How to fix this java mysql exception: Communications link failure?

Question

Here is the log of this exception:

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

The last packet successfully rec         


        

Answer 1

I had com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure reading more on the error message I realized it had to do with my SSL setting in the connection string. When you have SSL enabled in your connection string for MySQL.Please make sure your date and time are correct or in sync with the current date. If not the connection will be hit with javax.net.ssl.SSLHandshakeException resulting in The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server. . prompting you to do something about your date. This is how I solved mine. I had to set my date and time correctly. Sometimes the error messages say a lot about the problem.

Answer 2

Welcome to TCP/IP. Lots of things can cause loss of a TCP connection, especially one that has been idle for a while. One such thing, as you mention, is a firewall. The connection can be knocked down by some network entity even if both the client and the server agree it should be kept alive. Connection loss likelihood goes up when client and server are not on the same local network.

Figuring out why means doing lots of packet monitoring at various places in the network. That can use up a lot of time and effort and not teach you much. Plus, learning to read wireshark output is a real task.

Most client-server programmers who need long-lasting connections use some kind of keepalive operation to avoid having the connection sit idle for too long. Keepalive operations send something and get something back. In your case you could do this the easy way by issuing a SELECT NOW() query (or some other round-trip no-op) once every minute or two while your client sits otherwise idle.

The best way to handle this kind of thing is to open the database connection when you need it, then close it when you're done. If you use the connection pooling feature you can open and close your pooled connection upon every query, and still avoid churning the physical connections. The JDBC connector and MySQL server code are optimized for this approach; it is probably the best way to go.