MySQL Java Database Connectivity Communications Exception

Java Database Connectivity (JDBC) is the Java API that allows you to access and execute queries and commands on a database.

This article describes some steps to troubleshoot the com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
error, which occurs when you try to connect to a database by using JDBC.

The Communications link failure error occurs when the database cannot be reached. Following are the possible causes and their solutions:

The IP address or hostname in JDBC URL is wrong.

Verify and test that the hostname or the IP address can answer back by using
ping.

The local DNS server does not recognise the hostname in the JDBC URL

Refresh your DNS or use the IP address in JDBC URL instead.

The port number is missing or incorrect in JDBC URL

Verify that the IP is well configured in the MySQL® database my.cnf file by using your favorite
text editor.

To find the my.cnf file, run the following command to check your MySQL settings:

mysql --help | grep "Default options" -A 1 

If you still can't find the file, try running the following command:

find / -name my.cnf

You can also look in the default locations:

• /etc/my.cnf
• /etc/mysql/my.cnf
• /usr/etc/my.cnf
• ~/.my.cnf

The database server is not accepting TCP/IP connections

Verify that the mysqld service is rrunning. To verify the status of the mysqld service, run the following commands
according to your Linux® distribution:

For CentOS® 6 or Red Hat® Enterprise Linux® (RHEL) 6:

service mariadb status

For CentOS 7+ or RHEL 7+:

systemctl status mariadb.service

For the Ubuntu® operating system:

/etc/init.d/mysql status

The database is down or powered off

Run the following command to start the MySQL database:

For CentOS 6 / RHEL 6:

service mariadb start

For CentOS 7+ / RHEL 7+:

systemctl start mariadb.service

For the Ubuntu operating system:

/etc/init.d/mysql start

The database server has run out of connections

Restart the database and see if your code is having an issue when closing the connections.

Connections between Java and DB are blocking the communication

Disable the firewall or proxy. Alternately, configure them to allow or forward the port. If you are using CentOS 6/RHEL 6,
review the iptables article.