There are some activities related to Java implementation between 2002 and 2005.Some publications are:
A Java API for HL7 Version 3 RIM
Java SIG API Demo Release (including demo)
V3 Messaging Standard http://www.hl7.org/implement/standards/v3messages.cfm
Java API to HL7 Version 3 RIM
Afterwards, there are still updates to the standard, but java api development is not as active as before.
Reference link: HL7 Projects