OpenJava is an extensible language based on Java(TM). The OpenJava MOP (Metaobject Protocol) is the extension interface of the language. Through the MOP, the programmers can customize the language to implement a new language mechanism. It is fully written in Java of JDK 1.1, so it can be run on any platform which supports Java Virtual Machine of JDK 1.1. And, the generated codes are written in regular Java language, so the user’s products can be run on any platform which supports JVM of JDK 1.0 or JDK 1.1.