Eclipse:GWT connect to MYSQL:error:the driver has not received any packets from the server

Question

I'm using GWT to connect to MYSQL. But I every time I got the error as follows: 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. I have putthe driver files to the war\WEB-INF\lib. And the driver has been load. For a normal JAVA project I can use the same code to find file in MYSQL. But why can't use the same code in GWT project?

public class GreetingServiceImpl extends RemoteServiceServlet implements GreetingService {

public String greetServer(String input) throws IllegalArgumentException {
                            Connection conn = null;
                    String url ="jdbc:mysql://localhost:3306/pcmorder";
            String driver = "com.mysql.jdbc.Driver";
            String user     = "root";
            String pass     = "123456";


            //String query = "SELECT id FROM tb_user WHERE username = ";
            String sql=null ;
            String[] namelist;
            int name1=0 ;

            try {
                Class.forName(driver).newInstance();
                System.out.println("OK"); 
                conn = DriverManager.getConnection(url, user, pass);