SpringMVC 之处理请求

　　本篇研究下SpringMVC是怎么处理一条请求的

一 doDispatch 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.
                mappedHandler = getHandler(processedRequest);//1 获取HandlerExecutionChain
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());//2 获取HandlerAdapter

                // 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()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    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());//3 实际去执行

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);//拦截器在这里执行
            }

　　主要分为三部分

　　1  获取HandlerExecutionChain

　　2  获取HandlerAdapter

　　3  实际去执行

二  DispatcherServlet.getHandler

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        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;
    }

　　 AbstractHandlerMapping.getHandler 

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Object handler = getHandlerInternal(request);//主要的逻辑就是获取 HandlerMethod
        if (handler == null) {
            handler = getDefaultHandler();
        }
        if (handler == null) {
            return null;
        }
        // Bean name or resolved handler?
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }

        HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);//HandlerExecutionChain 就是HandlerMethod加上一些interceptor
        if (CorsUtils.isCorsRequest(request)) {
            CorsConfiguration globalConfig = this.corsConfigSource.getCorsConfiguration(request);
            CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
            CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
            executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
        }
        return executionChain;
    }

　　

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        if (logger.isDebugEnabled()) {
            logger.debug("Looking up handler method for path " + lookupPath);
        }
        this.mappingRegistry.acquireReadLock();
        try {
            HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);//该方法逻辑不复杂，但是实现很复杂，原因就是url可能含有通配符，
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　所以该方法用了大量的代码解决如何寻找最匹配的url的问题
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　最终找到HandlerMethod
            if (logger.isDebugEnabled()) {
                if (handlerMethod != null) {
                    logger.debug("Returning handler method [" + handlerMethod + "]");
                }
                else {
                    logger.debug("Did not find handler method for [" + lookupPath + "]");
                }
            }
            return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
        }
        finally {
            this.mappingRegistry.releaseReadLock();
        }
    }

　　这样就找到了  HandlerExecutionChain 

三   DispatcherServlet.getHandlerAdapter

　　拿到HandlerAdapter就会去执行，这里我们先看是怎么拿到的HandlerAdapter，具体逻辑我会再开一篇介绍

　　

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {//这里的handler是一个HandlerMethod，就是要调用的方法
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

　　这里的 handlerAdapters 有三种，都是系统默认初始化放进去的，处理HandlerMethod的第一种RequestMappingHandlerAdapter就负责处理

　　

　　然后就是执行了

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

　　

四 总结

　　总结一下就是，请求进来->根据url先找到MethodHandler->构造出RequestExecutionChain ->获取RequestMappingHandlerAdapter->执行方法->结果处理

　　在这个流程里，有两个环节没有细讲

　　1 RequestMappingHandlerAdapter是神马东西

　　2 结果处理比如@ResponseBody是怎样的原理

　　这两个我都会单独讲解