在过去的3个小时里，我尝试了很多代码来摆脱我在将数据插入java db（derby）时遇到的错误。请建议我一个正确的解决方案。单击addButton后，数据不会插入到db中。它显示以下错误。 plsease建议我一些想法。
桂班：
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import net.miginfocom.swing.*;
import javax.swing.border.*;
import javax.swing.BorderFactory;
import java.sql.*;
import java.util.*;
public class Gui {
JFrame frame;
JPanel mainPanel;
JPanel addPanel;
JPanel wordsPanel;
JPanel randomPanel;
JPanel wordTestPanel;
JTextField wordNameField;
JTextField wordMeaningField;
JLabel DialogLabel;
MigLayout layout;
TitledBorder title;
public static void main(String[] args) {
Gui gui = new Gui();
gui.showFrame();
}
public void showFrame() {
frame = new JFrame("My Words");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
layout = new MigLayout();
Container content = frame.getContentPane();
content.add(showMainPanel());
frame.pack();
frame.setVisible(true);
}
public JPanel showMainPanel() {
mainPanel = new JPanel();
mainPanel.setLayout(layout);
mainPanel.add(showAddPanel(),"wrap");
mainPanel.add(showRandomPanel(),"wrap");
mainPanel.add(showWordTestPanel(),"wrap");
mainPanel.add(showWordsPanel(),"east");
return mainPanel;
}
public JPanel showAddPanel() {
addPanel = new JPanel();
addPanel.setLayout(layout);
title = BorderFactory.createTitledBorder("Add Word");
title.setTitleJustification(TitledBorder.RIGHT);
addPanel.setBorder(title);
JLabel wordNameLabel = new JLabel("Word Name: ");
JLabel wordMeaningLabel = new JLabel("Word Meaning: ");
wordNameField = new JTextField(20);
wordMeaningField = new JTextField(20);
JButton addButton = new JButton("Add");
addButton.addActionListener(new addButtonListener());
JButton clearButton = new JButton("Clear");
DialogLabel = new JLabel(".");
addPanel.add(wordNameLabel,"align label");
addPanel.add(wordNameField,"wrap");
addPanel.add(wordMeaningLabel,"align label");
addPanel.add(wordMeaningField,"wrap");
addPanel.add(addButton,"span 2");
addPanel.add(clearButton,"span 2,wrap");
addPanel.add(DialogLabel,"span");
return addPanel;
}
public JPanel showRandomPanel() {
randomPanel = new JPanel();
randomPanel.setLayout(layout);
title = BorderFactory.createTitledBorder("Random Words");
title.setTitleJustification(TitledBorder.RIGHT);
randomPanel.setBorder(title);
JLabel wordName = new JLabel("Hello");
JLabel wordMeaning = new JLabel("this is the meaning");
randomPanel.add(wordName,"wrap");
randomPanel.add(wordMeaning,"span");
return randomPanel;
}
public JPanel showWordTestPanel() {
wordTestPanel = new JPanel();
wordTestPanel.setLayout(layout);
title = BorderFactory.createTitledBorder("Word Test");
title.setTitleJustification(TitledBorder.RIGHT);
wordTestPanel.setBorder(title);
return wordTestPanel;
}
public JPanel showWordsPanel() {
wordsPanel = new JPanel();
wordsPanel.setLayout(layout);
title = BorderFactory.createTitledBorder("List of Words");
title.setTitleJustification(TitledBorder.RIGHT);
wordsPanel.setBorder(title);
return wordsPanel;
}
class addButtonListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
String word = wordNameField.getText();
String meaning = wordMeaningField.getText();
Connection conn = null;
Statement stmt = null;
try {
JDBCUtil.getConnection();
stmt = conn.createStatement();
String SQL; SQL = "INSERT INTO words(wordName,wordMeaning) VALUES(" + word + "," + meaning + ")";
stmt.executeUpdate(SQL);
} catch(SQLException se) {
se.printStackTrace();
} finally {
JDBCUtil.closeStatement(stmt);
JDBCUtil.closeConnection(conn);
}
}
}
}
JDBCUtil类：
import java.sql.*;
public class JDBCUtil {
public static Connection getConnection() throws SQLException {
Driver derbyEmbeddedDriver = new org.apache.derby.jdbc.EmbeddedDriver();
DriverManager.registerDriver(derbyEmbeddedDriver);
String dbURL = "jdbc:derby:mywords;create = true;";
Connection con = DriverManager.getConnection(dbURL);
con.setAutoCommit(false);
return con;
}
public static void closeConnection(Connection con) {
try {
if(con != null) {
con.close();
}
} catch(SQLException e) {
e.printStackTrace();
}
}
public static void closeStatement(Statement stmt) {
try {
if(stmt != null) {
stmt.close();
}
} catch(SQLException e) {
e.printStackTrace();
}
}
public static void closeResultSet(ResultSet rs) {
try {
if(rs != null) {
rs.close();
}
} catch(SQLException e) {
e.printStackTrace();
}
}
public static void commit(Connection con) {
try {
if(con != null) {
con.commit();
}
} catch(SQLException e) {
e.printStackTrace();
}
}
public static void rollback(Connection con) {
try {
if(con!=null) {
con.rollback();
}
} catch(SQLException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Connection con = null;
try {
con = JDBCUtil.getConnection();
System.out.println("Connected to the Database");
} catch(SQLException e) {
e.printStackTrace();
} finally {
JDBCUtil.closeConnection(con);
}
}
}
错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Gui$addButtonListener.actionPerformed(Gui.java:114)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:20 18)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.jav a: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:3320)
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:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
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:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
答案 0 :(得分：2)
您获得了连接，但是您没有存储对它的引用，因此conn仍然为空。
尝试
conn = JDBCUtil.getConnection();
至少，这就是我可以通过格式化解决的问题。
答案 1 :(得分：2)
Connection conn = null;
JDBCUtil.getConnection();
stmt = conn.createStatement();
conn始终为空。
将conn设为JDBCUtil.getConnection();，如conn = JDBCUtil.getConnection();