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。

1. 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;
        }
    } 

2. 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());
        }
    } 

3. 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());
        }
    } 

4. 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("

"); } sb.append(tail); response.getWriter().write(sb.toString()); } }
5. 程序入口，北京java培训基本都会讲到这些，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。