Spring RestTemplate Behavior when handling responses with a status of NO_CONTENT

Question

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.

Answer 1

One more way to solve this would be to make response entity as null as shown below.

  ResponseEntity<?> response = restTemplate.exchange("http://localhost:8080/myapp/user/{userID}",
                                                             HttpMethod.DELETE, 
                                                             requestEntity,
                                                             null,
                                                             userID);

If you still need response headers, try implementing the ResponseErrorHandler.

Answer 2

I believe you should probably look at the ResponseExtractor interface & call execute on the RestTemplate providing your implementation of the extractor. To me it looks like a common requirement to do this so have logged this:

https://jira.springsource.org/browse/SPR-8016

Here's one I prepared earlier:

private class MyResponseExtractor extends HttpMessageConverterExtractor<MyEntity> {

    public MyResponseExtractor (Class<MyEntity> responseType,
      List<HttpMessageConverter<?>> messageConverters) {
        super(responseType, messageConverters);
    }

    @Override
    public MyEntity extractData(ClientHttpResponse response) throws IOException {

        MyEntity result;

        if (response.getStatusCode() == HttpStatus.OK) {
            result = super.extractData(response);
        } else {
            result = null;
        }

        return result;
    }
}

I've tested this & it seems to do what I want.

To create the instance of the ResponseExtractor I call the constructor & pass the converters from a RestTemplate instance that's been injected;

E.g.

ResponseExtractor<MyEntity> responseExtractor =
    new MyResponseExtractor(MyEntity.class, restTemplate.getMessageConverters());

Then the call is:

MyEntity responseAsEntity =
    restTemplate.execute(urlToCall, HttpMethod.GET, null, responseExtractor);

Your mileage may vary. ;-)