User root has two accounts in the mysql user table. Go in with the "mysql" command line query tool and you will see this. You must add the password to both. The mysql tools will use a local Unix socket, but jboss must use TCP/IP to connect. The second root@linuxserver record also has to have the password in its password column for your connection string to work. A quick and dirty way to update the one with the empty password, using the encrypted characters from the one with the password.