OpenFeign

OpenFeign is a Java library for declarative REST clients. It enables developers to define interfaces whose methods correspond to remote HTTP endpoints, with annotations specifying HTTP methods, paths, and parameters. At runtime, a dynamic proxy implements the interface and executes the HTTP requests, allowing client code to call remote services as if they were local objects. This approach reduces boilerplate and centralizes HTTP client configuration.

Key features include a declarative syntax via annotations, support for pluggable encoders and decoders (for JSON, XML, and other formats), and a pluggable contract that interprets interface annotations. OpenFeign clients can be customized with request interceptors, retry behavior, and custom error handling. It also supports integration with other ecosystems, notably Spring Cloud OpenFeign, which lets developers annotate interfaces with Spring MVC annotations and @FeignClient to leverage service discovery and load balancing.

Architecture and usage: OpenFeign builds a dynamic proxy from a configuration that includes the target URL, encoder/decoder, and the HTTP client implementation (such as OkHttp or Apache HttpClient). When a method is invoked, Feign converts the method call into an HTTP request, sends it, and decodes the response into the declared return type.

Context: OpenFeign is a community-maintained continuation of Netflix Feign, released under an open-source license. It is widely used in Java microservices architectures to simplify cross-service HTTP communication and to replace more boilerplate REST client code.