使用ResponseBodyAdvice返回值为String出现cannot be cast to java.lang.String异常

背景

由于项目中为了全局返回统一的JSON格式，使用ResponseBodyAdvice进行拦截，拦截的时候会将返回的信息统一一个对象返回到前端。但是有的同事将一个String的响应对象返回，结果报错com.example.demoweb.config.ApiResponse cannot be cast to java.lang.String

ResponseBodyAdvice 拦截器实现

ServletResponseBodyAdvice

package com.example.demoweb.config;import cn.hutool.extra.servlet.ServletUtil;
import cn.hutool.json.JSONUtil;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.core.MethodParameter;
import org.springframework.http.MediaType;
import org.springframework.http.converter.StringHttpMessageConverter;
import org.springframework.http.server.ServerHttpRequest;
import org.springframework.http.server.ServerHttpResponse;
import org.springframework.validation.BindException;
import org.springframework.validation.BindingResult;
import org.springframework.validation.FieldError;
import org.springframework.validation.ObjectError;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;import javax.servlet.http.HttpServletRequest;
import java.util.Objects;/*** @author tengwang8*/
@ControllerAdvice
@Slf4j
public class ServletResponseBodyAdvice implements ResponseBodyAdvice<Object> {@Autowiredprivate ApplicationContext applicationContext;@ExceptionHandler(BusinessException.class)@ResponseBodypublic ApiResponse businessException(BusinessException e) {log.error(" business  exception code = {} ,msg = {}", e.getCode(), e.getMsg());return ApiResponse.failed(e.getCode(), e.getMsg());}/*** 参数绑定异常** @param e 异常* @return 异常结果*/@ExceptionHandler(value = BindException.class)@ResponseBodypublic ApiResponse handleBindException(BindException e) {log.error("params bind exception: ", e);return wrapperBindingResult(e.getBindingResult());}/*** 参数校验异常，将校验失败的所有异常组合成一条错误信息** @param e 异常* @return 异常结果*/@ExceptionHandler(value = MethodArgumentNotValidException.class)@ResponseBodypublic ApiResponse handleValidException(MethodArgumentNotValidException e) {log.error("params valid exception:", e);return wrapperBindingResult(e.getBindingResult());}/*** 包装绑定异常结果** @param bindingResult 绑定结果* @return 异常结果*/private ApiResponse wrapperBindingResult(BindingResult bindingResult) {StringBuilder msg = new StringBuilder();for (ObjectError error : bindingResult.getAllErrors()) {msg.append(", ");if (error instanceof FieldError) {msg.append(((FieldError) error).getField()).append(": ");}msg.append(error.getDefaultMessage() == null ? "" : error.getDefaultMessage());}return ApiResponse.failed(ResultCode.UNAUTHORIZED, msg.substring(2));}/*** 这里直接返回true,表示对任何handler的responsebody都调用beforeBodyWrite方法** @param methodParameter* @param aClass* @return*/@Overridepublic boolean supports(MethodParameter methodParameter, Class aClass) {return true;}// 修改responsebody@Overridepublic Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType, Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {if (o instanceof ApiResponse) {return  o;} else {return ApiResponse.success(o);}}/*** 未定义异常** @param e 异常* @return 异常结果*/@ExceptionHandler(value = Exception.class)@ResponseBodypublic ApiResponse handleException(Exception e) {log.error("handleException :", e);return ApiResponse.failed(ResultCode.FAILED);}}

TestDemoController

/*** @author tengwang8* @date 2022年09月19日 11:02*/
@RestController
public class TestDemoController {@GetMapping("/tt")@ResponseBodypublic String test(){return "sadsadsad";}}

控制台输出：


java.lang.ClassCastException: com.example.demoweb.config.ApiResponse cannot be cast to java.lang.Stringat org.springframework.http.converter.StringHttpMessageConverter.addDefaultHeaders(StringHttpMessageConverter.java:44) ~[spring-web-5.3.22.jar:5.3.22]at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:211) ~[spring-web-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:293) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:183) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:78) ~[spring-web-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:135) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1070) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.3.22.jar:5.3.22]at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.3.22.jar:5.3.22]at javax.servlet.http.HttpServlet.service(HttpServlet.java:655) [tomcat-embed-core-9.0.65.jar:4.0.FR]at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) [spring-webmvc-5.3.22.jar:5.3.22]at javax.servlet.http.HttpServlet.service(HttpServlet.java:764) [tomcat-embed-core-9.0.65.jar:4.0.FR]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) [tomcat-embed-websocket-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) [spring-web-5.3.22.jar:5.3.22]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.22.jar:5.3.22]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) [spring-web-5.3.22.jar:5.3.22]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.22.jar:5.3.22]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) [spring-web-5.3.22.jar:5.3.22]at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.22.jar:5.3.22]at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:360) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:890) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1789) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.65.jar:9.0.65]at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.65.jar:9.0.65]at java.lang.Thread.run(Thread.java:745) [na:1.8.0_102]

通过错误信息进行分析

StringHttpMessageConverter我们看到有返回string的消息转换器报错的异常，我们断点看一下：
在这里插入图片描述
然后我们这边到是对字符串是重新包装成了一个对象，所以就会报错。

我们再尝试断点看一下如果返回值是实体对象的时候，发现这个转换器是org.springframework.http.converter.json.MappingJackson2HttpMessageConverter：

在这里插入图片描述
那说明Spring 内部是通过不同的消息转换器（MessageConverter）来处理不同的返回值，每个MessageConverer是根据返回类型和媒体类型来选择处理的。

解决方案

其实解决很简单，判断一下返回值是否是String，如果是String的话就将对象处理为Json串就好了,废话不多说，直接上代码：

public Object beforeBodyWrite(Object o, MethodParameter methodParameter, MediaType mediaType, Class aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {if (o instanceof ApiResponse) {return  o;} else {ApiResponse<Object> success = ApiResponse.success(o);return   o instanceof String ? JSONUtil.toJsonStr(success) : success;}
}