com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: How to handle this exception?

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I'm working on getting my database to talk to my Java programs.

Can someone give me a quick sample program using the JDBC?
Oct 8, 2018 in Java by Daisy
• 8,050 points
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1 answer to this question.

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You have a

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure java.net.ConnectException: Connection refused.

If you get a SQLException: Connection refused or Connection timed out or a MySQL specific CommunicationsException: Communications link failure, then it means that the DB isn't reachable at all. This can have one or more of the following causes:

  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. 

To solve the one or the other, follow the following advices:

  1. Verify and test them with ping.
  2. Refresh DNS or use IP address in JDBC URL instead.
  3. Verify it based on my.cnf of MySQL DB.
  4. Start the DB.
  5. Verify if mysqld is started without the --skip-networking option.
  6. Restart the DB and fix your code accordingly that it closes connections in finally.
  7. Disable firewall and/or configure firewall/proxy to allow/forward the port. 
answered Oct 8, 2018 by parth295
• 4,640 points

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