Feign simplifies the process of writing Java HTTP clients
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.
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
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]*}
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
@Paramparameters and their names:All expressions with the same name, regardless of their position on the
@RequestLine,@QueryMap,@BodyTemplate, or@Headerswill resolve to the same value. In the following example, the value ofcontentType, 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](#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](#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.<br> For request setting, you can use `options(Request.Options options)` on `target()` to set connectTimeout, connectTimeoutUnit, readTimeout, readTimeoutUnit, followRedirects.<br> For example: ```java 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!
Encoder/Decoder
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.
Moshi
Moshi includes an encoder and decoder you can use with a JSON API.
Add MoshiEncoder and/or MoshiDecoder to your Feign.Builder like so:
GitHub github = Feign.builder()
.encoder(new MoshiEncoder())
.decoder(new MoshiDecoder())
.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:
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");
}
}
Fastjson2
fastjson2 includes an encoder and decoder you can use with a JSON API.
Add Fastjson2Encoder and/or Fastjson2Decoder to your Feign.Builder like so:
public class Example {
public static void main(String[] args) {
GitHub github = Feign.builder()
.encoder(new Fastjson2Encoder())
.decoder(new Fastjson2Decoder())
.target(GitHub.class, "https://api.github.com");
}
}
Contract
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");
}
}
Client
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");
Breaker
Logger
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");
}
}
If any methods in your interface return type Stream, you’ll need to configure a StreamDecoder.
Here’s how to configure Stream decoder without delegate decoder:
public class Example {
public static void main(String[] args) {
GitHub github = Feign.builder()
.decoder(StreamDecoder.create((r, t) -> {
BufferedReader bufferedReader = new BufferedReader(r.body().asReader(UTF_8));
return bufferedReader.lines().iterator();
}))
.target(GitHub.class, "https://api.github.com");
}
}
Here’s how to configure Stream decoder with delegate decoder:
public class Example {
public static void main(String[] args) {
GitHub github = Feign.builder()
.decoder(StreamDecoder.create((r, t) -> {
BufferedReader bufferedReader = new BufferedReader(r.body().asReader(UTF_8));
return bufferedReader.lines().iterator();
}, (r, t) -> "this is delegate decoder"))
.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);
}
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"}
}
}
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.
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.
To filter out sensitive information like authorization or tokens
override methods shouldLogRequestHeader or shouldLogResponseHeader.
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. You can annotate a specific field of CustomPojo with the @Param annotation to specify a different name to the query parameter. The following POJO will generate query params of “/find?name={name}&number={number}®ion_id={regionId}” (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;
@Param("region_id")
private final String regionId;
public CustomPojo (String name, int number, String regionId) {
this.name = name;
this.number = number;
this.regionId = regionId;
}
}
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.
The following example shows how to refresh token and retry with ErrorDecoder and Retryer when received a 401 response.
public class Example {
public static void main(String[] args) {
var github = Feign.builder()
.decoder(new GsonDecoder())
.retryer(new MyRetryer(100, 3))
.errorDecoder(new MyErrorDecoder())
.target(Github.class, "https://api.github.com");
var contributors = github.contributors("foo", "bar", "invalid_token");
for (var contributor : contributors) {
System.out.println(contributor.login + " " + contributor.contributions);
}
}
static class MyErrorDecoder implements ErrorDecoder {
private final ErrorDecoder defaultErrorDecoder = new Default();
@Override
public Exception decode(String methodKey, Response response) {
// wrapper 401 to RetryableException in order to retry
if (response.status() == 401) {
return new RetryableException(response.status(), response.reason(), response.request().httpMethod(), null, response.request());
}
return defaultErrorDecoder.decode(methodKey, response);
}
}
static class MyRetryer implements Retryer {
private final long period;
private final int maxAttempts;
private int attempt = 1;
public MyRetryer(long period, int maxAttempts) {
this.period = period;
this.maxAttempts = maxAttempts;
}
@Override
public void continueOrPropagate(RetryableException e) {
if (++attempt > maxAttempts) {
throw e;
}
if (e.status() == 401) {
// remove Authorization first, otherwise Feign will add a new Authorization header
// cause github responses a 400 bad request
e.request().requestTemplate().removeHeader("Authorization");
e.request().requestTemplate().header("Authorization", "Bearer " + getNewToken());
try {
Thread.sleep(period);
} catch (InterruptedException ex) {
throw e;
}
} else {
throw e;
}
}
// Access an external api to obtain new token
// In this example, we can simply return a fixed token to demonstrate how Retryer works
private String getNewToken() {
return "newToken";
}
@Override
public Retryer clone() {
return new MyRetryer(period, maxAttempts);
}
}
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.
Response Interceptor
If you need to treat what would otherwise be an error as a success and return a result rather than throw an exception then you may use a ResponseInterceptor.
As an example Feign includes a simple RedirectionInterceptor that can be used to extract the location header from redirection responses.
public interface Api {
// returns a 302 response
@RequestLine("GET /location")
String location();
}
public class MyApp {
public static void main(String[] args) {
// Configure the HTTP client to ignore redirection
Api api = Feign.builder()
.options(new Options(10, TimeUnit.SECONDS, 60, TimeUnit.SECONDS, false))
.responseInterceptor(new RedirectionInterceptor())
.target(Api.class, "https://redirect.example.com");
}
}
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.
A Note on Metrics modules:
All the metric-integrations are built in separate modules and not available in the
feign-coremodule. You will need to add them to your dependencies.
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.builder()
.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.DefaultAsyncApacheHttp5Client
Maven’s Bill of Material (BOM)
Keeping all feign libraries on the same version is essential to avoid incompatible binaries. When consuming external dependencies, can be tricky to make sure only one version is present.
With that in mind, feign build generates a module called feign-bom that locks the versions for all feign-* modules.
The Bill Of Material is a special POM file that groups dependency versions that are known to be valid and tested to work together. This will reduce the developers’ pain of having to test the compatibility of different versions and reduce the chances to have version mismatches.
Here is one example of what feign BOM file looks like.