square/retrofit

Retrofit is a type-safe HTTP client library for Android and the Java Virtual Machine (JVM), developed and open-sourced by Square. The problem it solves is making REST API calls in Java and Android cleaner and safer. Without a library like Retrofit, calling a web API requires manually constructing URLs, building HTTP requests, sending them, parsing the raw JSON or XML response, and handling errors, all low-level, repetitive, and error-prone work. Retrofit lets you describe your entire API as a Java or Kotlin interface using annotations. Each method in the interface represents one API endpoint: you annotate it with the HTTP method and URL path, annotate the parameters to indicate which become URL segments, query parameters, or request body fields, and specify the return type. Retrofit then generates the actual implementation of that interface at runtime, handling URL construction, serialization, and deserialization automatically. Because the API contract is expressed as a regular Java/Kotlin interface with typed method signatures, your IDE can autocomplete method calls and your compiler can catch mistakes, for example, passing the wrong type of argument or forgetting a required parameter, before you ever run the code. This is what "type-safe" means in this context. Retrofit works with multiple serialization backends (converters) to handle different data formats: Gson, Jackson, Moshi, and others for JSON, as well as Protobuf and XML. For asynchronous calls it integrates with RxJava, Kotlin coroutines, and Java Futures, so you can choose the concurrency model that fits your project. Networking is built on top of Square's OkHttp library. Android developers and backend Java/Kotlin service developers reach for Retrofit whenever they need to consume a REST API. It is one of the most widely adopted networking libraries in the Android ecosystem. It requires Java 8 or newer, or Android API level 21 and above. It is distributed via Maven Central and licensed under Apache 2.0.