Spring MVC之视图呈现

     在Spring MVC DispatcherServlet.doDispatch中请求在经过handler处理返回一个ModelAndView.

那么ModelAndView又是如何最终转换为一个具体的View的呢？下面就对视图呈现部分作出一些简介。

视图的渲染是处理一个请求的最后阶段：

/**   根据ModelAndView中的视图名称进行解析，得到具体的视图对象
	 * 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 {
		// 确定请求的位置，并将位置信息应用到响应中
		Locale locale = this.localeResolver.resolveLocale(request);
		response.setLocale(locale);

		View view;
		// 如果只是一个视图的引用，则需要解析视图
		if (mv.isReference()) {
			// 解析视图名
			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 {
			// 不需要解析视图名，从ModleAndView中获取实际的视图对象
			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() + "'");
			}
		}

		// 将视图的渲染委派给视图对象,并通过HttpResponse把视图呈现给Http客户端
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		view.render(mv.getModelInternal(), request, response);
	}

  通过上面的代码可以发现对于视图的解析是使用如下方法：在该方法中遍历视图注册的视图解析器，如果优先级高的视图解析器可以解析出正确的视图对象，则直接放回视图对象。

	/**
	 * 将给定的视图名称解析成一个将被渲染的视图对象
         *  默认的实现要求调用所有的视图解析器。也可以基于特定的模型参数或请求参数使用自定义
         *  策略重写。 
	 * <p>The default implementations asks all ViewResolvers of this dispatcher.
	 * Can be overridden for custom resolution strategies, potentially based on
	 * specific model attributes or request parameters.
	 * @param viewName the name of the view to resolve
	 * @param model the model to be passed to the view
	 * @param locale the current locale
	 * @param request current HTTP servlet request
	 * @return the View object, or <code>null</code> if none found
	 * @throws Exception if the view cannot be resolved
	 * (typically in case of problems creating an actual View object)
	 * @see ViewResolver#resolveViewName
	 */
	protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
			HttpServletRequest request) throws Exception {

		for (ViewResolver viewResolver : this.viewResolvers) {
			View view = viewResolver.resolveViewName(viewName, locale);
			if (view != null) {
				return view;
			}
		}
		return null;
	}

自定义的一个JSON视图解析器：

**
 *
 * @author zhangwei_david
 * @version $Id: JsonViewResolver.java, v 0.1 2014��11��30�� ����12:09:44 zhangwei_david Exp $
 */
public class JsonViewResolver implements ViewResolver, Ordered {

    private static final Logger logger = LogManager.getLogger(JsonViewResolver.class);

    private int                 order  = Ordered.HIGHEST_PRECEDENCE;
    private UrlPathHelper       urlPathHelper;

    /**
     * @see org.springframework.web.servlet.ViewResolver#resolveViewName(java.lang.String, java.util.Locale)
     */
    public View resolveViewName(String viewName, Locale locale) throws Exception {

        RequestAttributes attrs = RequestContextHolder.getRequestAttributes();
        Assert.isInstanceOf(ServletRequestAttributes.class, attrs);
        HttpServletRequest request = ((ServletRequestAttributes) attrs).getRequest();
        String uri = urlPathHelper.getRequestUri(request);
        if (uri.contains(".json")) {
            LogUtils.info(logger, "Handler the Uri {0} and Return A JSON View", uri);
            return new MappingJacksonJsonView();
        }
        //
        return null;
    }

    /**
     * Getter method for property <tt>urlPathHelper</tt>.
     *
     * @return property value of urlPathHelper
     */
    public UrlPathHelper getUrlPathHelper() {
        return urlPathHelper;
    }

    /**
     * Setter method for property <tt>urlPathHelper</tt>.
     *
     * @param urlPathHelper value to be assigned to property urlPathHelper
     */
    public void setUrlPathHelper(UrlPathHelper urlPathHelper) {
        this.urlPathHelper = urlPathHelper;
    }

    /**
     * @see org.springframework.core.Ordered#getOrder()
     */
    public int getOrder() {
        return order;
    }

    /**
     * Setter method for property <tt>order</tt>.
     *
     * @param order value to be assigned to property order
     */
    public void setOrder(int order) {
        this.order = order;
    }

}