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前端控制器是整个MVC框架中最为核心的一块,它主要用来拦截符合要求的外部请求,并把请求分发到不同的控制器去处理,根据控制器处理后的结果,生成相应的响应发送到客户端。前端控制器既可以使用Filter实现(Struts2采用这种方式),也可以使用Servlet来实现(spring MVC框架)。

 ds

DispatcherServlet 作为前置控制器是web服务器的入口,是spring mvc最重要的一个类,通过它的生命周期可以加深对web服务器的理解。

servlet的生命周期

首先我们回忆一下servlet的生命周期:

Servlet生命周期分为三个阶段:【http://www.cnblogs.com/cuiliang/archive/2011/10/21/2220671.html】

  1,初始化阶段  调用init()方法。Servlet被装载后,Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期内,init()方法只被调用一次。

  2,响应客户请求阶段  调用service()方法

  3,终止阶段  调用destroy()方法

 

Servlet初始化阶段:

  在下列时刻Servlet容器装载Servlet:

    1,Servlet容器启动时自动装载某些Servlet,实现它只需要在web.XML文件中的<Servlet></Servlet>之间添加如下代码:  

<loadon-startup>1</loadon-startup> 

    2,在Servlet容器启动后,客户首次向Servlet发送请求

    3,Servlet类文件被更新后,重新装载Servlet

DispatcherServlet的结构

复习了上述知识后我们来看看DispatcherServlet的结构:

DispatcherServlet继承自抽象类:FrameworkServlet,间接继承了HttpServlet (FrameworkServlet继承自HttpServletBean,而HttpServletBean继承自HttpServlet )

Servlet的初始化

    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context); //文件上传解析,如果请求类型是multipart将通过MultipartResolver进行文件上传解析;
        initLocaleResolver(context);    //本地化解析
        initThemeResolver(context);   //主题解析
        initHandlerMappings(context);   //通过HandlerMapping,将请求映射到处理器
        initHandlerAdapters(context);   //通过HandlerAdapter支持多种类型的处理器
        initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
        initRequestToViewNameTranslator(context); //直接解析请求到视图名
        initViewResolvers(context);      //通过ViewResolver解析逻辑视图名到具体视图实现
        initFlashMapManager(context);   //flash映射管理器
    }

 

servlet如何处理请求

servlet的service方法处理http请求。

FrameworkServlet.java 定义了servlet的service和destroy方法,如下所示:

    /**
     * Override the parent class implementation in order to intercept PATCH
     * requests.
     */
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String method = request.getMethod();
        if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
            processRequest(request, response);
        }
        else {
            super.service(request, response);
        }
    }

 

我们知道http请求类型有七种(外加一个option选项),定义如下:

public enum RequestMethod {

GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE

}
FrameworkServlet的service()处理不同的请求,我们以常见的post来说明:
    /**
     * 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);
        }
    }
FrameworkServlet 抽象定义了处理流程,留待子类来实现该方法,完成具体的请求处理。
/**
     * Subclasses must implement this method to do the work of request handling,
     * receiving a centralized callback for GET, POST, PUT and DELETE.
     * <p>The contract is essentially the same as that for the commonly overridden
     * {@code doGet} or {@code doPost} methods of HttpServlet.
     * <p>This class intercepts calls to ensure that exception handling and
     * event publication takes place.
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception in case of any kind of processing failure
     * @see javax.servlet.http.HttpServlet#doGet
     * @see javax.servlet.http.HttpServlet#doPost
     */
    protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
            throws Exception;

具体实现如下:

    /**
     * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
     * for the actual dispatching.
     */
    @Override
    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("org.springframework.web.servlet")) {
                    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()) {
                return;
            }
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }

重头戏,作为请求分发器的实现:

功能:1. 把请求分发到handler(按照配置顺序获取servlet的映射关系获取handler);2. 根据servlet已安装的  HandlerAdapters 去查询第一个能处理的handler;3. handler激发处理请求

    /**
     * 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;
                }

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    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
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

servlet销毁

    /**
     * Close the WebApplicationContext of this servlet.
     * @see org.springframework.context.ConfigurableApplicationContext#close()
     */
    @Override
    public void destroy() {
        getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");
        // Only call close() on WebApplicationContext if locally managed...
        if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {
            ((ConfigurableApplicationContext) this.webApplicationContext).close();
        }
    }

小结:

本文因篇章限制,仅仅介绍了请求处理的流程,没有对代码进行深入的分析,接下来的文章将从细微处着手,分析spring的代码之美。

posted on 2014-11-11 17:14 一天不进步,就是退步 阅读(...) 评论(...) 编辑 收藏