一，概览

先简单介绍下springMVC，它是spring众多模块中的一员。

image

其中的核心处理类DispatcherServlet本质上是一个Servlet，是属于Java中web标准的javax包下面的一个接口，所以它是配置在web.xml下。每一个DispatcherServlet都有它自己的WebApplicationContext，继承自root WebApplicationContext，所以它可以使用root下配置的所以bean。(这里多说一句，因为这种继承关系，所以springmvc项目都存在2个WebApplicationContext，所以通常我们所有的bean都放在root中处理，否则可能出现bean无法注入等问题)

Context hierarchy in Spring Web MVC

以下是DispatcherServlet结构图：

image.png

DispatcherServlet使用以下这些类来处理请求以及展示等。

二，处理流程

简单说下web项目的启动。略
1，首先标准的HTTP请求进入HttpServlet的service方法，然后根据请求方式调用具体的doGet()等方法。然后FrameworkServlet重写了service ,doGet等方法，并且再将不同类型的请求归并到processRequest()方法（有空再解释这个方法的内容）。processRequest()最核心的方法是doService(request, response),这个方法是abstract方法，这用到了well-known的模板方法模式。

//HttpServlet的service方法
 protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
    {
    String method = req.getMethod();

    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
        // servlet doesn't support if-modified-since, no reason
        // to go through further expensive logic
        doGet(req, resp);
        } else {
        long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
        if (ifModifiedSince < (lastModified / 1000 * 1000)) {
            // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
            maybeSetLastModified(resp, lastModified);
            doGet(req, resp);
        } else {
            resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
        }
        }

    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);

    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);
        
    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);   
        
    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);
        
    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);
        
    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);
        
    } else {
        //
        // Note that this means NO servlet supports whatever
        // method was requested, anywhere on this server.
        //

        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);
        
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
    }

/**
     * Process this request, publishing an event regardless of the outcome.
     * <p>The actual event handling is performed by the abstract
     * {@link #doService} template method.
     */
    protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;

        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

        initContextHolders(request, localeContext, requestAttributes);

        try {
            doService(request, response);
        }
        catch (ServletException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }

        finally {
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }

            if (logger.isDebugEnabled()) {
                if (failureCause != null) {
                    this.logger.debug("Could not complete request", failureCause);
                }
                else {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        logger.debug("Leaving response open for concurrent processing");
                    }
                    else {
                        this.logger.debug("Successfully completed request");
                    }
                }
            }

            publishRequestHandledEvent(request, startTime, failureCause);
        }
    }

2,接下就进入到DispatcherServlet的doService方法。详解

3，doService调用了doDispatch方法。详解
抛开外部的异常处理，以及前后的Interceptor的处理，其中比较核心的是
根据request找到Handler;
根据Handler找到HandlerAdapter;
处理请求；

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, false);
                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 (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
               //执行Interceptor   PreHandle()
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }

                applyDefaultViewName(request, mv);
               //执行Interceptor   PostHandle()
                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);
            }
        }
    }

三，主要组件介绍

1，HandlerMapping ,主要是通过request找到Handler和Interceptors。

官方介绍：his maps a method to an URL, so that the DispatcherServlet will be able to invoke it when processing a request.And the DispatcherServlet uses the HandlerAdapter to actually invoke the method.

2，HandlerAdapter ,DispatcherServlet会通过Handler得到HandlerAdapter，HandlerAdapter再根据请求去定位请求的具体处理方法。

。。。。
to be continue