我的目的是使用Jframe创建登录页面。 我已经在一个单独的文件中创建了数据库连接类，如下所示，当我运行登录Jframe时，我收到一个错误，指出nullpointer异常。
请帮助我：）
dbConnection类--------------------------------------------- -
package vehicle_renting;
import java.sql.*;
import javax.swing.JOptionPane;
public class dbConnection {
Connection con;
Statement stmt;
ResultSet rs;
public dbConnection() {}
public void connect() {
try {
Class.forName("com.jdbc.mysql.Driver");
con=DriverManager.getConnection("jdbc:mysql://localhost:3306/vehicle_renting_1","root","qwer1234");
} catch (Exception e) {
e.printStackTrace();
}
}
}
以下代码放在jframe源代码中。
package vehicle_renting;
import java.sql.* ;
import javax.swing.* ;
public class Login extends javax.swing.JFrame {
Connection conn ;
ResultSet rs;
PreparedStatement pst;
/**
* Creates new form Login
*/
public Login() {
initComponents();
}
/**
* This method is called from within the constructor to initialize the form.
* WARNING: Do NOT modify this code. The content of this method is always
* regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
jLabel1 = new javax.swing.JLabel();
jLabel2 = new javax.swing.JLabel();
jButton1 = new javax.swing.JButton();
txt_username = new javax.swing.JTextField();
txt_password = new javax.swing.JPasswordField();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jLabel1.setText("Username");
jLabel2.setText("Password");
jButton1.setText("Ok");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton1ActionPerformed(evt);
}
});
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(110, 110, 110)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addGroup(layout.createSequentialGroup()
.addComponent(jLabel2)
.addGap(18, 18, 18)
.addComponent(txt_password))
.addGroup(layout.createSequentialGroup()
.addComponent(jLabel1)
.addGap(18, 18, 18)
.addComponent(txt_username, javax.swing.GroupLayout.PREFERRED_SIZE, 180, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addContainerGap(44, Short.MAX_VALUE))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addComponent(jButton1)
.addGap(143, 143, 143))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(60, 60, 60)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel1)
.addComponent(txt_username, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(18, 18, 18)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel2)
.addComponent(txt_password, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(18, 18, 18)
.addComponent(jButton1)
.addContainerGap(141, Short.MAX_VALUE))
);
pack();
}// </editor-fold>
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
String username=txt_username.getText();
String password=txt_password.getText();
String sql = "select * from login where username = ? and password = ?";
try
{
pst = conn.prepareStatement(sql);
pst.setString(1, username); // Passing the values to the username from the textbox
pst.setString(2, password); // Passing the values to the password from the textbox
rs = pst.executeQuery(); // Storing the results retrieved from the query
if (rs.next())
{
JOptionPane.showMessageDialog(null, "Username and Password correct");
}
else
{
JOptionPane.showMessageDialog(null, "invalid username and password");
}
}
catch (Exception e)
{
JOptionPane.showMessageDialog(null, e);
}
}
/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
try {
for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
javax.swing.UIManager.setLookAndFeel(info.getClassName());
break;
}
}
} catch (ClassNotFoundException ex) {
java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new Login().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton jButton1;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JPasswordField txt_password;
private javax.swing.JTextField txt_username;
// End of variables declaration
}
当我运行Jframe时，它会说＆＃34; java.lang.NullPointerException＆＃34; 。我无法在此找到错误。谢谢
try
{ pst = conn.prepareStatement(sql);
pst.setString(1, username); // Passing the values to the username from the textbox
pst.setString(2, password); // Passing the values to the password from the textbox
rs = pst.executeQuery(); // Storing the results retrieved from the query
if (rs.next())
{
JOptionPane.showMessageDialog(null, "Username and Password correct");
}
else
{
JOptionPane.showMessageDialog(null, "invalid username and password");
}
}
catch (Exception e)
{
JOptionPane.showMessageDialog(null, e);
}
我认为上面的代码段给了我错误。我不得不编辑JOptionPane来找到它。
答案 0 :(得分：0)
我说你没有在你的jframe代码中初始化属性conn