SPRING技术内幕-笔记（九）SpringMVC视图的呈现

9.1 DispatcherServlet视图呈现的设计
衔接上一章DisPatcherServlet的doDispatch,执行完handler之后得到ModelAndView对象，然后视图的展现交给了render方法：

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.
		 //如果配置了String类型的view名字，那么就解析出来
		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.
		//视图对象已经在ModelAndView 对象里边，直接拿来用
		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() + "'");
	}
	//不同的视图类型调用各自的render方法
	view.render(mv.getModelInternal(), request, response);
}

上面解析根据名字解析视图的方法resolveViewName：

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;
}

viewResolver.resolveViewName(viewName, locale)可以看子类BeanNameViewResolver的实现：

public View resolveViewName(String viewName, Locale locale) throws BeansException {
	ApplicationContext context = getApplicationContext();
	if (!context.containsBean(viewName)) {
		// Allow for ViewResolver chaining.
		return null;
	}
	return (View) context.getBean(viewName, View.class);
}

可以看出直接是从山下取出来的。
回到各种视图的解析SPringMVC有各种视图的支持，PDF，JSTL，等等，使用了第三方框架，从而让视图丰富多彩。
这是SPring的视图类型结构图：

9.2JSP视图的实现

在jsp中可以使用JSTL表达式来丰富jsp页面，在springmvc中需要Jstlview来作为view对象。
Jstlview没有实现render方法，其使用了基类的render方法：

public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
	if (logger.isTraceEnabled()) {
		logger.trace("Rendering view with name '" + this.beanName + "' with model " + model +
			" and static attributes " + this.staticAttributes);
	}
	//将所有的数据放到一个Map里边
	Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);

	prepareResponse(request, response);
	//展现模型数据到对应视图的方法
	renderMergedOutputModel(mergedModel, request, response);
}

renderMergedOutputModel使用的是模板方法，在InternalResourceView中，InternalResourceView也是Jstlview的基类，代码：

protected void renderMergedOutputModel(
		Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

	// Determine which request handle to expose to the RequestDispatcher.
	//判断将那个请求的处理交给RequestDispatcher
	HttpServletRequest requestToExpose = getRequestToExpose(request);

	// Expose the model object as request attributes.
	//对数据进行处理，放到servletcontext中
	exposeModelAsRequestAttributes(model, requestToExpose);

	// Expose helpers as request attributes, if any.
	exposeHelpers(requestToExpose);

	// Determine the path for the request dispatcher.
	//得到InternalResourceView定义的内部资源路径
	String dispatcherPath = prepareForRendering(requestToExpose, response);

	// Obtain a RequestDispatcher for the target resource (typically a JSP).
	 //把请求转发到前面获取的资源路径中去
	RequestDispatcher rd = getRequestDispatcher(requestToExpose, dispatcherPath);
	if (rd == null) {
		throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
				"]: Check that the corresponding file exists within your web application archive!");
	}

	// If already included or response already committed, perform include, else forward.
	if (useInclude(requestToExpose, response)) {
		response.setContentType(getContentType());
		if (logger.isDebugEnabled()) {
			logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
		}
		rd.include(requestToExpose, response);
	}

	else {
		// Note: The forwarded resource is supposed to determine the content type itself.
		//转发请求到之前内部定义好的资源上去，比如jsp页面，jsp页面的展现是有容器负责，在这种情况下，view只是起到转发  
		//请求的作用
		exposeForwardRequestAttributes(requestToExpose);
		if (logger.isDebugEnabled()) {
			logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
		}
		rd.forward(requestToExpose, response);
	}
}

exposeModelAsRequestAttributes(model, requestToExpose)方法是处理数据的关键，在AbstractView中他把ModelAndView中的数据以及其他请求的数据放到HttpServletRequest中去，这样就能在页面得到这些值。

protected void exposeModelAsRequestAttributes(Map<String, Object> model, HttpServletRequest request) throws Exception {
	for (Map.Entry<String, Object> entry : model.entrySet()) {
		String modelName = entry.getKey();
		Object modelValue = entry.getValue();
		if (modelValue != null) {
			request.setAttribute(modelName, modelValue);
			if (logger.isDebugEnabled()) {
				logger.debug("Added model object '" + modelName + "' of type [" + modelValue.getClass().getName() +
						"] to request in view with name '" + getBeanName() + "'");
			}
		}
		else {
			request.removeAttribute(modelName);
			if (logger.isDebugEnabled()) {
				logger.debug("Removed model object '" + modelName +
						"' from request in view with name '" + getBeanName() + "'");
			}
		}
	}
}

exposeHelpers包含所有关于 jstl的相关处理：

protected void exposeHelpers(HttpServletRequest request) throws Exception {
	if (this.messageSource != null) {
		JstlUtils.exposeLocalizationContext(request, this.messageSource);
	}
	else {
		JstlUtils.exposeLocalizationContext(new RequestContext(request, getServletContext()));
	}
}

得到转发路径后InternalResourceView进行重定向，那么这个路径的获得如下：

protected String prepareForRendering(HttpServletRequest request, HttpServletResponse response)
		throws Exception {

	String path = getUrl();
	if (this.preventDispatchLoop) {
	//从request中获取URL路径
		String uri = request.getRequestURI();
		if (path.startsWith("/") ? uri.equals(path) : uri.equals(StringUtils.applyRelativePath(uri, path))) {
			throw new ServletException("Circular view path [" + path + "]: would dispatch back " +
					"to the current handler URL [" + uri + "] again. Check your ViewResolver setup! " +
					"(Hint: This may be the result of an unspecified view, due to default view name generation.)");
		}
	}
	return path;
}

得到路径然后进行转发。

9.3 ExcelView的实现
实现类AbstractExcelView,springMvc使用了Apache开源的POI完成对excel的创建
主要实现：

protected final void renderMergedOutputModel(
		Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

	HSSFWorkbook workbook;
	if (this.url != null) {
	//使用一游模板创建HSSFWorkbook 对象，模板来自URL
		workbook = getTemplateSource(this.url, request);
	}
	else {
	//直接New
		workbook = new HSSFWorkbook();
		logger.debug("Created Excel Workbook from scratch");
	}
       //此方法留给应用重写，完成根据去填充这张表格
	buildExcelDocument(model, workbook, request, response);

	// Set the content type.
	response.setContentType(getContentType());

	// Should we set the content length here?
	// response.setContentLength(workbook.getBytes().length);

	// Flush byte array to servlet output stream.
	//输出到页面展现
	ServletOutputStream out = response.getOutputStream();
	workbook.write(out);
	out.flush();
}

从已有的文件创建 workbook对象：

protected HSSFWorkbook getTemplateSource(String url, HttpServletRequest request) throws Exception {
		LocalizedResourceHelper helper = new LocalizedResourceHelper(getApplicationContext());
		Locale userLocale = RequestContextUtils.getLocale(request);
		Resource inputFile = helper.findLocalizedResource(url, EXTENSION, userLocale);

		// Create the Excel document from the source.
		if (logger.isDebugEnabled()) {
			logger.debug("Loading Excel workbook from " + inputFile);
		}
		//创建excel文本对象
		POIFSFileSystem fs = new POIFSFileSystem(inputFile.getInputStream());
		return new HSSFWorkbook(fs);
	}

一下是一个举例解释对AbstractExcelView的使用：

public void testExcelWithTemplateAndLanguage() throws Exception {
	request.setAttribute(DispatcherServlet.LOCALE_RESOLVER_ATTRIBUTE,
			newDummyLocaleResolver("de", ""));

	AbstractExcelView excelView = new AbstractExcelView() {
	 //实现基类的buildExcelDocument
		protected void buildExcelDocument(Map model, HSSFWorkbook wb,
				HttpServletRequest request, HttpServletResponse response)
				throws Exception {
			HSSFSheet sheet = wb.getSheet("Sheet1");

			// test all possible permutation of row or column not existing
			HSSFCell cell = getCell(sheet, 2, 4);
			cell.setCellValue("Test Value");
			cell = getCell(sheet, 2, 3);
			setText(cell, "Test Value");
			cell = getCell(sheet, 3, 4);
			setText(cell, "Test Value");
			cell = getCell(sheet, 2, 4);
			setText(cell, "Test Value");
		}
	};

	excelView.setApplicationContext(webAppCtx);
	excelView.setUrl("template");
	//手动调用render，触发视图的呈现
	excelView.render(new HashMap(), request, response);

	POIFSFileSystem poiFs = new POIFSFileSystem(new ByteArrayInputStream(response.getContentAsByteArray()));
	HSSFWorkbook wb = new HSSFWorkbook(poiFs);
	HSSFSheet sheet = wb.getSheet("Sheet1");
	HSSFRow row = sheet.getRow(0);
	HSSFCell cell = row.getCell((short) 0);
	assertEquals("Test Template auf Deutsch", cell.getStringCellValue());
}

9.4 PDF视图的实现

实现类AbstractPdfView，主要的工程springMVC使用了itext：

protected final void renderMergedOutputModel(
		Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

	// IE workaround: write into byte array first.
	ByteArrayOutputStream baos = createTemporaryOutputStream();

	// Apply preferences and build metadata.
	//创建itext与PDF文件操作相关联的对象
	Document document = newDocument();
	PdfWriter writer = newWriter(document, baos);
	prepareWriter(model, writer, request);
	buildPdfMetadata(model, document, request);

	// Build PDF document.
	//根据应用业务构建对象，填充内容，buildPdfDocument有子类去实现
	document.open();
	buildPdfDocument(model, document, writer, request, response);
	document.close();

	// Flush to HTTP response.
	writeToResponse(response, baos);
}

SpringMvc提供的测试数据：

public void testPdf() throws Exception {
	final String text = "this should be in the PDF";
	MockHttpServletRequest request = new MockHttpServletRequest();
	MockHttpServletResponse response = new MockHttpServletResponse();

	AbstractPdfView pdfView = new AbstractPdfView() {
		protected void buildPdfDocument(Map model, Document document, PdfWriter writer,
				HttpServletRequest request, HttpServletResponse response) throws Exception {
			document.add(new Paragraph(text));
		}
	};

	pdfView.render(new HashMap(), request, response);
	byte[] pdfContent = response.getContentAsByteArray();
	assertEquals("correct response content type", "application/pdf", response.getContentType());
	assertEquals("correct response content length", pdfContent.length, response.getContentLength());

	// rebuild iText document for comparison
	Document document = new Document(PageSize.A4);
	ByteArrayOutputStream baos = new ByteArrayOutputStream();
	PdfWriter writer = PdfWriter.getInstance(document, baos);
	writer.setViewerPreferences(PdfWriter.AllowPrinting | PdfWriter.PageLayoutSinglePage);
	document.open();
	document.add(new Paragraph(text));
	document.close();
	byte[] baosContent = baos.toByteArray();
	assertEquals("correct size", pdfContent.length, baosContent.length);

	int diffCount = 0;
	for (int i = 0; i < pdfContent.length; i++) {
		if (pdfContent[i] != baosContent[i]) {
			diffCount++;
		}
	}
	assertTrue("difference only in encryption", diffCount < 70);
}