OJ, formerly named OpenJava, is a programming tool that parses and analyzes Java source code.
It uses a metaobject protocol (MOP) to provide services for language extensions.
Its first release was back to 1997, and won the Student Encouragement Prize at the Java Conference Grandprix '97 held in Japan.
[1] This isn't to be confused with OpenJDK, which is the open source release of the Java compiler runtime and tools.
OpenJava was renamed OJ in October 2007 at the request of Sun Microsystems.