[ec2-user@ip ~]$ nc -zv mydb.us-east-1.rds.amazonaws.com 5432
Connection to mydb.us-east-1.rds.amazonaws.com 5432 port [tcp/postgres] succeeded!


[ec2-user@ip ~]$ psql postgres -h mydb.us-east-1.rds.amazonaws.com -U userguy
Password for user userguy:
psql (9.5.4, server 9.6.1)
WARNING: psql major version 9.5, server major version 9.6.
         Some psql features might not work.
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384, bits: 256, compression: off)
Type "help" for help.

postgres=> q


[ec2-user@ip myjars]$ java -jar MyJar.jar param1
Connecting to jdbc:postgresql://mydb.us-east-1.rds.amazonaws.com:5432/postgres
org.postgresql.util.PSQLException: Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.