使用ObjectDB搜索空用户数据库时出现问题
我正在创建一个使用ObjectDB来创建和维护一组数据库的java应用程序。 我目前正在尝试实现一个DB来存储由用户名和密码字符串组成的用户对象。 在JFrame / swing类中，我有一个用于创建新用户的按钮，当单击此按钮时，我希望发生以下情况：
- 创建（或连接）数据库
- 搜索数据库以查看是否存在提供用户名的用户对象
- 如果用户已存在则显示对话框消息，否则创建用户
但是，单击此按钮时，我会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的数据库，但是我希望代码在程序第一次运行时工作，所以它需要处理一个空的数据库。 我已经尝试在首次运行查询之前更改代码以创建新用户，然后代码按照每次单击按钮时的工作方式运行，并且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”类型的用户，这样搜索就可以了，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function，我可以’执行查询以检查数据库是否为空（因此我只能在程序的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，对于使用ObjectDB的任何人来说，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 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.(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，也是为了记录我不会存储明文密码，我只是想在开始实施散列密码之前让基本数据库工作。
如果实体类尚未在数据库中（尚未保留该类的实例），并且未定义持久性单元，则可能发生这种情况。
如果这似乎是原因，可以在查询之前定义持久性单元或将类引入ObjectDB，例如：
em.getMetamodel().entity(User.class);