前言
上一篇文章介绍了SpringMvc的ControllerAdvice和ExceptionHandler，这里在介绍一下ViewResolver的使用，并介绍一下HandlerMethodReturnValueHandler和ViewResolver的关系。
ViewResolver和HandlerMethodReturnValueHandler
自定义ResponseBody这篇文章介绍过ResponseBody的编码规则，ViewResolver和ResponseBody是明显互斥的。
这两个不同类型的返回值就是通过不同的HandlerMethodReturnValueHandler来是实现的。
先看RequestResponseBodyMethodProcessor，这里设置了setRequestHandled为true，然后通过HttpMessageConverters编码对应的model。
public void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
mavContainer.setRequestHandled(true);
// Try even with null return value. ResponseBodyAdvice could get involved.
writeWithMessageConverters(returnValue, returnType, webRequest);
}
再看ViewNameMethodReturnValueHandler，这里没有设置setRequestHandled，而是取出CharSequence类型的返回值，并赋值viewName。
public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
if (returnValue instanceof CharSequence) {
String viewName = returnValue.toString();
mavContainer.setViewName(viewName);
if (isRedirectViewName(viewName)) {
mavContainer.setRedirectModelScenario(true);
}
}
else if (returnValue != null){
// should not happen
throw new UnsupportedOperationException("Unexpected return type: " + returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
}
}
再看RequestMappingHandlerAdapter中使用方法，假如mavContainer.isRequestHandled是true直接返回null，后面就不会调用ViewResolver了。假如是false，会取出mavContainer.getViewName，返回ModelAndView，后面会根据ViewName进行模板的映射。
private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {
modelFactory.updateModel(webRequest, mavContainer);
if (mavContainer.isRequestHandled()) {
return null;
}
ModelMap model = mavContainer.getModel();
ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model);
if (!mavContainer.isViewReference()) {
mav.setView((View) mavContainer.getView());
}
if (model instanceof RedirectAttributes) {
Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
}
return mav;
}
自定义ViewResolver
相比ExceptionHandler，自定义ViewResolver就比较简单了，只要注入一个ViewResolver的实现类就可以了。
不过为了介绍ViewResolver的原理，这里自定义了一个HandlerMethodReturnValueHandler来取代ViewNameMethodReturnValueHandler;
完整的代码还是在Github上了。
- 定义Controller。ViewName自定义类来包装viewName。
@Controller
public static class ControllerClass {
@RequestMapping
public ViewName index(ModelMap modelMap) {
modelMap.put("message", "hello world");
ViewName viewName = new ViewName();
viewName.setName("index");
return viewName;
}
@RequestMapping("html")
public ViewName htmlIndex(ModelMap modelMap) {
modelMap.put("message", "hello world");
ViewName viewName = new ViewName();
viewName.setName("html");
return viewName;
}
}
public static class ViewName {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
- 注入自定义的ViewResolvers和HandlerMethodReturnValueHandler
@Configuration
public static class MyWebMvcConfigurationSupport extends WebMvcConfigurationSupport {
public void configureViewResolvers(ViewResolverRegistry registry) {
MyViewResolver myViewResolver = new MyViewResolver();
registry.viewResolver(myViewResolver);
}
public void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers) {
returnValueHandlers.add(new MyHandlerMethodReturnValueHandler());
}
}
- 自定义HandlerMethodReturnValueHandler，指定只支持ViewName这个类，处理时，取出来ViewName的name，设置到ModelAndViewContainer中。
public static class MyHandlerMethodReturnValueHandler implements HandlerMethodReturnValueHandler {
public boolean supportsReturnType(MethodParameter returnType) {
return returnType.getParameterType().isAssignableFrom(ViewName.class);
}
public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
ViewName viewName = (ViewName) returnValue;
mavContainer.setViewName(viewName.getName());
}
}
- 自定义ViewResolver，定义了MyView和MyHtmlView一个处理index返回纯文字，另一个处理html返回html格式。
public static class MyViewResolver implements ViewResolver {
private View htmlView = new MyHtmlView();
private View view = new MyView();
public View resolveViewName(String viewName, Locale locale) throws Exception {
if (viewName.equals("index")) {
return view;
} else if (viewName.equals("html")) {
return htmlView;
}
return null;
}
}
public static class MyView implements View {
public String getContentType() {
return "text/html";
}
public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
StringBuilder stringBuilder = new StringBuilder();
for (Map.Entry<String, ?> entry : model.entrySet()) {
stringBuilder.append(entry.getKey()).append(":").append(entry.getValue());
}
response.getWriter().write(stringBuilder.toString());
}
}
public static class MyHtmlView implements View {
public String getContentType() {
return "text/html";
}
public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
String head = "<html><head><title>Hello World</title></head><body><ul>";
String tail = "</ul></body></html>";
StringBuilder sb = new StringBuilder();
sb.append(head);
for (Map.Entry<String, ?> entry : model.entrySet()) {
sb.append("<li>").append(entry.getKey()).append(":").append(entry.getValue()).append("</li>");
}
sb.append(tail);
response.getWriter().write(sb.toString());
}
}
- 程序入口，这里同时请求了/和/html分别用到了MyView和MyHtmlView。
@Configuration
public class CustomizeViewResolverTest {
public static void main(String[] args) throws ServletException, IOException {
MockServletContext mockServletContext = new MockServletContext();
MockServletConfig mockServletConfig = new MockServletConfig(mockServletContext);
AnnotationConfigWebApplicationContext annotationConfigWebApplicationContext = new AnnotationConfigWebApplicationContext();
annotationConfigWebApplicationContext.setServletConfig(mockServletConfig);
annotationConfigWebApplicationContext.register(CustomizeViewResolverTest.class);
DispatcherServlet dispatcherServlet = new DispatcherServlet(annotationConfigWebApplicationContext);
dispatcherServlet.init(mockServletConfig);
MockHttpServletResponse response = new MockHttpServletResponse();
MockHttpServletRequest request = new MockHttpServletRequest("GET", "/");
dispatcherServlet.service(request, response);
System.out.println(new String(response.getContentAsByteArray()));
MockHttpServletResponse htmlResponse = new MockHttpServletResponse();
MockHttpServletRequest htmlRequest = new MockHttpServletRequest("GET", "/html");
dispatcherServlet.service(htmlRequest, htmlResponse);
System.out.println(new String(htmlResponse.getContentAsByteArray()));
}
}
运行程序就会发现，一个输出了文章，另一个输出了html格式的文字。
结语
ViewResolver和ResponseBody都是用来处理HttpResponseBody的内容的，只不过处理的方式不同。
ResponseBody的编码方式是一样的，一般是处理JSON的编码。ViewResolver还可以一根据ViewName来路由到不用的View，每个View都可以自己的编码规则。
