Question

I tried out DAO Pattern after reading
http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html
but I get NullPointerException when I run the servlet

DAOFactory

package dao;
import java.sql.SQLException;
public abstract class DAOFactory {
// List of DAO types supported by the factory
public static final int MYSQL = 1;

public abstract UserDAO getUserDAO() throws SQLException;

public static DAOFactory getDAOFactory(int whichFactory) {

switch (whichFactory) {
  case MYSQL: 
      return new MySQLDAOFactory();
  default        : 
      return null;
}
}
}

MySQLDAOFactory

package dao;
import java.sql.*;
public class MySQLDAOFactory extends DAOFactory {

public MySQLDAOFactory() {
}
public static final String DRIVER="com.mysql.jdbc.Driver";
public static final String DBURL="jdbc:mysql://localhost:3306/musicompany";

public static Connection createConnection() {
    Connection conn=null;
    try{
        Class.forName(DRIVER);
        conn =  DriverManager.getConnection(DBURL,"root","toor");
    }catch(Exception e){}
    return conn;
}
public UserDAO getUserDAO() throws SQLException{
    return new MySQLUserDAO();
}
}

MySQLUserDAO

package dao;
import java.sql.*;
import java.util.ArrayList;
import model.User;

public class MySQLUserDAO implements UserDAO {

Connection conn=null;
Statement s=null;
public MySQLUserDAO() throws SQLException{
    conn = MySQLDAOFactory.createConnection();
    s = conn.createStatement();
}

public int insertUser(String fname, String lname) 
{
    try{
    //code to insertUser
    }catch(Exception e){}
    return key;
}


public ArrayList<User> selectUsers() {
    ResultSet rs=null;
    ArrayList customerList=null;
    try{
        rs =s.executeQuery("select * from users");
        customerList = new ArrayList<User>();
        while(rs.next())
        {
            customerList.add(new User(rs.getString("name"), rs.getString("pass"), rs.getInt("type"), rs.getString("email")));
        }
        return customerList;
    }catch(Exception e){}
    return customerList;
}
}

UserDAO

package dao;
import java.util.ArrayList;
import model.User;

public interface UserDAO {
public int insertUser(String fname, String lname);
public ArrayList<User> selectUsers();
} 

model.User.java

This class has the following fields and the corresponding accessor methods.

String name;
String pass;
short type;
String email;

My Servlet file

public class AddRetrieve extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException{
    response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    try {
        // create the required DAO Factory
        DAOFactory factoryObj = DAOFactory.getDAOFactory(DAOFactory.MYSQL);

        // Create a DAO
        UserDAO custDAO = factoryObj.getUserDAO();

        // print existing users
        ArrayList list = custDAO.selectUsers();
        Iterator i = list.iterator();
        while(i.hasNext())
        {
            User o = (User)i.next();
            out.println(o.getName());
        }

    } catch(Exception e)
    {
        out.println(e);
    }
    finally {            
        out.close();
    }
}

Answer 1

Stack trace points to MySQLUserDAO class and line s = conn.createStatement()

So, the conn is null.

Let’s look how you’re creating the conn:

public static Connection createConnection() {
    Connection conn=null;
    try{
        Class.forName(DRIVER);
        conn =  DriverManager.getConnection(DBURL,"root","toor");
    }catch(Exception e){}
    return conn;
}

So, you’re ignoring any exception thrown and returning a null connection instead of throwing an exception so that the code immediately stops.

Your technical problem is caused by the driver apparently not being in the classpath and your real design problem is you should never ignore exceptions unless you’re 100% sure that it’s safe to continue the code flow.

It’s by the way unnecessary to load the driver everytime. Just load it once on class’ initialization.

Rewrite that piece as follows:

static {
    try{
        Class.forName(DRIVER);
    } catch (ClassNotFoundException e) {
        throw new ExceptionInInitializerError("Driver " + DRIVER + " missing in classpath", e);
    }
}

public static Connection createConnection() throws SQLException {
    return DriverManager.getConnection(DBURL, "root", "toor");
}