Solving a “communications link failure” with JDBC

Question

This question already has an answer here:

• com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure 30 answers

I'm trying to connect to the local MySQL server but I keep getting an error.

Here is the code.

public class Connect {

    public static void main(String[] args) {
        Connection conn = null;

        try {
            String userName = "myUsername";
            String password = "myPassword";

            String url = "jdbc:mysql://localhost:3306/myDatabaseName";
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            conn = DriverManager.getConnection(url, userName, password);
            System.out.println("Database connection established");
        } catch (Exception e) {
            System.err.println("Cannot connect to database server");
            System.err.println(e.getMessage());
            e.printStackTrace();
        } finally {
            if (conn != null) {
                try {
                    conn.close();
                    System.out.println("Database Connection Terminated");
                } catch (Exception e) {}
            }
        }
    }
}

and the errors :

Cannot connect to database server
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.
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.
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
        at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1116)
        at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:344)
        at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2333)
        at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2370)
        at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2154)
        at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:792)
        at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
        at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:381)
        at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:305)
        at java.sql.DriverManager.getConnection(DriverManager.java:582)
        at java.sql.DriverManager.getConnection(DriverManager.java:185)
        at Connect.main(Connect.java:16)
    Caused by: java.net.ConnectException: Connection refused
        at java.net.PlainSocketImpl.socketConnect(Native Method)
        at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)
        at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)
        at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
        at java.net.Socket.connect(Socket.java:529)
        at java.net.Socket.connect(Socket.java:478)
        at java.net.Socket.<init>(Socket.java:375)
        at java.net.Socket.<init>(Socket.java:218)
        at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:257)
        at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:294)
        ... 15 more

I've set the classpath, made sure my.cnf had the skip network option commented out.

java version is 1.2.0_26 (64 bit) mysql 5.5.14 mysql connector 5.1.17

I made sure that the user had access to my database.

Answer 1

I just restarted MySQL (following a tip from here: https://stackoverflow.com/a/14238800) and it solved the issue.

I had the same issue on MacOS (10.10.2) and MySql (5.6.21) installed via homebrew.

The confusing thing was that one of my apps connected to the database fine and the other did not.

After trying many things on the app that threw the exception com.mysql.jdbc.CommunicationsException as suggested by the the accepted answer of this question to no avail, I was surprised that restarting MySQL worked.

The cause of my issue might have been the following as suggested in the answer in the aforementioned link:

Are you using connection pool ? If yes, then try to restart the server. Probably few of the connections in your connection pool are in closed state.

Answer 2

Setting the bind-address to the server's network IP instead of the localhost default, and setting privileges on my user worked for me.

my.cnf:

bind-address = 192.168.123.456

MySql Console:

GRANT ALL PRIVILEGES ON dbname.* to username@'%' IDENTIFIED BY 'password';

Answer 3

I was experiencing similar problem and the solution for my case was

1. changing bind-address = 0.0.0.0 from 127.0.0.1
2. changing url's localhost to localhost:3306

the thing i felt is we should never give up, i tried every options from this post and from other forums as well...happy it works @saurab

Answer 4

It is majorly because of weak connection between mysql client and remote mysql server.

In my case it is because of flaky VPN connection.

Answer 5

In my case it was an idle timeout, that caused the connection to be dropped on the server. The connection was kept open, but not used for a long period of time. Then a client restart works, while I believe a reconnect will work as well.

A not bad solution is to have a daemon/service to ping the connection from time to time.

Answer 6

As the detailed answer above says, this error can be caused by many things.

I had this problem too. My setup was Mac OSX 10.8, using a Vagrant managed VirtualBox VM of Ubuntu 12.04, with MySQL 5.5.34.

I had correctly setup port forwarding in the Vagrant config file. I could telnet to the MySQL instance both from my Mac and from within the VM. So I knew the MySQL daemon was running and reachable. But when I tried to connect over JDBC, I got the "Communications link failure" error.

In my case, the problem was solved by editing the /etc/mysql/my.cnf file. Specifically, I commented out the "#bind-address=127.0.0.1" line.