com.mysql.jdbc.exceptions.jdbc4.CommunicationsException:Communications link failure [duplicate]

My program that connects to a MySQL database was working fine. Then, without changing any code used to set up the connection, I get this exception:

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.

What happened?

The code used to get the connection:

private static Connection getDBConnection() throws SQLException, InstantiationException, IllegalAccessException, ClassNotFoundException {
    String username = "user";
    String password = "pass";
    String url = "jdbc:mysql://www.domain.com:3306/dbName?connectTimeout=3000";

    Class.forName("com.mysql.jdbc.Driver");
    Connection conn = DriverManager.getConnection(url, username, password);
    return conn;
}

This is a wrapped exception and not really interesting. It is the root cause of the exception which actually tells us something about the root cause. Please look a bit further in the stacktrace. The chance is big that you'll then face a SQLException: Connection refused or SQLException: Connection timed out.

If this is true in your case as well, then all the possible causes are:

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. Something in between Java and DB is blocking connections, e.g. a firewall or proxy.

To solve the one or the either, 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 it.
5. Verify if mysqld is started without the --skip-networking option.
6. Disable firewall and/or configure firewall/proxy to allow/forward the port.

By the way (and unrelated to the actual problem), you don't necessarily need to load the JDBC driver on every getConnection() call. Just only once during startup is enough.

check your wait timeout set on the DB server. Some times it defaults to 10 seconds. This looses the connection in 10 seconds.

mysql> show global variables like '%time%' ;

update it make it something like 28800

mysql> SET GLOBAL wait_timeout = 28800;