PL/Java is a free add-on module that brings Java™ Stored Procedures, Triggers, Functions, Aggregates, Operators, Types, etc., to the PostgreSQL™ backend.
Renamed the branch, as there's more doc-rotted documentation than just that one bit. GitHub closes the PR when the branch is renamed. Reopened as #454.
Stumbled on a comment that was not updated in keeping with another change four years ago, leaving it bewilderingly bogus.