CommunicationsException: Communications link failu

So, you have a

CommunicationsException: Communications link failure

I'm quoting from this answer which also contains a step-by-step MySQL+JDBC tutorial:

If you get a SQLException: Connection refused or Connection timed out or a MySQL specific CommunicationsException: Communications link failure, then it means that the DB isn't reachable at all. This can have one or more of the following causes:

1. IP address or hostname in JDBC URL is wrong.
2. Hostname in JDBC URL is not recognized by local DNS server.
3. Port number is missing or wrong in JDBC URL.
4. DB server is down.
5. DB server doesn't accept TCP/IP connections.
6. DB server has run out of connections.
7. Something in between Java and DB is blocking connections, e.g. a firewall or proxy. 
   
   

To solve the one or the other, follow the following advices:

1. Verify and test them with ping.
2. Refresh DNS or use IP address in JDBC URL instead.
3. Verify it based on my.cnf of MySQL DB.
4. Start the DB.
5. Verify if mysqld is started without the --skip-networking option.
6. Restart the DB and fix your code accordingly that it closes connections in finally.
7. Disable firewall and/or configure firewall/proxy to allow/forward the port.