The OpenDMK project now includes the SNMP API from Sun's Java DMK product. Kudos to Daniel Fuchs who put in the considerable work to make this happen!
This API includes the necessary interfaces to build both managers (clients) and agents (servers) that use SNMP. It also includes a tool called mibgen which takes an SNMP MIB (schema) and generates skeleton JMX MBeans. Implementing the MIB in an agent is then just a question of filling in the right behaviour in those MBeans. Daniel's bloghas much more information about SNMP and about OpenDMK.
Some parts of this SNMP code are still encumbered so we're not yet able to release full source. We're working on that, but in the meantime the missing parts are supplied as a binary plug, in the manner of OpenJDK.