Spring MVC 源码学习札记（二）ViewResolver处理viewname

第一节我们看到了ViewResolver对ModelAndView中属性Oject view为String时，将调用方法：

	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;
	}

 所以这一节就来了解下resolveViewName的机制。

来看看我们第一眼能懂的，Internal产生的是JstlView，Freemarker不用说，Velocity不用说，那就从这三个类来入手。

还是过程式的代码阅读，我们去找resovlveViewName 这个方法。

在AbstractCachingViewResolver类中：

 

public View resolveViewName(String viewName, Locale locale) throws Exception {
		if (!isCache()) {
			return createView(viewName, locale);



		}
		else {
			Object cacheKey = getCacheKey(viewName, locale);
			synchronized (this.viewCache) {
				View view = this.viewCache.get(cacheKey);
				if (view == null) {
					// Ask the subclass to create the View object.
					view = createView(viewName, locale);
					this.viewCache.put(cacheKey, view);
					if (logger.isTraceEnabled()) {
						logger.trace("Cached view [" + cacheKey + "]");
					}
				}
				return view;
			}
		}
	}

 

protected View createView(String viewName, Locale locale) throws Exception {
		return loadView(viewName, locale);



	}

 

protected abstract View loadView(String viewName, Locale locale) throws Exception;//子类必须实现这个方法

 

  loadView方法又将我们带入了AbstractCachingViewResolver的子类UrlBasedViewResolver类中，可以发现UrlBasedViewResolver类中：

 

/**
	 * Overridden to implement check for "redirect:" prefix.
	 * <p>Not possible in <code>loadView</code>, since overridden
	 * <code>loadView</code> versions in subclasses might rely on the
	 * superclass always creating instances of the required view class.
	 * @see #loadView
	 * @see #requiredViewClass
	 */
	@Override
	protected View createView(String viewName, Locale locale) throws Exception {
		// If this resolver is not supposed to handle the given view,
		// return null to pass on to the next resolver in the chain.
		if (!canHandle(viewName, locale)) {
			return null;
		}
		// Check for special "redirect:" prefix.
		if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
			String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
			return new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
		}
		// Check for special "forward:" prefix.
		if (viewName.startsWith(FORWARD_URL_PREFIX)) {
			String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
			return new InternalResourceView(forwardUrl);
		}
		// Else fall back to superclass implementation: calling loadView.
		return super.createView(viewName, locale);
	}

 这里千万别糊涂哦，这可是java基础知识多态哦。。。。

 因为在DispatcherServlet中的viewResolvers保存的都是ViewResolver的实现类的对象，那么调用接口的resolveViewName方法后，就得一层层来看这个方法了，从代码中我们看到的是只有AbstractCachingViewResolver有这个方法，那肯定调用的是它的。但是在resolveViewName中又调用了createView，那么这里调用的就是UrlBasedViewResolver的方法了。

 

所以在该方法前面的if都没得到满足时，才去显示的调用父类的createView方法。

// Else fall back to superclass implementation: calling loadView.
		return super.createView(viewName, locale);

 

这时父类的createView调用了loadView方法，而UrlBasedViewResolver又实现了这一方法，自然而然是调用它的loadView方法，而Internal暴露了buildView方法，则loadView执行时首先调用Internal的buildView。

@Override
	protected View loadView(String viewName, Locale locale) throws Exception {
		AbstractUrlBasedView view = buildView(viewName);
		View result = (View) getApplicationContext().getAutowireCapableBeanFactory().initializeBean(view, viewName);
		return (view.checkResource(locale) ? result : null);
	}

 UrlBasedViewResolver类中的buildView方法

 

protected AbstractUrlBasedView buildView(String viewName) throws Exception {
		AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(getViewClass());
		view.setUrl(getPrefix() + viewName + getSuffix());
		String contentType = getContentType();
		if (contentType != null) {
			view.setContentType(contentType);
		}
		view.setRequestContextAttribute(getRequestContextAttribute());
		view.setAttributesMap(getAttributesMap());
		return view;
	}

  InternalResourceViewResolver的buildView方法，忽然看到这个viewClass让我眼前一亮，终于知道Freemarker去生成静态页面是怎么做到的了，用自己实现的类配置在Resolver的属性中，这样等于是做了一次拦截。

<bean id="freemarkerViewResolver"
		class="org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver">
		<property name="contentType" value="text/html;charset=gbk" />
		<property name="viewClass" value="com.sha0k.util.MyFreeMarkerView">
		</property>
		<property name="exposeRequestAttributes">
			<value>true</value>
		</property>
		<property name="exposeSessionAttributes">
			<value>true</value>
		</property>
		<property name="order">
			<value>1</value>
		</property>
		<property name="suffix" value=".ftl" />
	</bean>

 

 

@Override
	protected AbstractUrlBasedView buildView(String viewName) throws Exception {
		InternalResourceView view = (InternalResourceView) super.buildView(viewName);
		if (this.alwaysInclude != null) {
			view.setAlwaysInclude(this.alwaysInclude);
		}
		if (this.exposeContextBeansAsAttributes != null) {
			view.setExposeContextBeansAsAttributes(this.exposeContextBeansAsAttributes);
		}
		if (this.exposedContextBeanNames != null) {
			view.setExposedContextBeanNames(this.exposedContextBeanNames);
		}
		view.setPreventDispatchLoop(true);
		return view;
	}