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

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.

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.

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

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

static class Contributor {
  String login;
  int contributions;
}

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

  // 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 + ")");
  }
}

Feign has several aspects that can be customized. For simple cases, you can use Feign.builder() to construct an API interface with your custom components. For example:

interface Bank {
  @RequestLine("POST /account/{id}")
  Account getAccountInfo(@Param("id") String id);
}
...
Bank bank = Feign.builder().decoder(new AccountDecoder()).target(Bank.class, "https://api.examplebank.com");

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.

Feign feign = Feign.builder().build();
CloudDNS cloudDNS = feign.target(new CloudIdentityTarget<CloudDNS>(user, apiKey));

Feign includes example GitHub and Wikipedia clients. The denominator project can also be scraped for Feign in practice. Particularly, look at its example daemon.

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

Gson includes an encoder and decoder you can use with a JSON API.

Add GsonEncoder and/or GsonDecoder to your Feign.Builder like so:

GsonCodec codec = new GsonCodec();
GitHub github = Feign.builder()
                     .encoder(new GsonEncoder())
                     .decoder(new GsonDecoder())
                     .target(GitHub.class, "https://api.github.com");

Jackson includes an encoder and decoder you can use with a JSON API.

Add JacksonEncoder and/or JacksonDecoder to your Feign.Builder like so:

GitHub github = Feign.builder()
                     .encoder(new JacksonEncoder())
                     .decoder(new JacksonDecoder())
                     .target(GitHub.class, "https://api.github.com");

SaxDecoder allows you to decode XML in a way that is compatible with normal JVM and also Android environments.

Here's an example of how to configure Sax response parsing:

api = Feign.builder()
           .decoder(SAXDecoder.builder()
                              .registerContentHandler(UserIdHandler.class)
                              .build())
           .target(Api.class, "https://apihost");

JAXB includes an encoder and decoder you can use with an XML API.

Add JAXBEncoder and/or JAXBDecoder to your Feign.Builder like so:

api = Feign.builder()
           .encoder(new JAXBEncoder())
           .decoder(new JAXBDecoder())
           .target(Api.class, "https://apihost");

JAXRSContract 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);
}

GitHub github = Feign.builder()
                     .contract(new JAXRSContract())
                     .target(GitHub.class, "https://api.github.com");           

OkHttpClient directs Feign's http requests to OkHttp, which enables SPDY and better network control.

To use OkHttp with Feign, add the OkHttp module to your classpath. Then, configure Feign to use the OkHttpClient:

GitHub github = Feign.builder()
                     .client(new OkHttpClient())
                     .target(GitHub.class, "https://api.github.com");

RibbonClient 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.builder().client(RibbonClient.create()).target(MyService.class, "https://myAppProd");

SLF4JModule allows directing Feign's logging to SLF4J, allowing you to easily use a logging backend of your choice (Logback, Log4J, etc.)

To use SLF4J with Feign, add both the SLF4J module and an SLF4J binding of your choice to your classpath. Then, configure Feign to use the Slf4jLogger:

GitHub github = Feign.builder()
                     .logger(new Slf4jLogger())
                     .target(GitHub.class, "https://api.github.com");

Feign.builder() allows you to specify additional configuration such as how to decode a response.

If any methods in your interface return types besides Response, String, byte[] or void, you'll need to configure a non-default Decoder.

Here's how to configure JSON decoding (using the feign-gson extension):

GitHub github = Feign.builder()
                     .decoder(new GsonDecoder())
                     .target(GitHub.class, "https://api.github.com");

Feign.builder() allows you to specify additional configuration such as how to encode a request.

If any methods in your interface use parameters types besides String or byte[], you'll need to configure a non-default Encoder.

Here's how to configure JSON encoding (using the feign-gson extension):

GitHub github = Feign.builder()
                     .encoder(new GsonEncoder())
                     .target(GitHub.class, "https://api.github.com");

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:

GitHub github = Feign.builder()
                     .decoder(new GsonDecoder())
                     .logger(new Logger.JavaLogger().appendToFile("logs/http.log"))
                     .logLevel(Logger.Level.FULL)
                     .target(GitHub.class, "https://api.github.com");

The SLF4JLogger (see above) may also be of interest.

When you need to change all requests, regardless of their target, you'll want to configure a RequestInterceptor. For example, if you are acting as an intermediary, you might want to propagate the X-Forwarded-For header.

static class ForwardedForInterceptor implements RequestInterceptor {
  @Override public void apply(RequestTemplate template) {
    template.header("X-Forwarded-For", "origin.host.com");
  }
}
...
Bank bank = Feign.builder()
                 .decoder(accountDecoder)
                 .requestInterceptor(new ForwardedForInterceptor())
                 .target(Bank.class, "https://api.examplebank.com");

Another common example of an interceptor would be authentication, such as using the built-in BasicAuthRequestInterceptor.

Bank bank = Feign.builder()
                 .decoder(accountDecoder)
                 .requestInterceptor(new BasicAuthRequestInterceptor(username, password))
                 .target(Bank.class, "https://api.examplebank.com");

Parameters annotated with Param expand based on their toString. By specifying a custom Param.Expander, users can control this behavior, for example formatting dates.

@RequestLine("GET /?since={date}") Result list(@Param(value = "date", expander = DateToMillis.class) Date date);