handle Exceptions from API server in client server - rest

Question

I am trying to learn how to handle exception in APIs servers so I followed this where he has built API for birds, he finally reached to APIs like this:
#GetMapping(value = "/params")
public Bird getBirdRequestParam(#RequestParam("birdId") Long birdId) throws EntityNotFoundException {
Bird bird = birdRepository.findOne(birdId);
if(bird == null){
throw new EntityNotFoundException(Bird.class, "id", birdId.toString());
}
return bird;
}
and the ControllerAdvice has a method:
#ExceptionHandler(EntityNotFoundException.class)
protected ResponseEntity<Object> handleEntityNotFound(
EntityNotFoundException ex) {
ApiError apiError = new ApiError(NOT_FOUND);
apiError.setMessage(ex.getMessage());
return buildResponseEntity(apiError);
}
the response will be a bird like this
{
"id": 1,
"scientificName": "Atlantic canary",
"specie": "serinus canaria",
"mass": 10,
"length": 11
}
or an exception details like this:
{
"apierror": {
"status": "NOT_FOUND",
"timestamp": "09-04-2018 10:11:44",
"message": "Bird was not found for parameters {id=2}"
}
but the problem is with my server that contacts with API
I am using :
public void gett(#RequestParam Long id) {
ResponseEntity<Bird> responseEntity = restTemplate.getForEntity("http://localhost:8181/params" + id, Bird.class);
bird = responseEntity.getBody();
model.addAttribute("birdform", bird);
return "bird";
}
the getForEntity is waiting for a response with bird body but an exception may be thrown in server and the response may be json of the error.
how to handle this problem in my client server?
in other words :
how to know in my client server that the api server has thrown an exception in json form.???
I have tried to get the response in "Object" variable and then try to know if it was excption or bird with "instance of" expression like this code
#GetMapping("/getbird")
public String getAll(#RequestParam Long id, Model model) {
ResponseEntity<Object> responseEntity = restTemplate.getForEntity("http://localhost:8181/api/bird/getone?id=" + id, Object.class);
if (responseEntity.getBody() instanceof Bird.class) {
Bird bird= (Bird) responseEntity.getBody();
model.addAttribute("Bird", bird);
return "bird-form";
}
else{
// something else
return "someview";
}
}
but first thing it didnot work (the instance of always return false)
the second thing is that this is a hard work to do with all my controllers' actions.
I hope that i could explain my problem clearly .
thanks....

Answer 1

You don't need to check the body of the response to see if you got an error. restTemplate.getForEntity() (and other methods) would throw an HttpClientErrorException if you get a 4XX response, or HttpServerErrorException if you get a 5XX response from the call.
To catch these exceptions, you can define a #ControllerAdvice with proper methods for exception handling:
#ControllerAdvice
class GlobalControllerExceptionHandler {
#ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR) // 500
#ExceptionHandler(HttpClientErrorException.class)
public ApiError handle4xxFromClient(HttpClientErrorException ex) {
// construct and return a custom ApiError object
}
}
See https://spring.io/blog/2013/11/01/exception-handling-in-spring-mvc for details.

Answer 2

Two solutions :
Use a custom implementation error handler in your reste template.
restTemplate.setErrorHandler(customerErrorHandler)
Use the http status, rest template will throw an exception if it gets a bad http status from the server it is querying. Catch the exception and get the ApiError Object from the exception, and then throw and exception of your own that will be handled by your exception handler in your client server.
For these solutions to work, the server your are querying needs to send the right http status code when something wrong happens.