我是一名学生，是JDBC概念的新手。我有个问题。我使用GUI创建了一个测试Java程序。
它将要求用户在文本字段中输入其名称，并且当用户单击“添加记录”按钮时，应将键入文本字段的文本上载到数据库。我已经创建了一个示例表，它仅包含一个字段。
当我运行上述程序时，异常语句将输出Null错误。但是，自从我 insert data method 包含System.out.print，它可以打印出在文本字段中键入的值。
这是我制作的示例GUI的代码：
import java.awt.BorderLayout;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import java.awt.FlowLayout;
import javax.swing.JTextField;
import javax.swing.JLabel;
import javax.swing.JButton;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.GroupLayout;
import javax.swing.GroupLayout.Alignment;
public class UIData extends JFrame {
private JPanel contentPane;
public static JTextField textField;
public static String Name;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
UIData frame = new UIData();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public UIData() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 823, 472);
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
JLabel lblInsertName = new JLabel("Insert Name: ");
textField = new JTextField();
textField.setColumns(10);
JButton btnAddRecord = new JButton("Add Record");
GroupLayout gl_contentPane = new GroupLayout(contentPane);
gl_contentPane.setHorizontalGroup(
gl_contentPane.createParallelGroup(Alignment.LEADING)
.addGroup(gl_contentPane.createSequentialGroup()
.addGap(65)
.addComponent(lblInsertName)
.addGap(107)
.addComponent(textField, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE))
.addGroup(gl_contentPane.createSequentialGroup()
.addGap(149)
.addComponent(btnAddRecord))
);
gl_contentPane.setVerticalGroup(
gl_contentPane.createParallelGroup(Alignment.LEADING)
.addGroup(gl_contentPane.createSequentialGroup()
.addGap(12)
.addGroup(gl_contentPane.createParallelGroup(Alignment.LEADING)
.addGroup(gl_contentPane.createSequentialGroup()
.addGap(3)
.addComponent(lblInsertName))
.addComponent(textField, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE))
.addGap(133)
.addComponent(btnAddRecord))
);
contentPane.setLayout(gl_contentPane);
btnAddRecord.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent arg0) {
UIDBConnect connect = new UIDBConnect();
connect.insertData();
}
});
}
}
这是我用于连接到所述数据库的代码
import java.sql.*;
public class UIDBConnect extends UIData {
private Connection con;
private Statement st;
private ResultSet rs;
private PreparedStatement pst;
public static String str = textField.getText();
public void DBConnect() {
try {
Class.forName("com.mysql.cj.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://localhost:3306/hospital?useUnicode=true&useJDBCCompliantTimezoneShift=true&useLegacyDatetimeCode=false&serverTimezone=UTC", "root", "");
st=con.createStatement();
} catch(Exception ex) {
System.out.println("Error : " + ex.getMessage());
}
}
public void insertData() {
try {
System.out.println("Inserting records into the table...");
System.out.println(str);
st = con.createStatement();
String sql = "INSERT INTO tbltest " +
"VALUES ('" + str + "')";
st.executeUpdate(sql);
} catch(Exception ex) {
System.out.println("Error : " + ex.getMessage());
}
}
}
错误打印出like this。
我希望我能清楚地解释。
编辑1：使用ex.printStackTrace（）时；该错误显示如下：
java.lang.NullPointerException
at UIDBConnect.insertData(UIDBConnect.java:43)
at UIData$2.mouseClicked(UIData.java:84)
at java.awt.AWTEventMulticaster.mouseClicked(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)