SpringMVC源码情操陶冶-ViewResolver视图解析

简单分析springmvc是如何解析view视图，并返回页面给前端

SpringMVC配置视图解析器

<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
	<property key="prefix" value="/WEB-INF/jsp/"/>
	<property key="suffix" value=".jsp" />
</bean>	

配置的为jsp的解析器

ViewResolver接口

其内部只有一个接口方法，具体如下

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

由上可知其是通过解析ViewName来得到View视图对象

ViewResolver实现类-BeanNameViewResolver

其意图是参数viewName就是springmvc上下文中的beanName对象，具体源码如下

	@Override
	public View resolveViewName(String viewName, Locale locale) throws BeansException {
		//获得springmvc上下文
		ApplicationContext context = getApplicationContext();
		if (!context.containsBean(viewName)) {
			//viewName不存在，则直接返回null
			return null;
		}
		if (!context.isTypeMatch(viewName, View.class)) {
			//viewName对应的beanName不是View.class的实现类，则直接返回
			return null;
		}
		return context.getBean(viewName, View.class);
	}

BeanNameResolver表示返回的viewName必须是springmvc上下文中的beanName并且对应的实体类必须是View.class的实现类，否则返回null

ViewResolver实现类-AbstractCachingViewResolver

其是我们常用解析器的抽象类，比如Freemarker/Groovy，我们直接去看其实现的接口方法，具体源码如下

	@Override
	public View resolveViewName(String viewName, Locale locale) throws Exception {
		//不采用缓存方案，则每次都进行创建View
		if (!isCache()) {
			return createView(viewName, locale);
		}
		else {
			//此处为使用缓存情况下的获取View对象
			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) {
							//默认会返回一个null的View对象
							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);
		}
	}

接以上代码我们接着分析AbstractCachingViewResolver#createView()方法

	protected View createView(String viewName, Locale locale) throws Exception {
		//此处的loadView便是模板方法，供子类去实现
		return loadView(viewName, locale);
	}

此处我们只分析其某个实现类UrlBasedViewResolver

UrlBasedViewResolver-对应请求url的解析器

1. 常用的内部属性

	public static final String REDIRECT_URL_PREFIX = "redirect:";
	
	public static final String FORWARD_URL_PREFIX = "forward:";
	//设置前缀
	private String prefix = "";
	//设置后缀
	private String suffix = "";
	//设置指定的viewName集合
	private String[] viewNames ;

2. createView()复写父类方法，即在创建view对象前做下跳转的请求检查

	@Override
	protected View createView(String viewName, Locale locale) throws Exception {
		//viewNames集合为null或者对应的viewName在viewNames集合内则返回true
		if (!canHandle(viewName, locale)) {
			return null;
		}
		// Check for special "redirect:" prefix.
		//检查handler返回的值为string类型时是否包含"redirect:"前缀
		//此处处理的便是跳转请求，比如"redirect:/user/list"
		if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
			String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
			RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
			return applyLifecycleMethods(viewName, view);
		}
		// Check for special "forward:" prefix.
		//同"redirect:"请求，此处为服务端直接跳转
		if (viewName.startsWith(FORWARD_URL_PREFIX)) {
			String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
			return new InternalResourceView(forwardUrl);
		}
		//通过父类再去调用loadView()方法，其实是本类也复写了此方法
		return super.createView(viewName, locale);
	}

3. loadView()复写方法

	@Override
	protected View loadView(String viewName, Locale locale) throws Exception {
		//创建View对象
		AbstractUrlBasedView view = buildView(viewName);
		//将view对象与viewName绑定注册至springmvc上下文中
		View result = applyLifecycleMethods(viewName, view);
		return (view.checkResource(locale) ? result : null);
	}

4. buildView()创建View对象主逻辑

	protected AbstractUrlBasedView buildView(String viewName) throws Exception {
		//获取类似FreemarkerView.class/GroovyView.class
		AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(getViewClass());
		//设置view对应的资源路径，此处便可知我们设置prefix和suffix的作用
		view.setUrl(getPrefix() + viewName + getSuffix());
		
		//下面都是设置与UrlBasedViewResolver的相关内部属性
		String contentType = getContentType();
		if (contentType != null) {
			view.setContentType(contentType);
		}

		view.setRequestContextAttribute(getRequestContextAttribute());
		view.setAttributesMap(getAttributesMap());
		//是否暴露路径变量
		Boolean exposePathVariables = getExposePathVariables();
		if (exposePathVariables != null) {
			view.setExposePathVariables(exposePathVariables);
		}
		//是否暴露springmvc的bean对象作为属性使用
		Boolean exposeContextBeansAsAttributes = getExposeContextBeansAsAttributes();
		if (exposeContextBeansAsAttributes != null) {
			view.setExposeContextBeansAsAttributes(exposeContextBeansAsAttributes);
		}
		String[] exposedContextBeanNames = getExposedContextBeanNames();
		if (exposedContextBeanNames != null) {
			view.setExposedContextBeanNames(exposedContextBeanNames);
		}

		return view;
	}

主要作用是设置prefix和suffix参数以及对应的内部属性，可自行查阅，并通过buildView()方法创建ViewClass属性指定的View对象

AbstractTemplateViewResolver-UrlBasedViewResolver子类

添加另外的属性

1. 内部属性

	//是否暴露request对象的attributes属性给前端引擎
	private boolean exposeRequestAttributes = false;
	//是否允许请求处理过程中复写request对象的attributes
	private boolean allowRequestOverride = false;
	//是否暴露session对象的attributes属性给前端引擎
	private boolean exposeSessionAttributes = false;
	//是否允许请求处理过程中复写session对象的attributes
	private boolean allowSessionOverride = false;
	//是否使用暴露springMacroRequestContext属性
	private boolean exposeSpringMacroHelpers = true;

2. buildView()复写方法，主要是额外添加以上的属性

	@Override
	protected AbstractUrlBasedView buildView(String viewName) throws Exception {
		//先调用父类的创建方法创建View对象
		AbstractTemplateView view = (AbstractTemplateView) super.buildView(viewName);
		//设置AbstractTemplateView的内部属性
		view.setExposeRequestAttributes(this.exposeRequestAttributes);
		view.setAllowRequestOverride(this.allowRequestOverride);
		view.setExposeSessionAttributes(this.exposeSessionAttributes);
		view.setAllowSessionOverride(this.allowSessionOverride);
		view.setExposeSpringMacroHelpers(this.exposeSpringMacroHelpers);
		return view;
	}

在UrlBasedViewResolver的基础上设置额外的属性，属性集合见上文

FreeMarkerViewResolver-AbstractTemplateViewResolver子类

为方便理解，我们选取常用的模板引擎Freemarker，其他的引擎工具则供读者自行分析

1. 构造函数-设置viewClass，满足上述的模板方法的viewClass的获取

	public FreeMarkerViewResolver() {
		//设置的为FreemarkView.class
		setViewClass(requiredViewClass());
	}

小结

本节只解析了ViewResolver的简单逻辑，其根据配置的ViewClass属性，将配置的其他属性都设置到ViewClass对应的实例中，具体的关于视图的渲染，也就是view#render()方法我们放在下节讲解