A high-performance and ease-of-use implementation of the Modbus protocol written in Java by Serotonin Software. Supports ASCII, RTU, TCP, and UDP transports as slave or master, automatic request partitioning and response data type parsing.
I had been looking for a higher-functionality replacement to jamod, which seems to have been completely abandoned, despite claims by the administrator that it has been. I had been hopeful that modbus4j would have more of the Modbus messages covered, but it seems that it is another far-less-than-complete implementation of the Modbus protocol. That it is GPL'd, instead of LGPL'd is the nail in the coffin -- libraries really need to be under the LGPL so developers don't have to worry about the viral nature of the GPL in their applications. I'd give two thumbs down if I could, but one will have to do.