Feign makes writing java http clients easier

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?

Feign uses tools like Jersey and CXF to write java clients for ReST or SOAP services. Furthermore, Feign allows you to write your own code on top of http libraries such as Apache HC. Feign connects your code to http APIs with minimal overhead and code via customizable decoders and error handling, which can be written to any text-based http API.

How does Feign work?

Feign works by processing annotations into a templatized request. Arguments are applied to these templates in a straightforward fashion before output. Although Feign is limited to supporting text-based APIs, it dramatically simplifies system aspects such as replaying requests. Furthermore, Feign makes it easy to unit test your conversions knowing this.

Java Version Compatibility

Feign 10.x and above are built on Java 8 and should work on Java 9, 10, and 11. For those that need JDK 6 compatibility, please use Feign 9.x

Feature overview

This is a map with current key features provided by feign:

Roadmap

Feign 11 and beyond

Making API clients easier

Short Term - What we’re working on now. ⏰

• Response Caching
  • Support caching of api responses. Allow for users to define under what conditions a response is eligible for caching and what type of caching mechanism should be used.
  • Support in-memory caching and external cache implementations (EhCache, Google, Spring, etc…)
• Complete URI Template expression support
  • Support level 1 through level 4 URI template expressions.
  • Use URI Templates TCK to verify compliance.
• Logger API refactor
  • Refactor the Logger API to adhere closer to frameworks like SLF4J providing a common mental model for logging within Feign. This model will be used by Feign itself throughout and provide clearer direction on how the Logger will be used.
• Retry API refactor
  • Refactor the Retry API to support user-supplied conditions and better control over back-off policies. This may result in non-backward-compatible breaking changes

Medium Term - What’s up next. ⏲

• Async execution support via CompletableFuture
  • Allow for Future chaining and executor management for the request/response lifecycle. Implementation will require non-backward-compatible breaking changes. However this feature is required before Reactive execution can be considered.
• Reactive execution support via Reactive Streams
  • For JDK 9+, consider a native implementation that uses java.util.concurrent.Flow.
  • Support for Project Reactor and RxJava 2+ implementations on JDK 8.

Long Term - The future ☁️

• Additional Circuit Breaker Support.
  • Support additional Circuit Breaker implementations like Resilience4J and Spring Circuit Breaker

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);

  @RequestLine("POST /repos/{owner}/{repo}/issues")
  void createIssue(Issue issue, @Param("owner") String owner, @Param("repo") String repo);

}

public static class Contributor {
  String login;
  int contributions;
}

public static class Issue {
  String title;
  String body;
  List<String> assignees;
  int milestone;
  List<String> labels;
}

public class MyApp {
  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("OpenFeign", "feign");
    for (Contributor contributor : contributors) {
      System.out.println(contributor.login + " (" + contributor.contributions + ")");
    }
  }
}

Interface Annotations

Feign annotations define the Contract between the interface and how the underlying client should work. Feign’s default contract defines the following annotations:

Overriding the Request Line

If there is a need to target a request to a different host then the one supplied when the Feign client was created, or you want to supply a target host for each request, include a java.net.URI parameter and Feign will use that value as the request target.

@RequestLine("POST /repos/{owner}/{repo}/issues")
void createIssue(URI host, Issue issue, @Param("owner") String owner, @Param("repo") String repo);

Templates and Expressions

Feign Expressions represent Simple String Expressions (Level 1) as defined by URI Template - RFC 6570. Expressions are expanded using their corresponding Param annotated method parameters.

Example

public interface GitHub {

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

  class Contributor {
    String login;
    int contributions;
  }
}

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

    /* The owner and repository parameters will be used to expand the owner and repo expressions
     * defined in the RequestLine.
     *
     * the resulting uri will be https://api.github.com/repos/OpenFeign/feign/contributors
     */
    github.contributors("OpenFeign", "feign");
  }
}

Expressions must be enclosed in curly braces {} and may contain regular expression patterns, separated by a colon : to restrict resolved values. Example owner must be alphabetic. {owner:[a-zA-Z]*}

Request Parameter Expansion

RequestLine and QueryMap templates follow the URI Template - RFC 6570 specification for Level 1 templates, which specifies the following:

• Unresolved expressions are omitted.
• All literals and variable values are pct-encoded, if not already encoded or marked encoded via a @Param annotation.

Undefined vs. Empty Values

Undefined expressions are expressions where the value for the expression is an explicit null or no value is provided. Per URI Template - RFC 6570, it is possible to provide an empty value for an expression. When Feign resolves an expression, it first determines if the value is defined, if it is then the query parameter will remain. If the expression is undefined, the query parameter is removed. See below for a complete breakdown.

Empty String

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   parameters.put("param", "");
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test?param=

Missing

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test

Undefined

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   parameters.put("param", null);
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test

See Advanced Usage for more examples.

What about slashes? /

@RequestLine templates do not encode slash / characters by default. To change this behavior, set the decodeSlash property on the @RequestLine to false.

What about plus? +

Per the URI specification, a + sign is allowed in both the path and query segments of a URI, however, handling of the symbol on the query can be inconsistent. In some legacy systems, the + is equivalent to the a space. Feign takes the approach of modern systems, where a + symbol should not represent a space and is explicitly encoded as %2B when found on a query string.

If you wish to use + as a space, then use the literal character or encode the value directly as %20

Custom Expansion

The @Param annotation has an optional property expander allowing for complete control over the individual parameter’s expansion. The expander property must reference a class that implements the Expander interface:

public interface Expander {
    String expand(Object value);
}

The result of this method adheres to the same rules stated above. If the result is null or an empty string, the value is omitted. If the value is not pct-encoded, it will be. See Custom @Param Expansion for more examples.

Request Headers Expansion

Headers and HeaderMap templates follow the same rules as Request Parameter Expansion with the following alterations:

• Unresolved expressions are omitted. If the result is an empty header value, the entire header is removed.
• No pct-encoding is performed.

See Headers for examples.

A Note on @Param parameters and their names:

All expressions with the same name, regardless of their position on the @RequestLine, @QueryMap, @BodyTemplate, or @Headers will resolve to the same value. In the following example, the value of contentType, will be used to resolve both the header and path expression:

public interface ContentService {
  @RequestLine("GET /api/documents/{contentType}")
  @Headers("Accept: {contentType}")
  String getDocumentByType(@Param("contentType") String type);
}

Keep this in mind when designing your interfaces.

Request Body Expansion

Body templates follow the same rules as Request Parameter Expansion with the following alterations:

• Unresolved expressions are omitted.
• Expanded value will not be passed through an Encoder before being placed on the request body.
• A Content-Type header must be specified. See Body Templates for examples.

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 request setting, you can use options(Request.Options options) on target() to set connectTimeout, connectTimeoutUnit, readTimeout, readTimeoutUnit, followRedirects. For example:

interface Bank {
  @RequestLine("POST /account/{id}")
  Account getAccountInfo(@Param("id") String id);
}

public class BankService {
  public static void main(String[] args) {
    Bank bank = Feign.builder()
        .decoder(new AccountDecoder())
        .options(new Request.Options(10, TimeUnit.SECONDS, 60, TimeUnit.SECONDS, true))
        .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.

public class CloudService {
  public static void main(String[] args) {
    CloudDNS cloudDNS = Feign.builder()
      .target(new CloudIdentityTarget<CloudDNS>(user, apiKey));
  }

  class CloudIdentityTarget extends Target<CloudDNS> {
    /* implementation of a Target */
  }
}

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 with other Open Source tools. 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:

public class Example {
  public static void main(String[] args) {
    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:

public class Example {
  public static void main(String[] args) {
      GitHub github = Feign.builder()
                     .encoder(new JacksonEncoder())
                     .decoder(new JacksonDecoder())
                     .target(GitHub.class, "https://api.github.com");
  }
}

For the lighter weight Jackson Jr, use JacksonJrEncoder and JacksonJrDecoder from the Jackson Jr Module.

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:

public class Example {
  public static void main(String[] args) {
      Api 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:

public class Example {
  public static void main(String[] args) {
    Api 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);
}

public class Example {
  public static void main(String[] args) {
    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:

public class Example {
  public static void main(String[] args) {
    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.

public class Example {
  public static void main(String[] args) {
    MyService api = Feign.builder()
          .client(RibbonClient.create())
          .target(MyService.class, "https://myAppProd");
  }
}

Java 11 Http2

Http2Client directs Feign’s http requests to Java11 New HTTP/2 Client that implements HTTP/2.

To use New HTTP/2 Client with Feign, use Java SDK 11. Then, configure Feign to use the Http2Client:

GitHub github = Feign.builder()
                     .client(new Http2Client())
                     .target(GitHub.class, "https://api.github.com");

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:

public class Example {
  public static void main(String[] args) {
    MyService api = HystrixFeign.builder().target(MyService.class, "https://myAppProd");
  }
}

SOAP

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

This module adds support for encoding and decoding SOAP Body objects via JAXB and SOAPMessage. It also provides SOAPFault decoding capabilities by wrapping them into the original javax.xml.ws.soap.SOAPFaultException, so that you’ll only need to catch SOAPFaultException in order to handle SOAPFault.

Add SOAPEncoder and/or SOAPDecoder to your Feign.Builder like so:

public class Example {
  public static void main(String[] args) {
    Api api = Feign.builder()
	     .encoder(new SOAPEncoder(jaxbFactory))
	     .decoder(new SOAPDecoder(jaxbFactory))
	     .errorDecoder(new SOAPErrorDecoder())
	     .target(MyApi.class, "http://api");
  }
}

NB: you may also need to add SOAPErrorDecoder if SOAP Faults are returned in response with error http codes (4xx, 5xx, …)

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:

public class Example {
  public static void main(String[] args) {
    GitHub github = Feign.builder()
                     .logger(new Slf4jLogger())
                     .logLevel(Level.FULL)
                     .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):

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

If you need to pre-process the response before give it to the Decoder, you can use the mapAndDecode builder method. An example use case is dealing with an API that only serves jsonp, you will maybe need to unwrap the jsonp before send it to the Json decoder of your choice:

public class Example {
  public static void main(String[] args) {
    JsonpApi jsonpApi = Feign.builder()
                         .mapAndDecode((response, type) -> jsopUnwrap(response, type), new GsonDecoder())
                         .target(JsonpApi.class, "https://some-jsonp-api.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);
}

public class Example {
  public static void main(String[] args) {
    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);
}

public class Example {
  public static void main(String[] args) {
    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);
}

public class Example {
  public static void main(String[] args) {
    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 key, V value);
}

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

public interface Api {
   @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.

public interface Api {
   @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

To customize headers for each request method on a Target, a RequestInterceptor can be used. RequestInterceptors can be shared across Target instances and are expected to be thread-safe. RequestInterceptors are applied to all request methods on a Target.

If you need per method customization, a custom Target is required, as the a RequestInterceptor does not have access to the current method metadata.

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();
    }
  }

  public class Example {
    public static void main(String[] args) {
      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 key);

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

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

public class Example {
  public static void main(String[] args) {
    GitHub github = Feign.builder()
                     .decoder(new GsonDecoder())
                     .logger(new Logger.JavaLogger("GitHub.Logger").appendToFile("logs/http.log"))
                     .logLevel(Logger.Level.FULL)
                     .target(GitHub.class, "https://api.github.com");
  }
}

A Note on JavaLogger: Avoid using of default JavaLogger() constructor - it was marked as deprecated and will be removed soon.

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");
  }
}

public class Example {
  public static void main(String[] args) {
    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.

public class Example {
  public static void main(String[] args) {
    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.

public interface Api {
  @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.

public interface Api {
  @RequestLine("GET /find")
  V find(@QueryMap Map<String, Object> queryMap);
}

This may also be used to generate the query parameters from a POJO object using a QueryMapEncoder.

public interface Api {
  @RequestLine("GET /find")
  V find(@QueryMap CustomPojo customPojo);
}

When used in this manner, without specifying a custom QueryMapEncoder, the query map will be generated using member variable names as query parameter names. The following POJO will generate query params of “/find?name={name}&number={number}” (order of included query parameters not guaranteed, and as usual, if any value is null, it will be left out).

public class CustomPojo {
  private final String name;
  private final int number;

  public CustomPojo (String name, int number) {
    this.name = name;
    this.number = number;
  }
}

To setup a custom QueryMapEncoder:

public class Example {
  public static void main(String[] args) {
    MyApi myApi = Feign.builder()
                 .queryMapEncoder(new MyCustomQueryMapEncoder())
                 .target(MyApi.class, "https://api.hostname.com");
  }
}

When annotating objects with @QueryMap, the default encoder uses reflection to inspect provided objects Fields to expand the objects values into a query string. If you prefer that the query string be built using getter and setter methods, as defined in the Java Beans API, please use the BeanQueryMapEncoder

public class Example {
  public static void main(String[] args) {
    MyApi myApi = Feign.builder()
                 .queryMapEncoder(new BeanQueryMapEncoder())
                 .target(MyApi.class, "https://api.hostname.com");
  }
}

Error Handling

If you need more control over handling unexpected responses, Feign instances can register a custom ErrorDecoder via the builder.

public class Example {
  public static void main(String[] args) {
    MyApi myApi = Feign.builder()
                 .errorDecoder(new MyErrorDecoder())
                 .target(MyApi.class, "https://api.hostname.com");
  }
}

All responses that result in an HTTP status not in the 2xx range will trigger the ErrorDecoder’s decode method, allowing you to handle the response, wrap the failure into a custom exception or perform any additional processing. If you want to retry the request again, throw a RetryableException. This will invoke the registered Retryer.

Retry

Feign, by default, will automatically retry IOExceptions, regardless of HTTP method, treating them as transient network related exceptions, and any RetryableException thrown from an ErrorDecoder. To customize this behavior, register a custom Retryer instance via the builder.

public class Example {
  public static void main(String[] args) {
    MyApi myApi = Feign.builder()
                 .retryer(new MyRetryer())
                 .target(MyApi.class, "https://api.hostname.com");
  }
}

Retryers are responsible for determining if a retry should occur by returning either a true or false from the method continueOrPropagate(RetryableException e); A Retryer instance will be created for each Client execution, allowing you to maintain state bewteen each request if desired.

If the retry is determined to be unsuccessful, the last RetryException will be thrown. To throw the original cause that led to the unsuccessful retry, build your Feign client with the exceptionPropagationPolicy() option.

Metrics

By default, feign won’t collect any metrics.

But, it’s possible to add metric collection capabilities to any feign client.

Metric Capabilities provide a first-class Metrics API that users can tap into to gain insight into the request/response lifecycle.

Dropwizard Metrics 4

public class MyApp {
  public static void main(String[] args) {
    GitHub github = Feign.builder()
                         .addCapability(new Metrics4Capability())
                         .target(GitHub.class, "https://api.github.com");

    github.contributors("OpenFeign", "feign");
    // metrics will be available from this point onwards
  }
}

Dropwizard Metrics 5

public class MyApp {
  public static void main(String[] args) {
    GitHub github = Feign.builder()
                         .addCapability(new Metrics5Capability())
                         .target(GitHub.class, "https://api.github.com");

    github.contributors("OpenFeign", "feign");
    // metrics will be available from this point onwards
  }
}

Micrometer

public class MyApp {
  public static void main(String[] args) {
    GitHub github = Feign.builder()
                         .addCapability(new MicrometerCapability())
                         .target(GitHub.class, "https://api.github.com");

    github.contributors("OpenFeign", "feign");
    // metrics will be available from this point onwards
  }
}

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");
  }
}

Async execution via CompletableFuture

Feign 10.8 introduces a new builder AsyncFeign that allow methods to return CompletableFuture instances.

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

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

    // Fetch and print a list of the contributors to this library.
    CompletableFuture<List<Contributor>> contributors = github.contributors("OpenFeign", "feign");
    for (Contributor contributor : contributors.get(1, TimeUnit.SECONDS)) {
      System.out.println(contributor.login + " (" + contributor.contributions + ")");
    }
  }
}

Initial implementation include 2 async clients:

• AsyncClient.Default
• AsyncApacheHttp5Client