SpringMVC系列之主要组件

一、组件说明

1. DispatcherServlet：前端控制器，用于请求到达前端控制器，由它调用其他组件处理用户的请求。
2. HandlerMapping：处理器映射器，负责根据用户请求找到Handler(处理器)，springmvc提供了不同的映射器实现方式。
3. Handler：处理器，对具体的用户请求进行处理。
4. HandlerAdapter：处理器适配器，通过HandlerAdapter对处理器进行执行。
5. View Resolver：视图解析器，负责将处理结果生成view视图。View Resolver首先根据逻辑视图名解析成物理视图名即具体的页面地址，再生成View视图对象，最后对View进行渲染将处理结果通过页面展示给用户。springmvc框架提供了很多的View视图类型，包括：jstlView、freemarkerView、pdfView等。

二、前端控制器

　　DispatcherServlet是springmvc的中央调度器，DispatcherServlet创建时会默认从DispatcherServlet.properties文件加载springmvc所用的各种组件。

　　

　　　

二、处理器映射器

　　处理器映射器负责根据request请求找到对应的Handler处理器以及Interceptor拦截器，将它们封装在HandlerExecutionChain对象中返回给前端控制器。

　　下面是两种常用的处理器映射器：

　　1、BeanNameURLHandlerMapping：根据请求的url与spring容器中定义的bean的name进行匹配，从而从spring容器中找到bean实例。

　　

　　2、simpleURLHandlerMapping：它可以将url和处理器bean的id进行统一映射配置。

　　

三、处理器适配器

　　HandlerAdapter会根据适配器接口对Handler进行包装适配，包装后即可对处理器进行执行(使用了适配器模式)。

　　下面是两种常用的处理器适配器：

　　1、SimpleControllerHandlerAdapter：简单控制器处理器适配器，所有实现了org.springframework.web.servlet.mvc.Controller 接口的bean通过此适配器进行适配执行。

　　

　　2、HttpRequestHandlerAdapter:http请求处理器适配器，所有实现了org.springframework.web.HttpRequestHandler接口的bean通过此适配器进行适配执行。

　　

　　注：处理器实现HttpRequestHandler接口，实现handleRequest方法，该方法没有返回、ModelAndView，可以通过response修改定义响应内容，比如返回json数据。

　　

四、注解处理器映射器和适配器

　　

　　可以使用组件扫描，而省去在spring容器中配置每个controller类。配置如下：

    <context:component-scan base-package="com.demo.ssm.controller"></context:component-scan>

　　1、注解处理器映射器，对类中标记的@RequestMapping的方法进行映射，根据RequestMapping定义的url匹配RequestMapping标记的方法，匹配成功返回HandlerMethod对象给前端控制器。

<!-- 注解映射器 -->
    <bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"/>

　　2、注解处理器适配器，对标记的@RequestMapping的方法进行适配。

<!-- 注解适配器 -->
    <bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"/>

　　注：还可以使用下面的配置代替上面处理器映射器和适配器的配置：

   <mvc:annotation-driven></mvc:annotation-driven>

五、总结

　　springmvc处理流程：

　　1、用户发送请求到DispatchServlet前端控制器。

/**
     * 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 || 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;
                    }
                }

                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

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

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

　　2、DispatchServlet调用HandlerMapping处理器映射器根据url查找Handler。

/**
     * Return the HandlerExecutionChain for this request.
     * <p>Tries all handler mappings in order.
     * @param request current HTTP request
     * @return the HandlerExecutionChain, or {@code null} if no handler could be found
     */
    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;
    }

　　3、DispatchServlet调用HandlerAdapter处理器适配器对HandlerMapping找到的Handler进行包装。

/**
     * Return the HandlerAdapter for this handler object.
     * @param handler the handler object to find an adapter for
     * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
     */
    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)) {
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

　　4、执行Handler，并返回一个ModelAndView。

/**
     * Determine an error ModelAndView via the registered HandlerExceptionResolvers.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handler the executed handler, or {@code null} if none chosen at the time of the exception
     * (for example, if multipart resolution failed)
     * @param ex the exception that got thrown during handler execution
     * @return a corresponding ModelAndView to forward to
     * @throws Exception if no error ModelAndView found
     */
    protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
            Object handler, Exception ex) throws Exception {

        // Check registered HandlerExceptionResolvers...
        ModelAndView exMv = null;
        for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
            exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
            if (exMv != null) {
                break;
            }
        }
        if (exMv != null) {
            if (exMv.isEmpty()) {
                request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
                return null;
            }
            // We might still need view name translation for a plain error model...
            if (!exMv.hasView()) {
                exMv.setViewName(getDefaultViewName(request));
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
            }
            WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
            return exMv;
        }

        throw ex;
    }

　　5、DispatchServlet调用ViewResolver视图解析器进行视图解析，解析完成后返回一个View，再进行视图渲染并展示。

/**
     * Render the given ModelAndView.
     * <p>This is the last stage in handling a request. It may involve resolving the view by name.
     * @param mv the ModelAndView to render
     * @param request current HTTP servlet request
     * @param response current HTTP servlet response
     * @throws ServletException if view is missing or cannot be resolved
     * @throws Exception if there's a problem rendering the view
     */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        // Determine locale for request and apply it to the response.
        Locale locale = this.localeResolver.resolveLocale(request);
        response.setLocale(locale);

        View view;
        if (mv.isReference()) {
            // We need to resolve the view name.
            view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
            if (view == null) {
                throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                        "' in servlet with name '" + getServletName() + "'");
            }
        }
        else {
            // No need to lookup: the ModelAndView object contains the actual View object.
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
                        "View object in servlet with name '" + getServletName() + "'");
            }
        }

        // Delegate to the View object for rendering.
        if (logger.isDebugEnabled()) {
            logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        try {
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
                        getServletName() + "'", ex);
            }
            throw ex;
        }
    }