Feign is a java to http client binder inspired by [Dagger](https://github.com/square/dagger), [Retrofit](https://github.com/square/retrofit), [jclouds](https://github.com/jclouds/jclouds), and [JAXRS-2.0](https://jax-rs-spec.java.net/nonav/2.0/apidocs/index.html). Feign's first goal was reducing the complexity of binding [Denominator](https://github.com/Netflix/Denominator) uniformly to http apis regardless of [restfulness](http://www.slideshare.net/adrianfcole/99problems).
### 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](https://github.com/square/retrofit/blob/master/retrofit-samples/github-client/src/main/java/com/example/retrofit/GitHubClient.java).
The last argument to `Feign.create` specifies how to decode the responses. You can plug-in your favorite library, such as gson, or use builtin RegEx Pattern decoders. Here's how the Gson module looks.
Feign doesn't offer a built-in json decoder as you can see above it is very few lines of code to wire yours in. If you are a jackson user, you'd probably thank us for not dragging in a dependency you don't use.
### 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.
You can find [several examples](https://github.com/Netflix/feign/tree/master/feign-core/src/test/java/feign/examples) in the test tree. Do take time to look at them, as seeing is believing!
[JAXRSModule](https://github.com/Netflix/feign/tree/master/feign-jaxrs) 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:
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.
Almost all configuration of Feign is represented as Map bindings, where the key is either the simple name (ex. `GitHub`) or the method (ex. `GitHub#contributors()`) in javadoc link format. For example, the following routes all decoding to gson: