深刻Spring:自定義ViewResolver

前言

上一篇文章介紹了SpringMvc的ControllerAdvice和ExceptionHandler，這裏在介紹一下ViewResolver的使用，並介紹一下HandlerMethodReturnValueHandler和ViewResolver的關係。

ViewResolver和HandlerMethodReturnValueHandler

自定義ResponseBody這篇文章介紹過ResponseBody的編碼規則，ViewResolver和ResponseBody是明顯互斥的。 這兩個不一樣類型的返回值就是經過不一樣的HandlerMethodReturnValueHandler來是實現的。 先看RequestResponseBodyMethodProcessor，這裏設置了setRequestHandled爲true，而後經過HttpMessageConverters編碼對應的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，這裏沒有設置setRequestHandled，而是取出CharSequence類型的返回值，並賦值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，後面就不會調用ViewResolver了。假如是false，會取出mavContainer.getViewName，返回ModelAndView，後面會根據ViewName進行模板的映射。

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;
	}

自定義ViewResolver

相比ExceptionHandler，自定義ViewResolver就比較簡單了，只要注入一個ViewResolver的實現類就能夠了。 不過爲了介紹ViewResolver的原理，這裏自定義了一個HandlerMethodReturnValueHandler來取代ViewNameMethodReturnValueHandler; 完整的代碼仍是在Github上了。

1. 定義Controller。ViewName自定義類來包裝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. 注入自定義的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. 自定義HandlerMethodReturnValueHandler，指定只支持ViewName這個類，處理時，取出來ViewName的name，設置到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. 自定義ViewResolver，定義了MyView和MyHtmlView一個處理index返回純文字，另外一個處理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());
        }
    }

5. 程序入口，這裏同時請求了*/和/html分別用到了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()));
        }
    }

運行程序就會發現，一個輸出了文章，另外一個輸出了html格式的文字。

結語

ViewResolver和ResponseBody都是用來處理HttpResponseBody的內容的，只不過處理的方式不一樣。 ResponseBody的編碼方式是同樣的，通常是處理JSON的編碼。ViewResolver還能夠一根據ViewName來路由到不用的View，每一個View均可以本身的編碼規則。