Troubleshooting com.mysql.jdbc.exceptions.jdbc4.communicationsexception: Communications Link Failure - Causes and Solutions

In this guide, we will discuss the common causes of the com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications Link Failure error and provide step-by-step solutions to resolve this issue.

Table of Contents

  1. Causes of Communications Link Failure
  1. Solutions
  1. FAQ

Network Issues

One of the common reasons for this error is network issues between the client and the MySQL server. This can be due to unstable network connections or the MySQL server being unreachable.

MySQL Server Configuration

Another common cause is the MySQL server configuration. The server may be configured to listen only on specific IP addresses or not listening on the default port (3306).

Firewall Restrictions

Firewall restrictions on the client or server side can block the communication between the client and the MySQL server.

Incorrect JDBC Connection URL

An incorrect JDBC connection URL can also cause this error. The URL may have a wrong hostname, port, or database name.


Solution 1: Check Network Connectivity

  1. Test the network connectivity between the client and the MySQL server using the ping command.
ping <MySQL_Server_IP>
  1. If the ping command does not return any response or shows packet loss, it indicates network issues. Contact your network administrator to resolve the issue.

Solution 2: Verify MySQL Server Configuration

  1. Check the MySQL server configuration file (usually my.cnf or my.ini) and ensure that the bind-address is set to the correct IP address, or to listen on all available IP addresses.
bind-address =
  1. Verify that the MySQL server is listening on the correct port (default is 3306). You can use the following command to check the port:
sudo netstat -tuln | grep 3306
  1. Restart MySQL server after making any changes to the configuration file.
sudo service mysql restart

Solution 3: Configure Firewall

Check the firewall settings on both the client and server sides. Ensure that the required port (default is 3306) is allowed in the firewall rules.

You can use the following command to allow the MySQL port on the server-side:

sudo iptables -A INPUT -p tcp --dport 3306 -j ACCEPT

Solution 4: Verify JDBC Connection URL

  1. Check the JDBC connection URL in your Java application. Ensure that the hostname, port, and database name are correct.
  1. Update the URL if necessary and restart your Java application.


1. What is the default port for MySQL server?

The default port for MySQL server is 3306.

2. How can I check the current MySQL server configuration?

You can check the current MySQL server configuration by locating and viewing the my.cnf or my.ini file, usually found in /etc/mysql/ on Linux systems or C:\ProgramData\MySQL\MySQL Server x.x\ on Windows systems.

3. How do I restart the MySQL server after making changes to the configuration?

To restart the MySQL server, use the following command:

sudo service mysql restart

4. How can I test the network connectivity between the client and the MySQL server?

You can test the network connectivity using the ping command:

ping <MySQL_Server_IP>

5. How do I allow the MySQL port in the firewall rules on the server side?

You can use the following command to allow the MySQL port on the server side:

sudo iptables -A INPUT -p tcp --dport 3306 -j ACCEPT

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