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

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

So, you have a

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

Im quoting from this answer which also contains a step-by-step MySQL+JDBC tutorial:

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 isnt 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 doesnt 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.

See also:

• How should I connect to JDBC database / datasource in a servlet based application?
• Is it safe to use a static java.sql.Connection instance in a multithreaded system?

In my case, I needed to do a replacement of Localhost to the actual database server IP address

Instead of

 Connection con = DriverManager.getConnection(
 jdbc:mysql://localhost:3306/DBname, root, root);

I needed

 Connection con = DriverManager.getConnection(
 jdbc:mysql://192.100.0.000:3306/DBname, root, root);

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

I catch this exception when Java out of heap. If I try to put in RAM many data items – first I catch Communications link failure and next OutOfMemoryError.

I logged it and I decrease memory consumption (delete 1/2 data) and all ok.