OpenJava is an extensible language based on Java. The extended features of OpenJava are specified by a meta-level program given at compile time. For distinction, programs written in OpenJava are called base-level programs. If no meta-level program is given, OpenJava is identical to regular Java.
The meta-level program extends OpenJava through the interface called the OpenJava MOP (Metaobject Protocol). The OpenJava compiler consists of three stages: preprocessor, source-to-source translator from OpenJava to Java, and the back-end Java compiler. The OpenJava MOP is an interface to control the translator at the second stage. It allows to specify how an extended feature of OpenJava is translated into regular Java code.
An extended feature of OpenJava is supplied as an add-on software for the compiler. The add-on software consists of not only the meta-level program but runtime support code. The runtime support code provides classes used by the base-level program translated into Java. The base-level program in OpenJava is first translated into Java by the meta-level program and is dynamically linked with the runtime support code.
The meta-level program is written in Java, accurately in OpenJava because OpenJava is a self-reflective language. It defines new metaobjects to control source-to-source translation. The metaobjects are the meta-level representation of the base-level program and they perform the translation. Details of the metaobjects are specified by the OpenJava MOP. In the following, we go through a few examples so that we illustrate how the OpenJava MOP is used to implement language extensions.