A high-performance implementation of the BACnet I/P and MS/TP protocol written for Java (minimum version 1.5) by Infinite Automation Systems. Supports all BACnet services and full message segregation. Can be used for field devices or for control platforms. For commercial licenses contact www.InfiniteAutomation.com
The most recent code is here:
For support see: http://infiniteautomation.com/forum/category/12/bacnet4j-general-discussion
Simple to use.
I have reviewed Mango M2M that uses this BACnet driver and so far it seems pretty solid, and actively supported.