SpringMVC运行原理

Arebirth

发布于 2019-09-24 15:22:38

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发布于 2019-09-24 15:22:38

文章被收录于专栏：arebirth重生者的IT之路

按照上边的执行流程图，我们可以看出一个SpringMVC整体的一个执行轮廓，下面我们具体来分析下

首先服务器接收到一个请求，匹配并调用了我们的前端控制器（DispatcherServlet）也叫中央处理器的请求路径，在web.xml文件中配置，我们来看下配置文件

<servlet>
        <servlet-name>springmvc</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:springmvc.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>springmvc</servlet-name>
　　<!-- / 代表匹配所有除了jsp文件的请求 -->
    <url-pattern>/</url-pattern>
</servlet-mapping>

因为DispatcherServlet实际上是间接的继承了我们的HttpServlet，所以它就相当于一个Servlet，SpringMVC帮我们封装好的一个Servlet，下面是继承体系

所以说，当来了一个请求后，WEB容器将把请求交给我们的DispatcherServlet进行处理

DispatcherServlet接收到请求后，会把我们的请求信息交给HandlerMapping处理器映射器进行处理

HandlerMapping根据请求的URL信息去查找匹配的URL的Handler，如果查找成，并返回一个执行链，下面来看下底层执行代码

 protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Iterator var2 = this.handlerMappings.iterator();

        HandlerExecutionChain handler;
        do {
            if (!var2.hasNext()) {
                return null;
            }

            HandlerMapping hm = (HandlerMapping)var2.next();
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + this.getServletName() + "'");
            }

            handler = hm.getHandler(request);//根据request对象获取handler，获取到的话将返回一个对象，没有的话将返回null  有兴趣童鞋自行深入查看
        } while(handler == null);//如果handler为null继续执行循环

        return handler;
    }

如果没有找到的话将执行以下代码

 protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (pageNotFoundLogger.isWarnEnabled()) {
            pageNotFoundLogger.warn("No mapping found for HTTP request with URI [" + getRequestUri(request) + "] in DispatcherServlet with name '" + this.getServletName() + "'");
        }

        if (this.throwExceptionIfNoHandlerFound) {
            ServletServerHttpRequest sshr = new ServletServerHttpRequest(request);
            throw new NoHandlerFoundException(sshr.getMethod().name(), sshr.getServletRequest().getRequestURI(), sshr.getHeaders());
        } else {
            response.sendError(404);//响应404
        }
    }

找到成功的情况下，DispatcherServlet再次请求处理器适配器（HandlerAdapter）调用相应的Handler 进行处理返回 ModelAndView 给 DispatcherServlet，底层代码，粗略看下即可，不是重点

 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 = this.checkMultipart(request);
                multipartRequestParsed = processedRequest != request;
                mappedHandler = this.getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    this.noHandlerFound(processedRequest, response);
                    return;
                }

                HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (this.logger.isDebugEnabled()) {
                        this.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;
                }

                mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); //在这里边调用相应的Handler，并通过这个方法返回ModelAndModel给DispatcherServlet
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                this.applyDefaultViewName(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            } catch (Exception var19) {
                dispatchException = var19;
            }

            this.processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        } catch (Exception var20) {
            this.triggerAfterCompletion(processedRequest, response, mappedHandler, var20);
        } catch (Error var21) {
            this.triggerAfterCompletionWithError(processedRequest, response, mappedHandler, var21);
        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }

    }

然后DispatcherServlet将ModelAndView请求对应的ViewResolver（视图解析器）解析视图，然后返回具体的View，底层实现

 protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale, HttpServletRequest request) throws Exception {
        Iterator var5 = this.viewResolvers.iterator();

        View view;
        do {
            if (!var5.hasNext()) {
                return null;
            }

            ViewResolver viewResolver = (ViewResolver)var5.next();
            view = viewResolver.resolveViewName(viewName, locale);
        } while(view == null);

        return view;
    }

DispatcherServlet对View视图进行渲染，也就是把模型数据填充到视图里边，底层执行代码

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        Locale locale = this.localeResolver.resolveLocale(request);
        response.setLocale(locale);
        View view;
        if (mv.isReference()) {
            view = this.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 '" + this.getServletName() + "'");
            }
        } else {
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + this.getServletName() + "'");
            }
        }

        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + this.getServletName() + "'");
        }

        try {
            view.render(mv.getModelInternal(), request, response);
        } catch (Exception var7) {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + this.getServletName() + "'", var7);
            }

            throw var7;
        }
    }

最后DispatcherServlet将视图传递给前台

PS：

Spring MVC核心组件： 1.DispatcherServlet(中央处理器)：将请求转发给控制器

2.Controller（控制器）：处理请求的控制器

3.HandlerMapping（映射处理器）：负责映射中央处理器转发给Controller时的映射策略

4.ModelAndView：服务器返回数据以及视图层的封装类

5.ViewResolver（视图解析器）：解析具体的视图

当用户发起请求后,执行DiapacherServlet,如果是JSP直接调用jsp页面.如果不是JSP,DiapacherServlet调用HandlerMapping判断请求URL是否合法,如果URL不存在报错,如果URL存在使用HandlerAdapter调用具体的HandlerMethod,当Handler执行完成后会返回ModelAndView,会被ViewResovler解析,调用具体的物理视图. 最终响应给客户端浏览器.

本文参与腾讯云自媒体同步曝光计划，分享自作者个人站点/博客。

原始发表：2019-09-14 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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