GNU classpath is currently used. There are however some minor differences.
It is intended that Classpath is used out of the box somewhere in the future. In the mean time, classpath is part of the JNode source repository and is synced on a somewhat regular based with the latest version from classpath.org.
Edited by Fabien D :
Since it is open sourced, we are moving to openjdk (instead of using GNU Classpath).