spring DispatcherServlet 分析

DispatcherServlet是spring进入调度的开始，下图展示了从tomcat进入到spring的的调用栈

1、从 tomcat 的 ApplicationFilterChain 的 doFilter 进入到再 web.xml 中配置的 springServlet

2、在 doDispatche 方法中，调度到具体的 Controller 中，并返回一个 ModelAndView 的对象

// Actually invoke the handler. 实际上调用处理程序。
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

3、应用视图解析

applyDefaultViewName(processedRequest, mv);

/**
 * Do we need view name translation?
 */
private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
    if (mv != null && !mv.hasView()) {
        String defaultViewName = getDefaultViewName(request);
        if (defaultViewName != null) {
            mv.setViewName(defaultViewName);
        }
    }
}

4、拦截器（前置拦截器和后置拦截器）

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);

可以注意到再调用具体的 controller 和解析视图的前后，分别有 applyPreHandle 和 applyPostHandle 两个方法

5、processDispatchResult 处理调度结果，例如异常捕获等

/**
* Handle the result of handler selection and handler invocation, which is
* either a ModelAndView or an Exception to be resolved to a ModelAndView.
*/
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
     @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
     @Nullable Exception exception) throws Exception {}

以下是 doDispatch 方法的源码

/**
 * Process the actual dispatching to the handler.
 * <p>The handler will be obtained by applying the servlet's HandlerMappings in  order.
 * The HandlerAdapter will be obtained by querying the servlet's installed  HandlerAdapters
 * to find the first that supports the handler class.
 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or  handlers
 * themselves to decide which methods are acceptable.
 * @param request current HTTP request
 * @param response current HTTP response
 * @throws Exception in case of any kind of processing failure
 */
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);
            if (mappedHandler == 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) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException =784 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()) {
            // 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);
            }
        }
    }
}