SpringMVC源码分析(九)--返回值解析器

1.返回值解析器介绍

返回值解析器用于解析Hanlder执行方法后的返回结果，例如将方法上标注有@ResponseBody注解的返回值解析成JSON、将方法返回的字符串作为视图名等

SpringMVC中默认的返回值解析器见RequestMappingHandlerAdapter#getDefaultReturnValueHandlers

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);handlers.add(new ModelAndViewMethodReturnValueHandler());handlers.add(new ModelMethodProcessor());handlers.add(new ViewMethodReturnValueHandler());handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));handlers.add(new StreamingResponseBodyReturnValueHandler());handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new HttpHeadersReturnValueHandler());handlers.add(new CallableMethodReturnValueHandler());handlers.add(new DeferredResultMethodReturnValueHandler());handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));handlers.add(new ModelAttributeMethodProcessor(false));handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new ViewNameMethodReturnValueHandler());handlers.add(new MapMethodProcessor());// 添加自定义的返回值解析器if (getCustomReturnValueHandlers() != null) {handlers.addAll(getCustomReturnValueHandlers());}// 兜底的解析器if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));}else {handlers.add(new ModelAttributeMethodProcessor(true));}return handlers;
}

这些返回值解析器会被封装到HandlerMethodReturnValueHandlerComposite中，处理返回值时，会按照加入的顺序依次判断每个返回值解析器能否解析此任务，如果其中某个解析器能够解析此参数，则返回解析结果

2.自定义返回值解析器

在某些场景下，我们可以自定义返回值解析器，将响应的返回值转成我们希望的格式，假如我们想要将响应转换为yml格式，这时我们可以自定义注解，然后再自定义返回值解析器实现HandlerMethodReturnValueHandler接口，示例如下：

1）创建自定义注解@Yml及自定义返回值处理器YmlReturnResolver实现HandlerMethodReturnValueHandler接口

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Yml {
}public class YmlReturnResolver implements HandlerMethodReturnValueHandler {// 判断返回值解析器是否支持@Overridepublic boolean supportsReturnType(MethodParameter methodParameter) {Yml yml = methodParameter.getMethodAnnotation(Yml.class);return yml != null;}@Overridepublic void handleReturnValue(Object o, MethodParameter methodParameter, ModelAndViewContainer modelAndViewContainer,NativeWebRequest nativeWebRequest) throws Exception {// 将返回值转换为yamlString str = new Yaml().dump(o);// 设置响应内容HttpServletResponse response = nativeWebRequest.getNativeResponse(HttpServletResponse.class);response.setContentType("text/plain;charset=utf-8");response.getWriter().print(str);// 设置请求已处理完毕modelAndViewContainer.setRequestHandled(true);}
}

2）配置类中加入自定义返回值解析器

@Configuration
@ComponentScan
public class WebConfig {@Beanpublic ServletWebServerFactory servletWebServerFactory() {return new TomcatServletWebServerFactory();}@Beanpublic DispatcherServlet dispatcherServlet() {return new DispatcherServlet();}@Beanpublic DispatcherServletRegistrationBean servletRegistrationBean(DispatcherServlet dispatcherServlet) {DispatcherServletRegistrationBean dispatcherServletRegistrationBean = new DispatcherServletRegistrationBean(dispatcherServlet, "/");dispatcherServletRegistrationBean.setLoadOnStartup(1);return dispatcherServletRegistrationBean;}@Beanpublic MyRequestMappingHandlerAdapter requestMappingHandlerAdapter() {MyRequestMappingHandlerAdapter handlerAdapter = new MyRequestMappingHandlerAdapter();// 添加自定义返回值解析器handlerAdapter.setCustomReturnValueHandlers(Arrays.asList(new YmlReturnResolver()));return handlerAdapter;}
}

3）创建控制器类及实体类

@Controller
public class Controller01 {@GetMapping("/test")@Ymlpublic User test() {