Activiti is a light-weight workflow and Business Process Management (BPM) Platform targeted at business people, developers and system admins. Its core is a super-fast and rock-solid BPMN 2 process engine for Java. It's open-source and distributed under the Apache license. Activiti runs in any Java application, on a server, on a cluster or in the cloud. It integrates perfectly with Spring, it is extremely lightweight and based on simple concepts.
Apache Camel is an open source integration framework with 350+ connectors. Write routes in Java, YAML, or XML. Run on Spring Boot, Quarkus, or standalone. Apache License 2.0.
OpenPDF is an open-source Java library for creating, editing, rendering, and encrypting PDF documents, as well as generating PDFs from HTML. It is licensed under the LGPL and MPL.
TLC is a model checker for specifications written in TLA+. The TLA+Toolbox is an IDE for TLA+.
An XMPP server licensed under the Open Source Apache License.
An enterprise-grade Java-based, Apache 2.0 licensed Ethereum client https://github.com/besu-eth/besu/wiki
CUBA Platform is a high level framework for enterprise applications development
YukiHub is a Galgame management tool for Android that supports synchronization of local games, shortcuts, and play records. The project uses the GPL-3.0 open-source license.
Pure Java LP, QP and MIP solvers plus fast linear algebra. Zero dependencies, MIT licensed. Mathematical optimization and linear algebra for the JVM.