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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:

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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:

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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:

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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:

这里写图片描述

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:

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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:

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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。

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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:

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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:

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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:

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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:

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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:

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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:

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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:

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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);
}