java - 为什么我不断收到对 ResultSet 进行非法操作的异常？

Question

为什么我总是在 ResultSet 上收到异常非法操作？

这是代码:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/*
 * SearchParts.java
 *
 * Created on 08-Mar-2010, 12:14:31
 */
package garits;

import java.sql.*;
import javax.swing.*;

/**
 *
 * @author Deniz
 */
public class SearchParts extends javax.swing.JFrame {

        /** Creates new form SearchParts */
        public SearchParts() {
                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")


        private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
                if (!jTextField1.getText().equals("")) {
                        String result = "";
                        int Partnumber = Integer.parseInt(jTextField1.getText());
                        DB db = new DB();
                        try {
                                db.connect();
                                String query = "Select * from Stock Where Part_no =" + "'" + jTextField1.getText() + "'";
                                ResultSet rs = db.execSQL(query);
                                if (rs.equals(null)) {
                                        PartNotFound nf = new PartNotFound();
                                        nf.setVisible(true);

                                } else {
                                        ResultSetMetaData rsmd = rs.getMetaData();
                                        int numberOfColumns = rsmd.getColumnCount();
                                        int RowCount = 0;
                                        for (int i = 1; i < numberOfColumns; i++) {
                                                rs.getString(i);
                                                result += i + "/n";
                                        }

                                        if (!result.equals("")) {
                                                Receptionist_FranchiseePartFound part = new Receptionist_FranchiseePartFound();
                                                part.setVisible(true);
                                                while (rs.next()) {
                                                        RowCount++;



                                                }

                                                part.getTable().addRowSelectionInterval(0, RowCount);


                                        } else {
                                                PartNotFound nf = new PartNotFound();


                                        }
                                }
                        } catch (Exception e) {
                                e.printStackTrace();
                                JOptionPane.showMessageDialog(jButton1, "More information needed for search", "Error Message", JOptionPane.ERROR_MESSAGE);
                        }


                } else if (!jTextField2.getText().equals("")) {
                        String result = "";

                        DB db = new DB();
                        try {
                                db.connect();
                                String query = "Select * from Stock Where Part_name =" + "'" + jTextField2.getText() + "'";
                                ResultSet rs = db.execSQL(query);
                                if (rs.equals(null)) {
                                        PartNotFound nf = new PartNotFound();
                                        nf.setVisible(true);

                                } else {
                                        ResultSetMetaData rsmd = rs.getMetaData();
                                        int numberOfColumns = rsmd.getColumnCount();
                                        int RowCount = 0;
                                        for (int i = 1; i < numberOfColumns; i++) {
                                                rs.getString(i);
                                                result += i + "/n";
                                        }


                                        // Receptionist_FranchiseePartFound part = new Receptionist_FranchiseePartFound();
                                        // part.setVisible(true);
                                        if (!result.equals("")) {
                                                Receptionist_FranchiseePartFound part = new Receptionist_FranchiseePartFound();
                                                part.setVisible(true);
                                                while (rs.next()) {
                                                        RowCount++;



                                                }
                                                part.getTable().addRowSelectionInterval(0, RowCount);


                                        } else {
                                                PartNotFound nf = new PartNotFound();
                                                nf.setVisible(true);

                                        }
                                }



                        } catch (Exception e) {
                                e.printStackTrace();
                                JOptionPane.showMessageDialog(jButton1, "More information needed for search", "Error Message", JOptionPane.ERROR_MESSAGE);
                        }
                } else if (jTextField1.getText().equals("") && jTextField2.getText().equals("")) {
                        String result = "";
                        DB db = new DB();
                        try {
                                db.connect();
                                String query = "Select * from Stock Where Manufacturer =" + "'" + jTextField3.getText() + "'AND Vehicle_type ='" + jTextField4.getText() + "'";
                                ResultSet rs = db.execSQL(query);
                                if (rs.equals(null)) {
                                        PartNotFound nf = new PartNotFound();
                                        nf.setVisible(true);

                                }
                                else{

                                ResultSetMetaData rsmd = rs.getMetaData();
                                int numberOfColumns = rsmd.getColumnCount();
                                int RowCount = 0;
                                for (int i = 1; i < numberOfColumns; i++) {
                                        rs.getString(i);
                                        result += i + "/n";
                                }


                                // Receptionist_FranchiseePartFound part = new Receptionist_FranchiseePartFound();
                                // part.setVisible(true);
                                if (!result.equals("")) {
                                        Receptionist_FranchiseePartFound part = new Receptionist_FranchiseePartFound();
                                        part.setVisible(true);
                                        while (rs.next()) {
                                                RowCount++;



                                        }
                                        part.getTable().addRowSelectionInterval(0, RowCount);


                                } else {
                                        PartNotFound nf = new PartNotFound();
                                        nf.setVisible(true);

                                }
                            }



                        } catch (Exception e) {
                                e.printStackTrace();
                                JOptionPane.showMessageDialog(jButton1, "More information needed for search", "Error Message", JOptionPane.ERROR_MESSAGE);
                        }


                } else if (jTextField3.getText().equals("") || jTextField4.getText().equals("")) {
                        JOptionPane.showMessageDialog(jButton1, "More information needed for search", "Error Message", JOptionPane.ERROR_MESSAGE);
                }

        }
        /**
         * @param args the command line arguments
         */
        // Variables declaration - do not modify
        private javax.swing.JButton jButton1;
        private javax.swing.JButton jButton2;
        private javax.swing.JLabel jLabel1;
        private javax.swing.JLabel jLabel2;
        private javax.swing.JLabel jLabel3;
        private javax.swing.JLabel jLabel4;
        private javax.swing.JLabel jLabel5;
        private javax.swing.JLabel jLabel6;
        private javax.swing.JLabel jLabel7;
        private javax.swing.JLabel jLabel8;
        private javax.swing.JTextField jTextField1;
        private javax.swing.JTextField jTextField2;
        private javax.swing.JTextField jTextField3;
        private javax.swing.JTextField jTextField4;
        // End of variables declaration
}

Answer 1

我看到了一些东西

• 第一个 rs 永远不会为空。如果没有行，则 rs.next() 将返回 false

• ResultSet 在第一行之前开始位置。您需要调用 rs.next() 移动到第一行，如上所述，如果没有行，则返回 false。

ResultSet 使用的典型模式是...

while (rs.next())
{
  rs.getXXX();
}

• 完成后，ResultSet 应该始终 close()d，否则可能会泄漏数据库资源。这也适用于 Connection、Statement、PreparedStatement，因此请检查其他类中的数据库代码。

• 循环列元数据时您需要执行的操作..

  for (int i = 1; i <= 列数; i++) {}

请注意 <= 而不是 <。正如您所正确发现的，JDBC 列索引从 1 开始。

• 值得考虑使用一个隐藏所有 JDBC 噪音的库。考虑一下 spring，它具有基本的 JDBC 抽象，同时保留所有功能或一直到 hibernate，它非常强大，但有相当长的学习曲线。对于你的情况，我建议只使用 Spring Material 。

参见:Spring docs