Feign makes writing java http clients easier

Feign is a java to http client binder inspired by Dagger, Retrofit, JAXRS-2.0, and WebSockets. Feign's first goal was reducing the complexity of binding Denominator uniformly to http apis regardless of restfulness.

Why Feign and not X?

You can use tools like Jersey and CXF to write java clients for ReST or SOAP services. You can write your own code on top of http transport libraries like Apache HC. Feign aims to connect your code to http apis with minimal overhead and code. Via customizable decoders and error handling, you should be able to write to any text-based http api.

How does Feign work?

Feign works by processing annotations into a templatized request. Just before sending it off, arguments are applied to these templates in a straightforward fashion. While this limits Feign to only supporting text-based apis, it dramatically simplified system aspects such as replaying requests. It is also stupid easy to unit test your conversions knowing this.

Basics

Usage typically looks like this, an adaptation of the canonical Retrofit sample.

interface GitHub {
  @RequestLine("GET /repos/{owner}/{repo}/contributors")
  List<Contributor> contributors(@Named("owner") String owner, @Named("repo") String repo);
}

static class Contributor {
  String login;
  int contributions;
}

public static void main(String... args) {
  GitHub github = Feign.create(GitHub.class, "https://api.github.com", new GsonModule());

  // Fetch and print a list of the contributors to this library.
  List<Contributor> contributors = github.contributors("netflix", "feign");
  for (Contributor contributor : contributors) {
    System.out.println(contributor.login + " (" + contributor.contributions + ")");
  }
}

Decoders

The last argument to Feign.create specifies how to decode the responses, modeled in Dagger. Here's how it looks to wire in a default gson decoder:

@Module(overrides = true, library = true)
static class GsonModule {
  @Provides(type = SET) Decoder decoder() {
    return new Decoder.TextStream<Object>() {
      Gson gson = new Gson();

      @Override public Object decode(Reader reader, Type type) throws IOException {
        try {
          return gson.fromJson(reader, type);
        } catch (JsonIOException e) {
          if (e.getCause() != null && e.getCause() instanceof IOException) {
            throw IOException.class.cast(e.getCause());
          }
          throw e;
        }
      }
    };
  }
}

Feign doesn't offer a built-in json decoder as you can see above it is very few lines of code to wire yours in. If you are a jackson user, you'd probably thank us for not dragging in a dependency you don't use.

Type-specific Decoders

The generic parameter of Decoder.TextStream<T> designates which The type parameter is either a concrete type, or Object, if your decoder can handle multiple types. To add a type-specific decoder, ensure your type parameter is correct. Here's an example of an xml decoder that will only apply to methods that return ZoneList.

@Provides(type = SET) Decoder zoneListDecoder(Provider<ListHostedZonesResponseHandler> handlers) {
  return new SAXDecoder<ZoneList>(handlers){};
}

Multiple Interfaces

Feign can produce multiple api interfaces. These are defined as Target<T> (default HardCodedTarget<T>), which allow for dynamic discovery and decoration of requests prior to execution.

For example, the following pattern might decorate each request with the current url and auth token from the identity service.

CloudDNS cloudDNS =  Feign.create().newInstance(new CloudIdentityTarget<CloudDNS>(user, apiKey));

You can find several examples in the test tree. Do take time to look at them, as seeing is believing!

Integrations

Feign intends to work well within Netflix and other Open Source communities. Modules are welcome to integrate with your favorite projects!

JAX-RS

JAXRSModule overrides annotation processing to instead use standard ones supplied by the JAX-RS specification. This is currently targeted at the 1.1 spec.

Here's the example above re-written to use JAX-RS:

interface GitHub {
  @GET @Path("/repos/{owner}/{repo}/contributors")
  List<Contributor> contributors(@PathParam("owner") String owner, @PathParam("repo") String repo);
}

Ribbon

RibbonModule overrides URL resolution of Feign's client, adding smart routing and resiliency capabilities provided by Ribbon.

Integration requires you to pass your ribbon client name as the host part of the url, for example myAppProd.

MyService api = Feign.create(MyService.class, "https://myAppProd", new RibbonModule());

Advanced usage and Dagger

Dagger

Feign can be directly wired into Dagger which keeps things at compile time and Android friendly. As opposed to exposing builders for config, Feign intends users to embed their config in Dagger.

Where possible, Feign configuration uses normal Dagger conventions. For example, Decoder bindings are of Provider.Type.SET, meaning you can make multiple bindings for all the different types you return. Here's an example of multiple decoder bindings.

@Provides(type = SET) Decoder recordListDecoder(Provider<RecordListHandler> handlers) {
  return new SAXDecoder<List<Record>>(handlers){};
}

@Provides(type = SET) Decoder directionalRecordListDecoder(Provider<DirectionalRecordListHandler> handlers) {
  return new SAXDecoder<List<DirectionalRecord>>(handlers){};
}

Logging

You can log the http messages going to and from the target by setting up a Logger. Here's the easiest way to do that:

@Module(overrides = true)
class Overrides {
  @Provides @Singleton Logger.Level provideLoggerLevel() {
    return Logger.Level.FULL;
  }

  @Provides @Singleton Logger provideLogger() {
    return new Logger.JavaLogger().appendToFile("logs/http.log");
  }
}
GitHub github = Feign.create(GitHub.class, "https://api.github.com", new GsonGitHubModule(), new Overrides());

Pattern Decoders

If you have to only grab a single field from a server response, you may find regular expressions less maintenance than writing a type adapter.

Here's how our IAM example grabs only one xml element from a response.

@Module(overrides = true, library = true)
static class IAMModule {
  @Provides(type = SET) Decoder arnDecoder() {
    return Decoders.firstGroup("<Arn>([\\S&&[^<]]+)</Arn>");
  }
}