GWT MySQL 连接问题

Question

I am working on a GWT project in which I need to make some MySQL queries. I have handled RPC properly and in the server-side I am trying to make a mysql connection but am running into an exception:

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server. at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) at java.lang.reflect.Constructor.newInstance(Unknown Source) at com.google.appengine.tools.development.agent.runtime.Runtime.newInstance_(Runtime.java:112) at com.google.appengine.tools.development.agent.runtime.Runtime.newInstance(Runtime.java:120) at com.mysql.jdbc.Util.handleNewInstance(Util.java:411) at com.mysql.jdbc.Util.getInstance(Util.java:386) at Z4D236D9A2D102C5FE6AD1C50D A4BEC50Z.mysql.jdbc.SQLError.createSQLException(SQLError.java:1013) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:982) at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:927) at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2412) at com.mysql.Z84BEFFD3A0D49636A 58CE6080CAA87C7Z.ConnectionImpl.createNewIO(ConnectionImpl.java:2154) at com.mysql.jdbc.ConnectionImpl.(ConnectionImpl.java:792) at com.mysql.jdbc.JDBC4Connection.(JDBC4Connection.java:47) at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) at java.lang.reflect.Constructor.newInstance(Unknown Source) at com.google.appengine.tools.development.agent.runtime.Runtime.newInstance_(Runtime.java:112) at com.google.appengine.tools.developmen t.agent.runtime.Runtime.newInstance(Runtime.java:120) 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(Unknown Source) at java.sql.DriverManager.getConnection(Unknown Source). . . more. . .

I created a very simple java class to test out my code to confirm that my connection syntax and use is proper, and have confirmed that syntax is not a problem because with the very same code in a simple only main method java app I can create a connection and query the database properly. I have made sure to have mysql-connector-java-5.1.16-bin.jar in the classpath as well as the /lib folder in the WEB-INF folder.

Here is the class that I am using in order to create a connection:

public class DB_Connection {

protected Connection getConnection() { Connection conn = null; try { Class.forName("com.mysql.jdbc.Driver").newInstance(); conn = DriverManager.getConnection("jdbc:mysql:///", "", "");
} catch (Exception ex) { ex.printStackTrace(); }

  return conn;

}

}

public class Login extends DB_Connection {

public User getUser(String email) { User user = new User(); user.setUserEmail(email); String query = "";

  try {
     Connection conn = getConnection();
     Statement statement = conn.createStatement();
     ResultSet result = statement.executeQuery(query);
     while(result.next()) {
        user.setUserId(result.getInt("rvuser_id"));
        System.out.println(user.getUserId());
     }
  }
  catch (Exception ex)
  {
     ex.printStackTrace();
  }

  return user;

}

It seems my problem happens all the way at the beginning in DB_Connection. Does anyone know what is making this happen? It is strange to me, because as a standalone the code works fine.

Another thing I just realized that I am at home now and don't even have access to that database server as it is on a local network and I am not connected through VPN. So it must be failing somehow before it even attempts to make the connection.

Thanks!

Answer 1

com.google.appengine.. Does it means you are deploying on Google App Engine? If so, MySQL,JDBC and many other things are restricted.