When the return value of ResponseBodyAdvice is String, an exception cannot be cast to java.lang.String occurs.
background
In order to return a unified JSON format globally in the project, ResponseBodyAdvice is used for interception. When intercepting, the returned information will be returned to the front end in one object. But some colleagues returned a String response object, and the result was an error
com.example.demoweb.config.ApiResponse cannot be cast to java.lang.String
ResponseBodyAdvice interceptor implementation
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> {
@Autowired
private ApplicationContext applicationContext;
@ExceptionHandler(BusinessException.class)
@ResponseBody
public 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)
@ResponseBody
public 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)
@ResponseBody
public 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
*/
@Override
public boolean supports(MethodParameter methodParameter, Class aClass) {
return true;
}
// 修改responsebody
@Override
public 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)
@ResponseBody
public 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")
@ResponseBody
public String test(){
return "sadsadsad";
}
}
Console output:
java.lang.ClassCastException: com.example.demoweb.config.ApiResponse cannot be cast to java.lang.String
at 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]
Analysis through error messages
StringHttpMessageConverterWe see an exception that the message converter that returns string reports an error. Let’s break it down and take a look:
Then we have repackaged the string into an object, so an error will be reported.
Let's try to breakpoint again to see if the return value is an entity object, and find that the converter is org.springframework.http.converter.json.MappingJackson2HttpMessageConverter:
That means that Spring internally handles different return values through different message converters (MessageConverter), each of which MessageConvereris selected and processed according to the return type and media type.
solution
In fact, the solution is very simple. Check whether the return value is a String. If it is a String, just process the object as a Json string. Without further ado, just add the code:
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;
}
}
Summarize
Although this problem has been solved, I actually think that the return should not be written casually here. The return should be defined according to the specification. Whether the Stringtype of data is suitable for return is not easy to understand the response field of the interface, so this is a technical problem. , It’s actually a matter of specification. It should be regulated that everyone’s response needs to return an entity object. Whether it is returning one field or multiple fields, the entity must be used to return.