SpringMVC源码情操陶冶-View视图渲染

本节简单分析View视图对象的render方法

View接口

最重要的就是render()方法，具体源码如下

	/**
	 * 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
	 */
	void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception;

下面我们就此接口方法展开分析

AbstractView

直接看源码

	@Override
	public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
		//综合model、内部属性staticAttributes和request对象中的View.PATH_VARIABLES，都封装在同一个Map集合中
		Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
		//为response做准备，默认是针对download请求
		prepareResponse(request, response);
		//真实处理render操作，供子类实现调用
		renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
	}

AbstractTemplateView-AbstractView的子类

抽象模板视图，源码如下

	@Override
	protected final void renderMergedOutputModel(
			Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
		//判断是否暴露request属性给前端，是则将所有的request属性加到model中
		if (this.exposeRequestAttributes) {
			for (Enumeration<String> en = request.getAttributeNames(); en.hasMoreElements();) {
				String attribute = en.nextElement();
				//当不允许request对象中的属性被覆盖且model存在相同key时，会抛异常
				if (model.containsKey(attribute) && !this.allowRequestOverride) {
					throw new ServletException("Cannot expose request attribute '" + attribute +
						"' because of an existing model object of the same name");
				}
				//允许则直接通过
				Object attributeValue = request.getAttribute(attribute);
				
				model.put(attribute, attributeValue);
			}
		}
		//等同request
		if (this.exposeSessionAttributes) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				for (Enumeration<String> en = session.getAttributeNames(); en.hasMoreElements();) {
					String attribute = en.nextElement();
					if (model.containsKey(attribute) && !this.allowSessionOverride) {
						throw new ServletException("Cannot expose session attribute '" + attribute +
							"' because of an existing model object of the same name");
					}
					Object attributeValue = session.getAttribute(attribute);
					if (logger.isDebugEnabled()) {
						logger.debug("Exposing session attribute '" + attribute +
								"' with value [" + attributeValue + "] to model");
					}
					model.put(attribute, attributeValue);
				}
			}
		}
		//设置SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE属性到model中
		if (this.exposeSpringMacroHelpers) {
			if (model.containsKey(SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE)) {
				throw new ServletException(
						"Cannot expose bind macro helper '" + SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE +
						"' because of an existing model object of the same name");
			}
			// Expose RequestContext instance for Spring macros.
			model.put(SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE,
					new RequestContext(request, response, getServletContext(), model));
		}
		
		//设置返回给前端的内容类型，可在ViewResolver中设置contentType属性
		applyContentType(response);

		//抽象方法供子类调用实现
		renderMergedTemplateModel(model, request, response);
	}

由以上代码分析可得，此类主要是判断是否将request和session的对象中的所有属性添加到Map类型的model对象中
判断条件为：

1. exposeRequestAttributes设置为true，表明将request中的Attributes属性添加到model中，这样前端可以直接引用request对象中的attribute属性。其默认为false
2. exposeSessionAttributes设置为true，表明将session中的Attributes属性添加到model中，这样前端可以直接引用session对象中的attribute属性。其默认为false
3. allowRequestOverride/allowSessionOverride设置为true，表明是否允许request/session对象中的attribute属性覆盖model中的同key键。其默认为false，在出现上述情况则会抛出异常

FreeMakerView-AbstractTemplateView实现类

关于AbstractTemplateView的实现类有很多，本文则选取常用的FreemarkerView来进行简析

FreeMarkerView#initServletContext()-初始化方法

此方法在父类WebApplicationObjectSupport#initApplicationContext()中被调用

	protected void initServletContext(ServletContext servletContext) throws BeansException {
		if (getConfiguration() != null) {
			this.taglibFactory = new TaglibFactory(servletContext);
		}
		else {
			//查询springmvc上下文中是否存在FreeMarkerConfig接口bean，其一般是通过FreeMarkerConfigurer来注册的
			//如果不存在FreeMarkerConfig bean对象则会抛异常
			FreeMarkerConfig config = autodetectConfiguration();
			setConfiguration(config.getConfiguration());
			this.taglibFactory = config.getTaglibFactory();
		}
		
		//初始servlet对象，其实是创建ServletContextResourceLoader对象，帮助可以获取"/WEB-INF/"下的资源
		GenericServlet servlet = new GenericServletAdapter();
		try {
			servlet.init(new DelegatingServletConfig());
		}
		catch (ServletException ex) {
			throw new BeanInitializationException("Initialization of GenericServlet adapter failed", ex);
		}
		this.servletContextHashModel = new ServletContextHashModel(servlet, getObjectWrapper());
	}

此处主要获取FreeMarkerConfig对象，其一般由FreeMarkerConfigurer对象生成，可指定加载资源的路径和设置输出的一些属性，且这是必须注册到springmvc的bean工厂的

FreeMarkerView#renderMergedTemplateModel()-渲染视图

源码奉上

	protected void renderMergedTemplateModel(
			Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
		//默认为空，待实现
		exposeHelpers(model, request);
		//启动渲染程序
		doRender(model, request, response);
	}

FreeMarkerView#doRender()-渲染操作

源码奉上

	protected void doRender(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
		// 这里其实是将model中的所有属性都注册到request对象中
		exposeModelAsRequestAttributes(model, request);
		// Expose all standard FreeMarker hash models.将FreeMarker的属性放到一个Map中
		SimpleHash fmModel = buildTemplateModel(model, request, response);

		//此处的getUrl()由ViewResolver来设定，为prefix+viewName+suffix		
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering FreeMarker template [" + getUrl() + "] in FreeMarkerView '" + getBeanName() + "'");
		}
		// Grab the locale-specific version of the template.
		Locale locale = RequestContextUtils.getLocale(request);
		//此处的操作比较复杂，需要慢慢分析
		processTemplate(getTemplate(locale), fmModel, response);
	}

上述的代码具体解析分为两部分：模板对象Template获取和Template对象的处理方法process()，下面将从这两部分进行简单的展开

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1.FreeMarkerView#getTemplate()
获取模板对象

	protected Template getTemplate(Locale locale) throws IOException {
		//templateName为prefix+viewName+suffix
		return getTemplate(getUrl(), locale);
	}

	protected Template getTemplate(String name, Locale locale) throws IOException {
		//最终是通过FreeMarkerConfigurer的内部属性Configuration来获取模板对象
		return (getEncoding() != null ?
				getConfiguration().getTemplate(name, locale, getEncoding()) :
				getConfiguration().getTemplate(name, locale));
	}

此处限于FreeMaker相关类Configuration获取Template模板对象的步骤过于冗长，这里作下总结，具体读者可自行去阅读源码分析

• 获取Template对象是由TemplateCache来实现的，其是通过prefix+viewName+suffix作为整个文件名去找寻，其中prefix参数值不能以/为开头，这在FreeMarker/Velocity是约定的，而jsp页面引擎InternalView支持/开头的prefix，这点需要区别开来

• 具体的获取资源文件是通过Configuration指定的templateLoader去加载实际的资源，一般此处的templateLoader为SpringTemplateLoader，支持templateLoaderPath为classpath:搜索

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2.FreeMarker#process()

1. 此处主要是通过获取到的Template对象，对其持有的实际资源进行读取渲染后再重新写入以完成${}这样字符的含义

2. model中所有的数据都会保留到request的Attributes对象中，所以FreeMarker可直接通过${attribute}获取相应的参数值

小结

1. ViewResolver帮助我们设置好对应的视图解析器比如FreeMarkerView，包含加载资源的prefix/suffix，以及是否将request/session对象属性绑定到model对象中

2. 本文则以FreeMarkerView如何解析资源入手，简单的分析其中的操作逻辑，具体的细节读者可自行查询