1. Overview

In this short tutorial, we’ll discuss how to implement and inject the ResponseErrorHandler interface in a RestTemplate instance to gracefully handle the HTTP errors returned by remote APIs.

2. Default Error Handling

By default, the RestTemplate will throw one of these exceptions in the case of an HTTP error:

1. HttpClientErrorException – in the case of HTTP status 4xx
2. HttpServerErrorException – in the case of HTTP status 5xx
3. UnknownHttpStatusCodeException – in the case of an unknown HTTP status

All of these exceptions are extensions of RestClientResponseException.

Obviously, the simplest strategy to add custom error handling is to wrap the call in a try/catch block. Then we can process the caught exception as we see fit.

However, this simple strategy doesn’t scale well as the number of remote APIs or calls increases. It would be more efficient if we could implement a reusable error handler for all of our remote calls.

3. Implementing a ResponseErrorHandler

A class that implements ResponseErrorHandler will read the HTTP status from the response and either:

1. Throw an exception that is meaningful to our application
2. Simply ignore the HTTP status and let the response flow continue without interruption

We need to inject our ResponseErrorHandler implementation into the RestTemplate instance. To do this, we use the RestTemplateBuilder to replace the default error handler in the response flow.

3.1. Handling Common HTTP Errors

Let’s first implement a simple RestTemplateResponseErrorHandler that handles common HTTP errors such as 4xx and 5xx statuses. For example, we handle 404 errors by throwing a custom NotFoundException:

@Component
public class RestTemplateResponseErrorHandler implements ResponseErrorHandler {

 @Override
 public boolean hasError(ClientHttpResponse httpResponse) throws IOException {
 return httpResponse.getStatusCode().is5xxServerError() || 
 httpResponse.getStatusCode().is4xxClientError();
 }

 @Override
 public void handleError(ClientHttpResponse httpResponse) throws IOException {
 if (httpResponse.getStatusCode().is5xxServerError()) {
 //Handle SERVER_ERROR
 throw new HttpClientErrorException(httpResponse.getStatusCode());
 } else if (httpResponse.getStatusCode().is4xxClientError()) {
 //Handle CLIENT_ERROR
 if (httpResponse.getStatusCode() == HttpStatus.NOT_FOUND) {
 throw new NotFoundException();
 }
 }
 }
}

Then we can build the RestTemplate instance using the RestTemplateBuilder to introduce our RestTemplateResponseErrorHandler:

@Service
public class BarConsumerService {

 private RestTemplate restTemplate;

 @Autowired
 public BarConsumerService(RestTemplateBuilder restTemplateBuilder) {
 RestTemplate restTemplate = restTemplateBuilder
 .errorHandler(new RestTemplateResponseErrorHandler())
 .build();
 }

 public Bar fetchBarById(String barId) {
 return restTemplate.getForObject("/bars/4242", Bar.class);
 }

}

3.2. Handling 401 Unauthorized and Parsing the Response Body

Sometimes, when an API returns a 401 Unauthorized status, the response body includes useful details such as error messages that we may want to extract and use.

Since the ResponseErrorHandler.handleError() method does not automatically deserialize the response body, a common approach is to catch the HttpStatusCodeException thrown by RestTemplate and manually extract the body. To handle a 401 Unauthorized status more effectively, we can wrap the request in a try-catch block and inspect the response body when the exception is thrown:

public Bar fetchBarById(String barId) {
 try {
 return restTemplate.getForObject("/bars/" + barId, Bar.class);
 } catch (HttpStatusCodeException e) {
 if (e.getStatusCode() == HttpStatus.UNAUTHORIZED) {
 String responseBody = e.getResponseBodyAsString();
 throw new UnauthorizedException("Unauthorized access: " + responseBody);
 }
 throw e;
 }
}

This approach allows us to handle 401 Unauthorized errors in a controlled and informative way. By catching the exception and manually extracting the response body, we can inspect any error details returned by the server, such as a message indicating why authentication failed or instructions for recovering from the issue. This gives us the flexibility to provide clearer feedback to the user, log more descriptive messages for troubleshooting, or even trigger fallback mechanisms depending on the contents of the response.

4. Testing Our Implementation

Finally, we’ll test this handler by mocking a server and returning a NOT_FOUND status:

@ExtendWith(SpringExtension.class)
@ContextConfiguration(classes = { NotFoundException.class, Bar.class })
@RestClientTest
public class RestTemplateResponseErrorHandlerIntegrationTest {

 @Autowired 
 private MockRestServiceServer server;
 
 @Autowired 
 private RestTemplateBuilder builder;

 @Test
 public void givenRemoteApiCall_when404Error_thenThrowNotFound() {
 Assertions.assertNotNull(this.builder);
 Assertions.assertNotNull(this.server);

 RestTemplate restTemplate = this.builder
 .errorHandler(new RestTemplateResponseErrorHandler())
 .build();

 this.server
 .expect(ExpectedCount.once(), requestTo("/bars/4242"))
 .andExpect(method(HttpMethod.GET))
 .andRespond(withStatus(HttpStatus.NOT_FOUND));

 Assertions.assertThrows(NotFoundException.class, () -> {
 Bar response = restTemplate.getForObject("/bars/4242", Bar.class);
 });
 }
}

5. Conclusion

In this article, we presented a solution to implement and test a custom error handler for a RestTemplate that converts HTTP errors into meaningful exceptions.