SpringMVC源码解析从service到doDispatch（上））

请求在被Servlet处理之前会先被过滤器处理，之后调用Servlet的service方法来对相应的请求进行处理响应。所以我们这里分析的入口是Servlet的service方法。

我们在用SpringMVC的时候，通常都会在web.xml中进行这样的配置：

<servlet>
    <servlet-name>spring-mvc</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:learn-spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>spring-mvc</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>      

所有的请求(除静态资源)将由DispatcherServlet处理。

DispatcherServlet继承了FrameworkServlet，FrameworkServlet继承了HttpServletBean，HttpServletBean继承了HttpServlet，HttpServlet继承了GenericServlet，GenericServlet则实现了我们最顶级的接口Servlet和ServletConfig。

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (logger.isDebugEnabled()) {
            String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
            logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
                    " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        // Keep a snapshot of the request attributes in case of an include,
        // to be able to restore the original attributes after the include.
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap<String, Object>();
            Enumeration<?> attrNames = request.getAttributeNames();
            while (attrNames.hasMoreElements()) {
                String attrName = (String) attrNames.nextElement();
                if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }

        // Make framework objects available to handlers and view objects.
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }
        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

        try {
            doDispatch(request, response);
        }
        finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
                // Restore the original attribute snapshot, in case of an include.
                if (attributesSnapshot != null) {
                    restoreAttributesAfterInclude(request, attributesSnapshot);
                }
            }
        }
    }      

从DispatcherServlet的源码中我们没有找到service(ServletRequest req, ServletResponse res)这个方法，但是我们在DispatcherServlet的父类HttpServlet中找到了这个方法，我们去HttpServlet中看看这个方法的内容：

HttpServlet#service

将ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse

因为web开发，用HTTP协议，所以需要HttpServletRequest和HttpServletResponse

接下来就是调用service(HttpServletRequest request, HttpServletResponse response)，

HttpServlet和FrameworkServlet中都找到了这个方法，但是HttpServlet是FrameworkServlet的父类，即FrameworkServlet中重写了service这个方法，所以我们这里取FrameworkServlet中去看看这个方法的内容：

FrameworkServlet#service

根据请求的方法类型转换对应的枚举类

HttpMethod这个定义了这样的几种枚举类型：GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;而这些也是RFC标准中几种请求类型。如果请求类型为PATCH或者没有找到相应的请求类型的话，则直接调用processRequest这个方法。但是这种情况我们很少很少会遇到。

所以这里会执行super.service这个方法。即调用HttpServlet中的service方法。我们看一下HttpServlet中这个service方法的内容：