Why MySQL Connections Fail on CentOS with Spring Boot and How to Fix Them

Problem Overview

A Spring Boot project runs fine locally and can connect to the MySQL server from the command line, but when the same application is deployed on a CentOS server it throws a CommunicationsException indicating a communications link failure.

Exception Details

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
... (stack trace) ...
Caused by: java.net.ConnectException: Connection refused: connect

Root Causes & Solutions

1. Incompatible JDK Version

The server originally used the OpenJDK package installed via yum, while the local machine used the Oracle JDK 1.8.0_151. Reinstalling the Oracle JDK 1.8.0_241 on the server resolved the connection problem.

2. Incorrect mysql.sock Path

After moving MySQL data to a new disk and updating datadir, the socket file location changed (default locations are /var/lib/mysql/mysql.sock or /tmp/mysql.sock). Updating all MySQL client sections ([client], [mysql], [mysqld]) to use the same socket path fixed the Communications link failure error.

3. SSL Connection Issues

When the JDK was upgraded to 1.8, an SSLHandshakeException may appear:

javax.net.ssl.SSLHandshakeException: No appropriate protocol (protocol is disabled or cipher suites are inappropriate)
    at sun.security.ssl.Handshaker.activate(Handshaker.java:529)
    ...

Removing SSLv3 from the jdk.tls.disabledAlgorithms entry in $JAVA_HOME/jre/lib/security/java.security resolves the issue. If MySQL is configured to use SSL, follow the standard MySQL SSL setup instructions.

4. MySQL Wait Timeout

The default wait_timeout is 8 hours (28800 s). When a connection stays idle longer, MySQL closes it, but the connection pool may still consider it valid, leading to the same exception during runtime.

Fix: increase the timeout in my.ini or add &autoReconnect=true to the JDBC URL. Example configuration:

wait_timeout=31536000
interactive_timeout=31536000

Restart MySQL after editing.

5. Connection Pool Idle Time

If the pool’s maxIdleTime is set to 0, idle connections are never discarded, causing stale connections to be reused. Setting maxIdleTime to a reasonable value (e.g., 20 seconds) prevents the error.

6. Other Common Causes

Database account permissions – ensure the IP and user are authorized.

Network firewall – open the MySQL port (default 3306).

Incorrect port number or blocked port.

Wrong password or missing IP‑specific privileges.

Driver version mismatch with the MySQL server.

Unstable network causing intermittent drops.

Connection pool size too large for the MySQL max connections.

IPv4 vs IPv6 address conflicts.

By checking each of these items, developers can quickly pinpoint and resolve MySQL connection failures on CentOS deployments.