在使用为了在controller层中只关心我业务数据，而不去关心数据的形式。一般来说，在http接口响应的json数据时，我都回去设置一个响应码code，描述msg，数据data。形式如下：

public class ResponseResult implements Serializable{    private static final long serialVersionUID = 1638693422783183022L;    private String code;    private String msg;    private Object data;    public ResponseResult(CodeConstant codeConstant) {        this.code = codeConstant.getCode();        this.msg = codeConstant.getMsg();    }    public ResponseResult(CodeConstant codeConstant, Object data) {        this(codeConstant);        this.data = data;    }}    

而在controller层中，我只关心我想要返回的数据，也是上面响应实体的部分，我不想在controller中还要处理响应数据包装这种重复无聊的工作。所以controller中我想要的形式是，我只返回我业务需要返回的数据，效果如下：

@RestController@RequestMapping("/auth")@Slf4jpublic class UserController {      @RequestMapping("test1")   public UserEntity test1() {      return new UserEntity();   }      @RequestMapping("test")    public String test2() {      return "12312";   }}

所以为了统一接口响应的报文，我现实了ResponseBodyAdvice接口，通过这个接口的实现类来统一处理报文

public class BaseGlobalResponseBodyAdvice implements ResponseBodyAdvice

然而在测试返回值是String类型的时候，程序抛出一个类转换的异常：

java.lang.ClassCastException: com.jinhualun.moguhou.merchant.console.base.common.ResponseResult cannot be cast to java.lang.String	at org.springframework.http.converter.StringHttpMessageConverter.getContentLength(StringHttpMessageConverter.java:41) ~[spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.http.converter.AbstractHttpMessageConverter.addDefaultHeaders(AbstractHttpMessageConverter.java:260) ~[spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:205) ~[spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:247) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81) ~[spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:113) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) ~[spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) [spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) [spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) [spring-webmvc-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at com.jinhualun.moguhou.merchant.console.base.filter.CorsFilter.doFilter(CorsFilter.java:47) [classes/:na]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:105) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.10.RELEASE.jar:4.3.10.RELEASE]	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) [tomcat-embed-core-8.5.16.jar:8.5.16]	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.16.jar:8.5.16]	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_131]	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_131]	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.16.jar:8.5.16]	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_131]

后来经过debug发现这个跟MessageConverter有关系。Springmvc内部定义了九个不同的MessageConverter用来处理不同的返回值。在AbstractMessageConverterMethodProcessor类下面的writeWithMessageConverters方法可以看出来，每个MessageConverer是根据返回类型和媒体类型来选择处理的MessageConverter的，下面是代码片段：

if (selectedMediaType != null) {   selectedMediaType = selectedMediaType.removeQualityValue();   for (HttpMessageConverter
     messageConverter : this.messageConverters) {      if (messageConverter instanceof GenericHttpMessageConverter) {         if (((GenericHttpMessageConverter) messageConverter).canWrite(               declaredType, valueType, selectedMediaType)) {            outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,                  (Class
    >) messageConverter.getClass(),                  inputMessage, outputMessage);            if (outputValue != null) {               addContentDispositionHeader(inputMessage, outputMessage);               ((GenericHttpMessageConverter) messageConverter).write(                     outputValue, declaredType, selectedMediaType, outputMessage);               if (logger.isDebugEnabled()) {                  logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +                        "\" using [" + messageConverter + "]");               }            }            return;         }      }      else if (messageConverter.canWrite(valueType, selectedMediaType)) {         outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,               (Class
    >) messageConverter.getClass(),               inputMessage, outputMessage);         if (outputValue != null) {            addContentDispositionHeader(inputMessage, outputMessage);            ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);            if (logger.isDebugEnabled()) {               logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +                     "\" using [" + messageConverter + "]");            }         }         return;      }   }}

所以导致这个问题的原因就是，controller层中返回的类型是String，但是在ResponseBodyAdvice实现类中，我们把响应的类型修改成了ResponseResult。这就导致了，上面的这段代码在选择处理MessageConverter的时候，依旧根据之前的String类型选择对应String类型的StringMessageConverter。而在StringMessageConverter类型，他只接受String类型的返回类型，我们在ResponseBodyAdvice中将返回值从String类型改成ResponseResult类型之后，调用StringMessageConverter方法发生类型强转。ReponseResult无法转换成String，发生类型转换异常。

所以解决该异常最好的方式就是重写StringMessageConverter方法，让他可以解决ResonseResult类型的转化。后来我没有用这种方式，我觉得这样相对比较麻烦，所以换了一种思路，在ResponseBodyAdvice中做了针对String类型返回值的修改：

public class BaseGlobalResponseBodyAdvice implements ResponseBodyAdvice

最终解决了这个问题。