Feign makes writing java http clients easier

Feign is a java to http client binder inspired by Dagger, Retrofit, JAXRS-2.0, and WebSocket. 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(@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 + ")");
  }
}

Customization

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");

For further flexibility, you can use Dagger modules directly. See the Dagger section for more details.

Request Interceptors

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");

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.builder().target(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!

Gson

GsonModule adds default encoders and decoders so you get get started 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

JacksonModule adds 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");

Sax

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

JAXBModule allows you to encode and decode XML using JAXB.

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");

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);
}

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

OkHttp

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");

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());

SLF4J

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");

Decoders

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");

Encoders

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");

Advanced usage and Dagger

Base Apis

In many cases, apis for a service follow the same conventions. Feign supports this pattern via single-inheritance interfaces.

Consider the example:

interface BaseAPI {
  @RequestLine("GET /health")
  String health();

  @RequestLine("GET /all")
  List<Entity> all();
}

You can define and target a specific api, inheriting the base methods.

interface CustomAPI extends BaseAPI {
  @RequestLine("GET /custom")
  String custom();
}

In many cases, resource representations are also consistent. For this reason, type parameters are supported on the base api interface.

@Headers("Accept: application/json")
interface BaseApi<V> {

  @RequestLine("GET /api/{key}")
  V get(@Param("key") String);

  @RequestLine("GET /api")
  List<V> list();

  @Headers("Content-Type: application/json")
  @RequestLine("PUT /api/{key}")
  void put(@Param("key") String, V value);
}

interface FooApi extends BaseApi<Foo> { }

interface BarApi extends BaseApi<Bar> { }

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, RequestInterceptor bindings are of Provider.Type.SET, meaning you can have multiple interceptors. Here's an example of multiple interceptor bindings.

@Provides(type = SET) RequestInterceptor forwardedForInterceptor() {
  return new RequestInterceptor() {
    @Override public void apply(RequestTemplate template) {
      template.header("X-Forwarded-For", "origin.host.com");
    }
  };
}

@Provides(type = SET) RequestInterceptor userAgentInterceptor() {
  return new RequestInterceptor() {
    @Override public void apply(RequestTemplate template) {
      template.header("User-Agent", "My Cool Client");
    }
  };
}

Custom Parameter Expansion

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);

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:

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 SLF4JModule (see above) may also be of interest.