：
1.DispatcherServelt作为前端控制器，拦截request对象。
2.DispatcherServlet接收到request对象之后，查询HandlerMapping,得到一个HandlerExecutionChain对象。
3.DispatcherServlet将Handler对象交由HandlerAdapter(适配器模式的典型应用)，调用相应的controller方法。
4.Controller方法返回ModelAndView对象，DispatcherServlet将view交由ViewResolver进行解析，得到相应的视图。用model渲染视图。
5.返回响应结果。

整个过程的流程其实就是DispatcherServelt中doDispatch()方法的调用过程。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        //第一步拦截request对象，doDispatch()方法在doService()方法中被调用，request对象是经过处理的。
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                //和文件的上传和下载有关系，判断请求的类型是否是multipart类型
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                //主要看这里，这里是得到HandlerExecutionChain的方法。关于Handler()方法向下看
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                //这里已经获取到HandlerExecutionChain对象，接下来就要获取HandlerAdapter对象，调用Handler对象的方法。
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler
                //有关浏览器缓存的设定（304）
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
                //pan'du
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //解析视图，数据渲染
                applyDefaultViewName(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        //遍历HandlerMappingList对象（存储若干个HandlerMapping对象），不断调用，直到不为空为止，否则返回null
        for (HandlerMapping hm : this.handlerMappings) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
            }
            HandlerExecutionChain handler = hm.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
        return null;
    }