Spring RestTemplate Behavior when handling responses with a status of NO_CONTENT

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Okay, I have a class NamedSystems, that has as its only field a Set of NamedSystem.

I have a method to find NamedSystems by certain criteria. That's not really important. When it gets results, everything works fine. However, when it can't find anything, and thus returns a null (or empty -- I've tried both ways) set, I get problems. Let me explain.

I'm using the Spring RestTemplate class and I'm making a call like this in a unit test:

ResponseEntity<?> responseEntity = template.exchange(BASE_SERVICE_URL + "?
  alias={aliasValue}&aliasAuthority={aliasAssigningAuthority}", 
  HttpMethod.GET, makeHttpEntity("xml"), NamedSystems.class, 
  alias1.getAlias(), alias1.getAuthority());

Now, since this would normally return a 200, but I want to return a 204, I have an interceptor in my service that determines if a ModelAndView is a NamedSystem and if its set is null. If so, I then the set the status code to NO_CONTENT (204).

When I run my junit test, I get this error:

org.springframework.web.client.RestClientException: Cannot extract response: no Content-Type found

Setting the status to NO_CONTENT seems to wipe the content-type field (which does make sense when I think about it). So why is it even looking at it?

Spring's HttpMessageConverterExtractor extractData method:

public T extractData(ClientHttpResponse response) throws IOException {
    MediaType contentType = response.getHeaders().getContentType();
    if (contentType == null) {
        throw new RestClientException("Cannot extract response: no Content-Type found");
    }
    for (HttpMessageConverter messageConverter : messageConverters) {
        if (messageConverter.canRead(responseType, contentType)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Reading [" + responseType.getName() + "] as \"" + contentType
                    +"\" using [" + messageConverter + "]");
            }
            return (T) messageConverter.read(this.responseType, response);
        }
    }
    throw new RestClientException(
        "Could not extract response: no suitable HttpMessageConverter found for response type [" +
        this.responseType.getName() + "] and content type [" + contentType + "]");
}

Going up the chain a bit to find out where that Extractor is set, I come to RestTemplate's exchange() method that I used in the test:

public <T> ResponseEntity<T> exchange(String url, HttpMethod method,
  HttpEntity<?> requestEntity, Class<T> responseType, Object... uriVariables) throws RestClientException {
    HttpEntityRequestCallback requestCallback = new HttpEntityRequestCallback(requestEntity, responseType);
    ResponseEntityResponseExtractor<T> responseExtractor = new ResponseEntityResponseExtractor<T>(responseType);
    return execute(url, method, requestCallback, responseExtractor, uriVariables);
}

So, it's trying to convert what amounts to nothing because of the supplied response type from the exchange call. If I change the responseType from NamedSystems.class to null, it works as expected. It doesn't try to convert anything. If I had tried to set the status code to 404, it also executes fine.

Am I misguided, or does this seem like a flaw in RestTemplate? Sure, I'm using a junit right now so I know what's going to happen, but if someone is using RestTemplate to call this and doesn't know the outcome of the service call, they would naturally have NamedSystems as a response type. However, if they tried a criteria search that came up with no elements, they'd have this nasty error.

Is there a way around this without overriding any RestTemplate stuff? Am I viewing this situation incorrectly? Please help as I'm a bit baffled.

spring-mvc rest web-services unit-testing java
7 Answers

I came along a workaround (not sure if it meets your case):

First define a custom interceptor class which implements ClientHttpRequestInterceptor. and check if response.getStatusCode() meets your case (my case is != HttpStatus.NOT_FOUND and response.getBody() length is 0), define a custom class (e.x. DefaultResponseForEmptyRestTemplateBody) which has a static method of type MockClientHttpResponse:

    public class RequestResponseInterceptor implements ClientHttpRequestInterceptor {
@Override
public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {

    ClientHttpResponse response = execution.execute(request, body);
     if(response.getStatusCode()!=HttpStatus.NOT_FOUND && response.getBody().readAllBytes().length==0){
        response = DefaultResponseForEmptyRestTemplateBody.getResponse(response.getStatusCode());

    }

    return response;
}


}

    public static class DefaultResponseForEmptyRestTemplateBody  {
MockClientHttpResponse response;
private static byte[] content = new byte[0];

public static MockClientHttpResponse getResponse(HttpStatus statusCode){
    content = "response body is empty".getBytes();
  return new MockClientHttpResponse(content, statusCode);
}

}

finally add this interceptor to your restTemplate object as below:

restTemplate.setInterceptors(Collections.singletonList(new RequestResponseLoggingInterceptor()));

and call your restTemplate.postForEntity:

ResponseEntity<String> response = this.restTemplate.postForEntity(baseUrl, requestParams,String.class);
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