使用 Java derby DB 进行 Java 登录

Question

这是我的数据库连接类

import java.sql.*;

public class connectWithDB {


public static void DBconnection(){

  Connection conn = null;

  String url = "jdbc:derby://localhost:1527/";
  String dbName = "MyTinyShopDB";
  String driver = "org.apache.derby.jdbc.ClientDriver";
  String userName = "root"; 
  String password = "root";

  try {
  Class.forName(driver).newInstance();
  conn = DriverManager.getConnection(url+dbName,userName,password);

  //conn.close();

  } 

 catch (Exception e)
 {
  e.printStackTrace();
  }

 }

}

这是登录按钮的代码

    private void btnLoginActionPerformed(java.awt.event.ActionEvent evt) {                                         
   Connection conn = null;
   PreparedStatement prestmnt = null;
   ResultSet Reltset = null;


       try {
           String sql = "SELECT * FROM LOGINDETAILS WHERE LOGINID='"+txtFieldUserName.getText()+"'AND USERPASSWORD='"+txtFieldPassword.getText()+"'";
           prestmnt=conn.prepareStatement(sql);
           Reltset=prestmnt.executeQuery();

           if (Reltset.next()){

           AdminMainForm adminform = new AdminMainForm();
           adminform.setVisible(true);
           }

           else
           {
           JOptionPane.showMessageDialog(null, "User Name or Password is Wrong");
           }


       } catch (SQLException ex) {
           Logger.getLogger(Login.class.getName()).log(Level.SEVERE, null, ex);
       }

    }

程序正在运行，但是当我单击登录按钮时，netbeans 中会出现这些错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mytinyshop.Login.btnLoginActionPerformed(Login.java:159)
at mytinyshop.Login.access$200(Login.java:18)
at mytinyshop.Login$3.actionPerformed(Login.java:77)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341 )
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java: 402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

以及更多错误

我已在数据库中添加了用户名和密码，它必须检查数据库，如果用户名和密码正确，它必须转到其他表单，否则它必须显示错误的密码对话框，请有人帮助我..

Answer 1

您在 btnLoginActionPerformed() 开始时将连接设置为 null

Connection conn = null;

然后你尝试在 5 行后使用它

prestmnt=conn.prepareStatement(sql);

在 null 引用上调用方法将导致 NullPointerException