spring-mvc 视图模式之freemarker整合解析

梦里梦外; 2022-01-14 13:37 442阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

spring-mvc 版本4.04

今天翻项目中freemarker相关代码，疑惑springmvc是怎么发现freemarker的，于是单步进去。  
DispatcherServlet的doDispatch方法里有这么一句：

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

可以猜到，这是handler业务处理后，到渲染页面的阶段了，也就是freemarker该出场的时候了。

进入processDispatchResult这个方法，看到这么一段

// Did the handler return a view to render? 返回一个view 去渲染
    if (mv != null && !mv.wasCleared()) {
    	render(mv, request, response);//就是渲染了
    	if (errorView) {
    		WebUtils.clearErrorRequestAttributes(request);
    	}
    }

然后render方法，看到

View view;
    if (mv.isReference()) {//mv 是String类型的，比如url
    	// We need to resolve the view name.
    	view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);//获取来渲染页面的view
    	if (view == null) {
    		throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
    				"' in servlet with name '" + getServletName() + "'");
    	}
    }

看到resolveViewName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);
    		//创建根据viewName（其实就是controller里返回的url，比如/wiew/test.ftl）创建view
    		//那随便一个url都能有相应的view吗？当然不是。比如要先匹配，这个下面说，
    		if (view != null) {
    			return view;
    		}
    	}
    	return null;
    }

原来view解决方案已经都存在viewResolvers对象里了，那什么时候存的呢，代码里搜索下，找到如下方法：

/**
     * Initialize the ViewResolvers used by this class.
     * <p>If no ViewResolver beans are defined in the BeanFactory for this
     * namespace, we default to InternalResourceViewResolver.
     */
    private void initViewResolvers(ApplicationContext context) {
    	this.viewResolvers = null;
    
    	if (this.detectAllViewResolvers) {//boolean值，是否自动检测所有的ViewResolver，
    		// Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
    		//是自动检测，就用BeanFactoryUtils的beansOfTypeIncludingAncestors方法，找所有的ViewResolver
    		//可以看到这个方法很有用，在项目也可以用，，可你找到所有ViewResolver.class类型或子类的bean，很好用。
    //这也体现了mvc框架的v的部分。任何实现了ViewResolver接口的类，都可作为视图用
    		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);//如果不自动检测
    			//String VIEW_RESOLVER_BEAN_NAME = "viewResolver";  就找名字为viewResolver的bean作为ViewResolver
    			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.
            //以上都没找到视图，只有获取一个默认的。
    	if (this.viewResolvers == null) {
    		this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
    		if (logger.isDebugEnabled()) {
    			logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default");
    		}
    	}
    }

initViewResolvers这个方法，是在onRefresh的调用的，其实是重写了父类的方法，  
在spring的初始化时自动被调用。spring模板方法的神力开始起作用了。

protected void onRefresh(ApplicationContext context) {
    	initStrategies(context);
    }

\--------------------------------------------------------------------------  
以上是spring-mvc怎么发现第三方viewResolver（不限freemarker）的，然后看看，请求来的url怎么找到匹配的view的，接着说上面遗留的问题  
再看这句代码：

View view = viewResolver.resolveViewName(viewName, locale);

这个方法是org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver父类AbstractCachingViewResolver实现的。  
跟进去：

@Override
    public View resolveViewName(String viewName, Locale locale) throws Exception {
    	if (!isCache()) {//缓存无处不在
    		return createView(viewName, locale);//跟进去
    	}
    	else {
    		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);
    	}
    }

//由于UrlBasedViewResolver类重新写了这个方法所以，是UrlBasedViewResolver的createView方法
    @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.
    	//就在这里，代码首先检查resolver是否支持这个viewName（url）
    	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());
    		RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
    		return applyLifecycleMethods(viewName, view);
    	}
    	// 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);
    }
    //在这里
    protected boolean canHandle(String viewName, Locale locale) {
    	String[] viewNames = getViewNames();
    	//viewNames就是在xml文件里配置的如，freemarker
    	//<property name="viewNames">
    	//<array>
    	//<value>*.ftl</value>
    		//<value>*.html</value>
    	//</array>
    	//</property>
    	return (viewNames == null || PatternMatchUtils.simpleMatch(viewNames, viewName));//做模式匹配
    }

最后类图，注意方法的重写。子类总是调用最近上级节点方法。

![143058_THea_146130.jpg][]

转载于:https://my.oschina.net/u/146130/blog/997764

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220114/7551568dddd74440ba4c4d338fa82816.png
[143058_THea_146130.jpg]: https://static.oschina.net/uploads/space/2017/0623/143058_THea_146130.jpg