Spring Boot: returned HTTP response code 400, nested exception is java.io.IOException

Question

Consider the below code.Here, when the status code returned by the response is 200, then code execution works perfectly.

But if I try to return the status code 400, then Exception is caught at the specified line.

* Error Response : Internal Server Error in: getResponse I/O error on POST request for "http://localhost:8090/get-data": Server returned HTTP response code: 400 for URL: http://localhost:8090/get-data; nested exception is java.io.IOException: Server returned HTTP response code: 400 for URL: http://localhost:8090/get-data

connection: close, content-length: 485, content-type: application/json, date: Thu, 08 Apr 2021 20:25:47 GMT*

Why is an exception thrown when status 400 is returned?

 @RestController
    Class ABC{
    
        @Autowired
        RestTemplate template;
    
        @PostMapping("....")
        ResponseEntity<Object> getResponse(){
            ResponseEntity<String> response
            try{
              HttpEntity<Object> body=new HttpEntity(obj,header); // data obj & httpheader
              response=template.postForEntity("http://localhost:8090/get- 
                       data",body,String.class);
            }catch(HttpStatusCodeException ex){
                  throw....
            }catch(Exception ex){
                throw Internal Server Error.....  //Exception is thrown here
            }       
            if(response.getStatusCodeValue()==200){
                 ...
            }
               ...
    
        }
    }
    
    @RestController
    Class XYZ{
       
       @PostMapping("/get-data")
       ResponseEntity<Object> getTheStatus(@RequestHeader HttpHeaders headers ,@ResponseBody MyData data){
    
              return new ResponseEntity<>("",HttpStatus.BAD_REQUEST);
    
       }
    
    }

Answer 1

In HTTP context response status 200 represent success response. Statuses 4xx and 5xx represent some problems. The problem can be related to a request, connection, server, or another source. Some HTTP client implementations like RestTemplate will throw exceptions in case of non-success status code because non-success response status represents failed execution .

If you want to avoid exceptions you can add an error handler.

public class RestTemplateResponseErrorHandler 
  implements ResponseErrorHandler {

    @Override
    public boolean hasError(ClientHttpResponse httpResponse) throws IOException {
      return false;
    }

    @Override
    public void handleError(ClientHttpResponse httpResponse) throws IOException {}
}

RestTemplate restTemplate = restTemplateBuilder
          .errorHandler(new RestTemplateResponseErrorHandler())
          .build();

template.postForEntity("http://localhost:8090/get-data", body, String.class);

Answer 2

it's happen because of passing HttpStatus.BAD_REQUEST

Answer 3

IMHO returning 4xx statuses like this isn't the best way, use @ControllerAdvice instead