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Jayden Bell
Jayden Bell

Total Post:105

Points:739
Posted on    June-12-2013 12:23 AM

 Java Java 
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I have a simple login screen that connects to a MySQL database. It connects just fine, but it throws a null pointer exception when it tries to execute. Here are two snippets of my code as well as the stack trace. I bolded the line where the exception is being thrown.

package GUI;

 

import java.sql.*;

import javax.swing.JOptionPane;

import java.awt.event.*;

 

public class LogInFrame extends javax.swing.JFrame

{

 

    Connection con;

    Statement st;

    ResultSet rs;

 

    // Login Constructor

    public LogInFrame()

    {

        connect();

        initComponents();

        setLocationRelativeTo(null);

    }

 

    public void connect(){

         try

        {

            con = DriverManager.getConnection(

                    "jdbc:mysql://instance44668.db.xeround.com:5719/obliquedb?"

                    + "user=NAME2&password=PASS");

        }

        catch (SQLException ex)

        {

            System.err.println("Cannot connect to database server");

            System.err.println(ex.getMessage());

        }

        finally

        {

            try

            {

                con.close();

            }

            catch (SQLException ex)

            {

                System.out.println("Better luck next time");

            }

        }

    }

and here is the method that executes the statement

private void acceptButtonActionPerformed(java.awt.event.ActionEvent evt)                                         
    {                                             
        try 
        {
            String user = userTextField.getText().trim();
            char[] password = passwordTextField.getPassword();
            String pass = new String(password);
            String query = "SELECT username,password FROM users WHERE username = '"+user+"'and password = '"+pass+"'";
  EXCEPTION THROWN-**rs = st.executeQuery(query);**
            int count = 0;
            while(rs.next())
            {
                count++;
            }
            if(count == 1) {
                JOptionPane.showMessageDialog(this, 
                        "Login Successful", 
                        "Error Message", JOptionPane.ERROR_MESSAGE);
            }
            else if(count > 1) {
                JOptionPane.showMessageDialog(this, 
                        "Duplicate Records Found!", 
                        "Error Message", JOptionPane.ERROR_MESSAGE);
            }
            else {
                JOptionPane.showMessageDialog(this, 
                        "User Not Found", 
                        "Error Message", JOptionPane.ERROR_MESSAGE);
            }
        }
        catch (Exception e) {
            System.out.println("Error Retrieving Records" + e);
    }                                        
    }

and stack trace

Error Retrieving Recordsjava.lang.NullPointerException
java.lang.NullPointerException
    at GUI.LogInFrame.acceptButtonActionPerformed(LogInFrame.java:178)
    at GUI.LogInFrame.access$000(LogInFrame.java:7)
    at GUI.LogInFrame$1.actionPerformed(LogInFrame.java:94)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2713)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
    at java.awt.EventQueue.access$000(EventQueue.java:101)
    at java.awt.EventQueue$3.run(EventQueue.java:666)
    at java.awt.EventQueue$3.run(EventQueue.java:664)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:680)
    at java.awt.EventQueue$4.run(EventQueue.java:678)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)


Vijay Shukla

Total Post:100

Points:700
Posted on    June-12-2013 8:04 AM

Hello Jayden Bell!
In connect the connection is immediately closed.
The statement better should be a PreparedStatement (having miscellaneous advantages).
So create the statement in connect too and remove the finally.
Make a disconnect called on application exit closing the statement and connection.
The result set rs should be close too.
String query = "SELECT username,password "
           + "FROM users "
           + "WHERE BINARY username = ? "
           + "AND BINARY password = ?";

For case sensitive comparison, either specify a binary collation for the columns, or use BINARY.

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