We present JavaGo 4.0, a library for thread migration and checkpointing in Java.
In version 4.0, we have re-designed the JavaGo Language Specification using
standard Java 5 annotations such that no additional keywords are used, which
eases the developers' work as they can keep using their favorite IDE and can
produce both a migratable and a non-migratable version of their program out of
the same source code.

Java source code with JavaGo specific annotations is transformed into migratable
Java code by the JavaGo source-to-source compiler jgoc. In version 4.0, it has
been re-developed from scratch, now supporting the Java 6 standard, producing user-
friendly, meaningful error / warning messages on compilation problems, and running
much faster than its previous version.

In this paper we present the JavaGo 4.0 Language Specification, compiler internals,
especially the implementation of the unfolding technique, and some evaluations
concerning code growth etc.