Feign makes writing java http clients easier

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.

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");

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.

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

Examples

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

Integrations

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

Gson

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

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");

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

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");

JAX-RS

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");

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

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");

Hystrix

HystrixFeign configures circuit breaker support provided by Hystrix.

To use Hystrix with Feign, add the Hystrix module to your classpath. Then use the HystrixFeign builder:

MyService api = HystrixFeign.builder().target(MyService.class, "https://myAppProd");

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

The simplest way to send a request body to a server is to define a POST method that has a String or byte[] parameter without any annotations on it. You will likely need to add a Content-Type header.

interface LoginClient {
  @RequestLine("POST /")
  @Headers("Content-Type: application/json")
  void login(String content);
}
...
client.login("{\"user_name\": \"denominator\", \"password\": \"secret\"}");

By configuring an Encoder, you can send a type-safe request body. Here's an example using the feign-gson extension:

static class Credentials {
  final String user_name;
  final String password;

  Credentials(String user_name, String password) {
    this.user_name = user_name;
    this.password = password;
  }
}

interface LoginClient {
  @RequestLine("POST /")
  void login(Credentials creds);
}
...
LoginClient client = Feign.builder()
                          .encoder(new GsonEncoder())
                          .target(LoginClient.class, "https://foo.com");

client.login(new Credentials("denominator", "secret"));

@Body templates

The @Body annotation indicates a template to expand using parameters annotated with @Param. You will likely need to add a Content-Type header.

interface LoginClient {

  @RequestLine("POST /")
  @Headers("Content-Type: application/xml")
  @Body("<login \"user_name\"=\"{user_name}\" \"password\"=\"{password}\"/>")
  void xml(@Param("user_name") String user, @Param("password") String password);

  @RequestLine("POST /")
  @Headers("Content-Type: application/json")
  // json curly braces must be escaped!
  @Body("%7B\"user_name\": \"{user_name}\", \"password\": \"{password}\"%7D")
  void json(@Param("user_name") String user, @Param("password") String password);
}
...
client.xml("denominator", "secret"); // <login "user_name"="denominator" "password"="secret"/>
client.json("denominator", "secret"); // {"user_name": "denominator", "password": "secret"}

Headers

Feign supports settings headers on requests either as part of the api or as part of the client depending on the use case.

Set headers using apis

In cases where specific interfaces or calls should always have certain header values set, it makes sense to define headers as part of the api.

Static headers can be set on an api interface or method using the @Headers annotation.

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

Methods can specify dynamic content for static headers using variable expansion in @Headers.

 @RequestLine("POST /")
 @Headers("X-Ping: {token}")
 void post(@Param("token") String token);

In cases where both the header field keys and values are dynamic and the range of possible keys cannot be known ahead of time and may vary between different method calls in the same api/client (e.g. custom metadata header fields such as "x-amz-meta-*" or "x-goog-meta-*"), a Map parameter can be annotated with HeaderMap to construct a query that uses the contents of the map as its header parameters.

 @RequestLine("POST /")
 void post(@HeaderMap Map<String, Object> headerMap);

These approaches specify header entries as part of the api and do not require any customizations when building the Feign client.

Setting headers per target

In cases where headers should differ for the same api based on different endpoints or where per-request customization is required, headers can be set as part of the client using a RequestInterceptor or a Target.

For an example of setting headers using a RequestInterceptor, see the Request Interceptors section.

Headers can be set as part of a custom Target.

  static class DynamicAuthTokenTarget<T> implements Target<T> {
    public DynamicAuthTokenTarget(Class<T> clazz,
                                  UrlAndTokenProvider provider,
                                  ThreadLocal<String> requestIdProvider);
    ...
    @Override
    public Request apply(RequestTemplate input) {
      TokenIdAndPublicURL urlAndToken = provider.get();
      if (input.url().indexOf("http") != 0) {
        input.insert(0, urlAndToken.publicURL);
      }
      input.header("X-Auth-Token", urlAndToken.tokenId);
      input.header("X-Request-ID", requestIdProvider.get());

      return input.request();
    }
  }
  ...
  Bank bank = Feign.builder()
          .target(new DynamicAuthTokenTarget(Bank.class, provider, requestIdProvider));

These approaches depend on the custom RequestInterceptor or Target being set on the Feign client when it is built and can be used as a way to set headers on all api calls on a per-client basis. This can be useful for doing things such as setting an authentication token in the header of all api requests on a per-client basis. The methods are run when the api call is made on the thread that invokes the api call, which allows the headers to be set dynamically at call time and in a context-specific manner -- for example, thread-local storage can be used to set different header values depending on the invoking thread, which can be useful for things such as setting thread-specific trace identifiers for requests.

Advanced usage

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> { }

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 SLF4JLogger (see above) may also be of interest.

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");

Custom @Param 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);

Dynamic Query Parameters

A Map parameter can be annotated with QueryMap to construct a query that uses the contents of the map as its query parameters.

@RequestLine("GET /find")
V find(@QueryMap Map<String, Object> queryMap);

Static and Default Methods

Interfaces targeted by Feign may have static or default methods (if using Java 8+). These allows Feign clients to contain logic that is not expressly defined by the underlying API. For example, static methods make it easy to specify common client build configurations; default methods can be used to compose queries or define default parameters.

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

  @RequestLine("GET /users/{username}/repos?sort={sort}")
  List<Repo> repos(@Param("username") String owner, @Param("sort") String sort);

  default List<Repo> repos(String owner) {
    return repos(owner, "full_name");
  }

  /**
   * Lists all contributors for all repos owned by a user.
   */
  default List<Contributor> contributors(String user) {
    MergingContributorList contributors = new MergingContributorList();
    for(Repo repo : this.repos(owner)) {
      contributors.addAll(this.contributors(user, repo.getName()));
    }
    return contributors.mergeResult();
  }

  static GitHub connect() {
    return Feign.builder()
                .decoder(new GsonDecoder())
                .target(GitHub.class, "https://api.github.com");
  }
}