While diagnosing a problem on a big installation of OpenNMS, I found that there are several threads blocked by org.snmp4j.security.SecurityProtocols.addDefaultProtocols.
That method is invoked inside SNMP4J, it is typically related with SNMPv3 stuff, but it doesn't seem to be a way to avoid calling it when creating an SNMP session, and the system on which this problem was detected doesn't use SNMPv3 at all.
Potential Workarounds (could have side effects):
1. Go back to JoeSNMP
2. Comment org.snmp4j.smisyntaxes in opennms.properties