Apache Pulsar - distributed pub-sub messaging system

JeroMQ is a pure Java implementation of the ZeroMQ messaging library, offering high-performance asynchronous messaging for distributed or concurrent applications.

EventMesh is a new generation serverless event middleware for building distributed event-driven applications.

WebSocket & WAMP in Java for Android and Java 8

Domain-Driven-Design Pub/Sub Domain-Events framework

HiveMQ CE is a Java-based open source MQTT broker that fully supports MQTT 3.x and MQTT 5. It is the foundation of the HiveMQ Enterprise Connectivity and Messaging Platform

Fast and reliable message broker built on top of Kafka.

An open source Valkey client library that supports Valkey, and Redis open source 6.2, 7.0 and 7.2. Valkey GLIDE is designed for reliability, optimized performance, and high-availability, for Valkey and Redis OSS based applications. GLIDE is a multi language client library, written in Rust with programming language bindings, such as Java and Python

Kafka-on-Pulsar - A protocol handler that brings native Kafka protocol to Apache Pulsar

A JVM library for all Twitch APIs, including Chat, Helix, and EventSub (webhook/websocket/conduit). It includes advanced features for large-scale apps such as connection pooling, rate-limiting, and more!

(M) Highly optimized, extremely modular and very flexible XMPP/Jabber server

NYBus (RxBus) - A pub-sub library for Android and Java applications

Divolte Collector

Streaming Anomaly Detection Solution by using Pub/Sub, Dataflow, BQML & Cloud DLP

Sample project to get started with the Pub/Sub API

MemQ is an efficient, scalable cloud native PubSub system

Java and Android class communication library: New and improved Pub-Sub

AMQP on Pulsar protocol handler

Ballerina MQTT Module.

Plugin to simplify communication between multiple Minecraft servers (and proxies) in a network. Supports plugin messages, redis and MQTT (e.g. using RabbitMQ or Google Cloud. MQTT is untested rn!). RedisBungee alternative.