SpringMvc @RequestMapping原理

讲这个之前，我们得先知道在SpringMvc启动时，会加载所有的Bean类，就是加了@Controller，@Component等组件标识的类，然后会把@RequestMapping的方法也加入到一个集合。放入到上下文环境中。

发起请求后的执行流程是：检查request类型-->获取匹配的Handlemethod-->查找拦截器-->组成HandlerExecutionChain执行链-->获取方法执行链对象的适配器（HandlerAdapter）-->然后反射执行业务方法。

有一个大概的了解之后，开始源码分析：

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;

                // Determine handler for the current request.
　　　　　　　　　　//获取匹配的执行链
                 mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                //根据匹配到的执行链对象，获取合适的适配器
               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 (logger.isDebugEnabled()) {
                        String requestUri = urlPathHelper.getRequestUri(request);
                        logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
      
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                try {
                    // Actually invoke the handler.
　　　　　　　　　　　　//在这里面反射执行业务方法
                   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }

                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
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

从标红部分点进去，到AbstractHandlerMethodAdapter类的handle方法-->RequestMappingHandlerAdapter类的handleInternal方法-->RequestMappingHandlerAdapter类的invokeHandleMethod

-->RequestMappingHandlerAdapter类的invokeAndHandle-->ServletInvocableHandlerMethod类的invokeForRequest-->InvocableHandlerMethod类的invokeForRequest。该方法代码如下：

public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
        if (logger.isTraceEnabled()) {
            StringBuilder sb = new StringBuilder("Invoking [");
            sb.append(this.getBeanType().getSimpleName()).append(".");
            sb.append(getMethod().getName()).append("] method with arguments ");
            sb.append(Arrays.asList(args));
            logger.trace(sb.toString());
        }
　　　　　//执行方法
        Object returnValue = invoke(args);
        if (logger.isTraceEnabled()) {
            logger.trace("Method [" + getMethod().getName() + "] returned [" + returnValue + "]");
        }
        return returnValue;
    }

然后就可以执行方法了。需要注意的是匹配方法时，根据@RequestMapping里面的value路径来匹配的，如果匹配到的有多个，如你配置了通配符，也配置了精确配置，他都会匹配到放在一个集合中，根据规则排序，然后取集合的第一个元素。有兴趣的可以看看这个排序的规则，理论上肯定是路径越精确的会优先，具体代码实现如下：

    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        List<Match> matches = new ArrayList<Match>();

        List<T> directPathMatches = this.urlMap.get(lookupPath);
        if (directPathMatches != null) {
            addMatchingMappings(directPathMatches, matches, request);
        }

        if (matches.isEmpty()) {
            // No choice but to go through all mappings
            addMatchingMappings(this.handlerMethods.keySet(), matches, request);
        }

        if (!matches.isEmpty()) {
　　　　　　  //排序规则
            Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
           //进行排序
            Collections.sort(matches, comparator);

            if (logger.isTraceEnabled()) {
                logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches);
            }

            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                Match secondBestMatch = matches.get(1);
                if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                    Method m1 = bestMatch.handlerMethod.getMethod();
                    Method m2 = secondBestMatch.handlerMethod.getMethod();
                    throw new IllegalStateException(
                            "Ambiguous handler methods mapped for HTTP path '" + request.getRequestURL() + "': {" +
                            m1 + ", " + m2 + "}");
                }
            }

            handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        }
        else {
            return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
        }
    }