JDA (Java Discord API) is a comprehensive Java library for building Discord bots. It provides a full-featured client for Discord's REST API and Gateway (WebSocket) protocol, including support for slash commands, message components (buttons, select menus, modals), voice/audio (Opus, DAVE encryption), auto-moderation, scheduled events, forum channels, polls, stickers, soundboard, and sharding. The architecture is event-driven with a rich hierarchy of typed events, an asynchronous request system (RestAction) with combinators (map, flatMap, delay), and a fluent manager pattern for entity modifications. It includes a custom component tree with immutable replacement, ETF (Erlang External Term Format) encoding for gateway payloads, and a thread-safe caching layer using Trove maps. The codebase is well-structured with extensive unit tests, snapshot testing, and architecture compliance tests. It is actively maintained with regular releases and a large community.
Discord4J is a reactive, modular Java library for building Discord bots. It provides a high-level abstraction over Discord's REST API, Gateway WebSocket, and Voice protocols, built on Project Reactor and Netty. The library is structured into separate modules: `common` (shared utilities, store abstraction, rate limiting), `rest` (HTTP client with rate limiting and retry), `gateway` (WebSocket client with state management), `core` (domain objects, event system, spec builders), `voice` (voice connection with encryption and DAVE protocol), and `oauth2` (OAuth2 authentication). The architecture follows a layered design with pluggable components for caching (StoreLayout), rate limiting (GlobalRateLimiter), serialization (ExchangeStrategies), and event dispatching. The codebase demonstrates strong software engineering practices: extensive use of immutable specs via the Immutables library, reactive streams for all I/O, and a comprehensive event hierarchy mirroring Discord's API. However, it contains a mix of deprecated legacy specs and newer immutable specs, some experimental features, and a few duplicated utilities. Overall, it is a well-crafted, production-ready library with active maintenance and good reusability in its core patterns.