RxJava

RxJava is a Java VM implementation of Reactive Extensions (ReactiveX) for composing asynchronous and event-based programs using observable sequences. It provides a library of operators that enable functional-style composition and a way to manage threading with schedulers. It runs on the Java Virtual Machine and is used in Java and Android applications.

Core concepts include observables, which emit items over time, and observers, which receive them via onNext, onError, and onComplete. Operators transform, filter, combine, or otherwise manipulate sequences. Backpressure support is provided in RxJava 2 and 3 via the Flowable type, addressing scenarios where producers emit items faster than consumers can process. Subscriptions are represented by Disposable objects, enabling cancellation and resource management. Schedulers control where work runs, such as IO, computation, or the Android main thread.

Versions and ecosystem: RxJava 2 introduced a separate Flowable type and improved backpressure handling, along with updated error handling and API changes. RxJava 3 is the current major version with updated dependencies and Java 8+ requirements. RxJava is part of the ReactiveX family, and RxAndroid provides Android-specific bindings for UI-thread interactions.

Usage patterns typically involve creating observables from various sources, then composing with operators such as map, flatMap, filter, and zip. Subscribing to receive events and managing the resulting Disposable helps prevent leaks. Operators can be chained into reusable sequences, and patterns that align with component lifecycles are commonly used to coordinate with UI elements.