When Zabbix Java Gateway uses RMI JMX it does not close connections fast. By default

sun.rmi.transport.connectionTimeout

parameter has 15 seconds to close, so it is possible to reach max FD in case of RMI JMX monitoring. Probably it is a good idea to add the argument with 1000 (one second) to default Zabbix Java packages and, maybe, to sources as well.

sun.rmi.transport.connectionTimeout (1.1.6 and later):

The value of this property represents the period (in milliseconds) for which socket connections may reside in an "unused" state, before the Java RMI runtime will allow those connections to be freed (closed). The default value is 15000 milliseconds (15 seconds). See also sun.rmi.transport.tcp.readTimeout.