我正在创建一个 java 应用程序，它使用 ObjectDB 来创建和维护一组数据库。我目前正在尝试实现一个数据库来存储由用户名和密码字符串组成的用户对象。在 JFrame/swing 类上，我有一个用于创建新用户的按钮，单击此按钮时，我希望发生以下情况：
- 创建（或连接到）数据库
- 搜索数据库以查看是否存在具有提供的用户名的用户对象
- 如果用户已存在，则显示对话框消息，否则创建用户
但是，当单击此按钮时，我在使用查询对象结果的行上收到“找不到用户”错误。我很确定这是因为我有一个空数据库，但是我希望代码在程序第一次运行时能够工作，因此它需要处理一个空数据库。我尝试在首次运行查询之前更改代码以创建新用户，然后每次单击按钮时代码都会按照我希望的方式工作，并且它可以检测是否需要创建新用户。
我试图创建一个“默认”或“管理员”类型的用户，以便搜索可以工作，但这意味着每次运行程序时都会创建一个重复的“默认”用户，这显然是一个不需要的功能，我可以' t 执行查询来检查数据库是否为空（因此我只能在程序第一次运行时创建默认用户），因为这是我首先遇到的问题！
那么，对于任何有使用 ObjectDB 经验的人来说，有没有办法可以处理搜索空数据库的情况？
这是我的代码的相关部分：
public class FrameLogIn extends JFrame {
private JPanel contentPane;
private String username;
char[] password;
private static EntityManager em;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
FrameLogIn frame = new FrameLogIn();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public FrameLogIn() {
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("user.odb");
em = emf.createEntityManager();
final JTextField txtUsername = new JTextField();
txtUsername.setFont(new Font("Segoe UI Light", Font.PLAIN, 30));
txtUsername.setHorizontalAlignment(SwingConstants.CENTER);
txtUsername.setToolTipText("username");
txtUsername.setText("");
txtUsername.setBounds(220, 30, 177, 52);
contentPane.add(txtUsername);
txtUsername.setColumns(10);
final JPasswordField passwordField = new JPasswordField();
passwordField.setFont(new Font("Tahoma", Font.PLAIN, 30));
passwordField.setHorizontalAlignment(SwingConstants.CENTER);
passwordField.setBounds(220, 93, 177, 52);
contentPane.add(passwordField);
JButton btnNewUser = new JButton("New user");
btnNewUser.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
username = txtUsername.getText();
password = passwordField.getPassword();
System.out.println(username);
TypedQuery<Long> q = em.createQuery(
"SELECT COUNT(u) FROM User u "
+ "WHERE u.name = '" + username + "'", Long.class);
if (q.getSingleResult()>0) {
JOptionPane.showMessageDialog(contentPane.getParent(), "A user with this name already exists.");
} else {
em.getTransaction().begin();
User newUser = new User(username, password.toString());
em.persist(newUser);
em.getTransaction().commit();
JOptionPane.showMessageDialog(contentPane.getParent(), "User created.");
}
}
});
btnNewUser.setFont(new Font("Segoe WP Semibold", Font.PLAIN, 25));
btnNewUser.setBackground(Color.WHITE);
btnNewUser.setBounds(71, 175, 149, 63);
contentPane.add(btnNewUser);
}
}
和错误：
Exception in thread "AWT-EventQueue-0" [ObjectDB 2.5.3_03] SELECT COUNT(u) FROM ==> User <== u WHERE u.name = ''
javax.persistence.PersistenceException
Type User is not found (error 301)
(position 21) at com.objectdb.jpa.JpaQuery.getSingleResult(JpaQuery.java:723)
at sg.FrameLogIn$2.actionPerformed(FrameLogIn.java:113)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(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$200(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$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.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$1.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)
Caused by: com.objectdb.o.TEX: Type User is not found
at com.objectdb.o.MSG.e(MSG.java:107)
at com.objectdb.o.TRS.g(TRS.java:212)
at com.objectdb.o.SYR.q(SYR.java:259)
at com.objectdb.o.SYR.n(SYR.java:188)
at com.objectdb.o.QRC.<init>(QRC.java:152)
at com.objectdb.o.QRM.U6(QRM.java:250)
at com.objectdb.o.MST.U6(MST.java:933)
at com.objectdb.o.WRA.U6(WRA.java:293)
at com.objectdb.o.WSM.U6(WSM.java:114)
at com.objectdb.o.QRR.g(QRR.java:245)
at com.objectdb.o.QRR.f(QRR.java:154)
at com.objectdb.jpa.JpaQuery.getSingleResult(JpaQuery.java:716)
... 37 more
谢谢，还要记录一下，我不会存储纯文本密码，我只是想在开始实施哈希密码之前让基本数据库正常工作。
