com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

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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.

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