Springboot使用了ResponseBodyAdvice处理返回值异常？

摘要: 在用Springmvc的ResponseBodyAdvice接口处理controller的返回值的时候，程序报错了一个类转化异常。

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")
@Slf4j
public 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<Object> {

//这个方法表示对于哪些请求要执行beforeBodyWrite，返回true执行，返回false不执行
@Override
public boolean supports(MethodParameter methodParameter, Class aClass) {
return true;
}

//对于返回的对象如果不是最终对象ResponseResult，则选包装一下
@Override
public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType,
Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
if(!(o instanceof ResponseResult)) {
ResponseResult responseResult = new ResponseResult(CodeConstant.SUCCESS, o);
return responseResult;
}
return o;
}

}

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<? extends HttpMessageConverter<?>>) 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<? extends HttpMessageConverter<?>>) 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<Object> {

//这个方法表示对于哪些请求要执行beforeBodyWrite，返回true执行，返回false不执行
@Override
public boolean supports(MethodParameter methodParameter, Class aClass) {
return true;
}

//对于返回的对象如果不是最终对象ResponseResult，则选包装一下
@Override
public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType,
Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
if(!(o instanceof ResponseResult)) {
ResponseResult responseResult = new ResponseResult(CodeConstant.SUCCESS, o);
//因为handler处理类的返回类型是String，为了保证一致性，这里需要将ResponseResult转回去
if(o instanceof String) {
return JSON.toJSONString(responseResult);
}
return responseResult;
}
return o;
}

}

最终解决了这个问题。