SpringMVC源码总结（十二）ViewResolver介绍

首先我们先看看ModelAndView中重要的View接口。
View接口：

String getContentType();

	/**
	 * Render the view given the specified model.
	 * <p>The first step will be preparing the request: In the JSP case,
	 * this would mean setting model objects as request attributes.
	 * The second step will be the actual rendering of the view,
	 * for example including the JSP via a RequestDispatcher.
	 * @param model Map with name Strings as keys and corresponding model
	 * objects as values (Map can also be {@code null} in case of empty model)
	 * @param request current HTTP request
	 * @param response HTTP response we are building
	 * @throws Exception if rendering failed
	 */
//上面说的很清楚，对于jsp来说，第一步就是将model作为request的attributes；第二步才开始渲染view
	void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception;

再看下ViewResolver接口：

View resolveViewName(String viewName, Locale locale) throws Exception;

它是对给定的viewName找到对应的View对象，然后使用该view对象的render方法将本身的内容写到response中。
然后就看下，当我们的处理函数返回一个viewName时，SpringMVC是如何渲染的。

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;
			}
//这里是我们的关注重点，就是进行视图渲染的过程
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
		}
		catch (Error err) {
			triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
		}

继续看下processDispatchResult是如何来渲染的

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

		boolean errorView = false;

		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				mv = processHandlerException(request, response, handler, exception);
				errorView = (mv != null);
			}
		}

		// Did the handler return a view to render?
//这里是我们关注的重点
		if (mv != null && !mv.wasCleared()) {
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
						"': assuming HandlerAdapter completed request handling");
			}
		}

		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Concurrent handling started during a forward
			return;
		}

		if (mappedHandler != null) {
			mappedHandler.triggerAfterCompletion(request, response, null);
		}
	}

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
		// Determine locale for request and apply it to the response.
		Locale locale = this.localeResolver.resolveLocale(request);
		response.setLocale(locale);

		View view;
		if (mv.isReference()) {
			// We need to resolve the view name.
			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 {
			// No need to lookup: the ModelAndView object contains the actual View object.
			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() + "'");
			}
		}

		// Delegate to the View object for rendering.
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		try {
			view.render(mv.getModelInternal(), request, response);
		}
		catch (Exception ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
						getServletName() + "'", ex);
			}
			throw ex;
		}
	}

这里可以看到整体的处理流程。首先判断view是不是一个视图的名称，若是需要找到这个视图名称对应的View对象，然后便是调用view对象的render方法，渲染到response中。
由于我们的处理函数经常仅仅是返回一个view名称，所以我们重点要看看它是如何根据视图名称来找到对应的View对象的，即resolveViewName方法内容。其实上文已经说明了View接口和ViewResolver 接口，ViewResolver 接口就是根据view名称来找到对应的View对象的，所以看下面就会很清晰明白

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;
	}

这里就是对DispatcherServlet的private List<ViewResolver> viewResolvers属性进行遍历找到一个能够获取View对象的ViewResolver，并返回这个view对象。
至此整个流程便走通了，接下来就是要看看有哪些ViewResolver以及它们的注册来源是什么？

常用的ViewResolver有：FreeMarkerViewResolver、InternalResourceViewResolver、VelocityViewResolver等。

接下来就是如何来注册这些ViewResolver：

protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
//我们关注的重点
		initViewResolvers(context);
		initFlashMapManager(context);
	}

还是在DispatcherServlet的初始化策略中，调用了initViewResolvers，如下：

private void initViewResolvers(ApplicationContext context) {
		this.viewResolvers = null;

		if (this.detectAllViewResolvers) {
			// Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
			Map<String, ViewResolver> matchingBeans =
					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
			if (!matchingBeans.isEmpty()) {
				this.viewResolvers = new ArrayList<ViewResolver>(matchingBeans.values());
				// We keep ViewResolvers in sorted order.
				OrderComparator.sort(this.viewResolvers);
			}
		}
		else {
			try {
				ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
				this.viewResolvers = Collections.singletonList(vr);
			}
			catch (NoSuchBeanDefinitionException ex) {
				// Ignore, we'll add a default ViewResolver later.
			}
		}

		// Ensure we have at least one ViewResolver, by registering
		// a default ViewResolver if no other resolvers are found.
		if (this.viewResolvers == null) {
			this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
			if (logger.isDebugEnabled()) {
				logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default");
			}
		}
	}

这和HandleMapping和HandlerAdapter的初始化过程基本类似。this.detectAllViewResolvers是DispatcherServlet的一个boolean属性，可以在web.xml文件中修改这个值，默认是true。

/** Detect all ViewResolvers or just expect "viewResolver" bean? */
	private boolean detectAllViewResolvers = true;

当detectAllViewResolvers为true,意味着就会获取从xml文件中解析出来的ViewResolver。如果为false,则直接去找bean name为"viewResolver"并且是ViewResolver类型的作为DispatcherServlet的ViewResolver。
当上述两种情况都没有找到，则会启用默认的ViewResolver，在this.viewResolvers = getDefaultStrategies(context, ViewResolver.class)中，这个过程已经多次说过，可以见本系列第一篇HandleMapping的来源。它就是依据DispatcherServlet.properties文件中所配置的ViewResolver，如下：

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

也就是默认采用的是InternalResourceViewResolver。
再说说在xml文件中配置ViewResolver的情况，如下：

<bean class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer">
		<property name="templateLoaderPath" value="/WEB-INF/views" />
		<property name="defaultEncoding" value="utf-8" />
		<property name="freemarkerSettings">
			<props>
				<prop key="locale">zh_CN</prop>
			</props>
		</property>
	</bean>
	<bean class="org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver">
		<property name="suffix" value=".html" />
		<property name="contentType" value="text/html;charset=utf-8" />
		<property name="requestContextAttribute" value="request" />
		<property name="exposeRequestAttributes" value="true" />
		<property name="exposeSessionAttributes" value="true" />
	</bean>

这里是以FreeMarkerViewResolver为例来说明，它的配置内容还是需要有待继续研究。这里只是粗略的说下它的继承情况。
FreeMarkerViewResolver继承AbstractTemplateViewResolver继承UrlBasedViewResolver继承AbstractCachingViewResolver。
首先是抽象类AbstractCachingViewResolver：它加入了缓存功能，它有几个重要的属性。

/** Default maximum number of entries for the view cache: 1024 */
	public static final int DEFAULT_CACHE_LIMIT = 1024;

	/** The maximum number of entries in the cache */
	private volatile int cacheLimit = DEFAULT_CACHE_LIMIT;

	

	/** Fast access cache for Views, returning already cached instances without a global lock */
	private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);

	/** Map from view key to View instance, synchronized for View creation */
	@SuppressWarnings("serial")
	private final Map<Object, View> viewCreationCache =
			new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {
				@Override
				protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {
					if (size() > getCacheLimit()) {
						viewAccessCache.remove(eldest.getKey());
						return true;
					}
					else {
						return false;
					}
				}
			};

属性一：cacheLimit 最大的缓存数量，默认为1024。
属性二：viewAccessCache 是ConcurrentHashMap类型的，适合高并发。
属性三：viewCreationCache是LinkedHashMap类型的
我们再来看下，由view名称来解析到view视图对象的具体过程：

public View resolveViewName(String viewName, Locale locale) throws Exception {
//这里进行了是否进行缓存的判断，即cacheLimit是否大于0
		if (!isCache()) {
                        //不进行缓存，始终每次都创建
			return createView(viewName, locale);
		}
		else {
                        //viewAccessCache viewCreationCache两者的key
			Object cacheKey = getCacheKey(viewName, locale);
			View view = this.viewAccessCache.get(cacheKey);
			if (view == null) {
				synchronized (this.viewCreationCache) {
					view = this.viewCreationCache.get(cacheKey);
					if (view == null) {
						// Ask the subclass to create the View object.
						view = createView(viewName, locale);
						if (view == null && this.cacheUnresolved) {
							view = UNRESOLVED_VIEW;
						}
						if (view != null) {
							this.viewAccessCache.put(cacheKey, view);
							this.viewCreationCache.put(cacheKey, view);
							if (logger.isTraceEnabled()) {
								logger.trace("Cached view [" + cacheKey + "]");
							}
						}
					}
				}
			}
			return (view != UNRESOLVED_VIEW ? view : null);
		}
	}

对于Object cacheKey = getCacheKey(viewName, locale);默认为viewName + "_" + locale;
但是可以被子类覆盖，子类UrlBasedViewResolver覆盖了它，变成只有viewName。

先从viewAccessCache中看能否找到已缓存的view视图，若能找到则返回。若未找到则加上同步锁synchronized (this.viewCreationCache)，进入这个方法之后，最关键的是仍需要进行一次判断view = this.viewCreationCache.get(cacheKey)，看看是否已经创建过了，并不是viewAccessCache和viewCreationCache他们所缓存的内容不一样而是如果没有这个判断，则会有多线程问题。

如线程1和线程2同时要解析相同的view名称，他们都来到同步锁synchronized (this.viewCreationCache)之前，线程2先拿到锁，线程1等待，线程2创建好view视图后，加入viewCreationCache和viewAccessCache，并释放锁。此时线程1获得锁，进入同步锁synchronized (this.viewCreationCache)内部，若不进行判断，则线程1又会去创建一次view视图。所以view = this.viewCreationCache.get(cacheKey)并判断view是否为null这一步骤是十分有用的。

创建View视图的任务就交给了子类来实现。resolveViewName这个方法基本上就分析完了，应该还会想到，它的那个cacheLimit限制好像还没发挥出作用。
继续回看

private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT);

	private final Map<Object, View> viewCreationCache =
			new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {
				@Override
				protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {
					if (size() > getCacheLimit()) {
						viewAccessCache.remove(eldest.getKey());
						return true;
					}
					else {
						return false;
					}
				}
			};

viewCreationCache 的类型是LinkedHashMap，但是它复写了protected boolean removeEldestEntry(Map.Entry<Object, View> eldest)方法，当该方法返回true时，LinkedHashMap则会删除最老的key。在这里我们可以看到，当viewCreationCache 的所存的View数量达到cacheLimit时，就会删除最老的那个key和value，同时也会使viewAccessCache删除这个key和value。

viewAccessCache主要是用来高并发的访问，viewCreationCache 则是用来统计最老的key。他们所存储的view都是一样的。