1. Yes, but you need to install proxies; only one Java gateway can be installed per Zabbix server or per proxy.
2. The Java gateway will retrieve items from the JAVA application and the Zabbix server will pull the data from the gateway and process the data. We don't use Zabbix agents.
3. Ports we need to open for JMX monitoring are: 
   • For communication between Zabbix server/proxy and the gateway, we need port 10052 (not IANA registered).
   • For communication between the Java gateway and an application, we need to open another port (standard in Zabbix 12345).