For developers interested in contributing to the Hibernate O/RM project and may be new to the Hibernate codebase, Git and/or GitHub be sure to read the CONTRIBUTING.md file in the project root.
Additionally, if you are unfamiliar with Gradle (the build tool we use), be sure to read the README.md file in the project root.
For testing Hibernate O/RM we use JUnit 4, however we have developed a set of extensions to JUnit that make it easier to run tests on multiple environments (different databases, jvms, etc) as well as supplying some additional testing features we felt were lacking. For details on these extensions, see the JUnit Infastructure page. Our testing approach also leverages extensions of Gradle via a custom Gradle plugin whose code is currently in the buildSrc project directory; see its README.md as well for details.
For members of the development team, details on one approach for applying pull requests can be seen at Handling-Pull-Requests.
Taking a page from the Gradle development team, I want to take a look at moving to a more formalized process for defining, discussing and scoping proposals for enhancements and new features. The idea is that each such change would have its own wiki page. At the very least this would make it easy for contributors to help implement these changes and ideally make it easier to document them later.
See Development Proposals for this list of proposals.