Question

我正在研究java项目的身份验证，我有两个文件login.java，它是JFrame和Auth.java，它是一个处理身份验证的java类。 Auth.java有一个方法login(...)，它返回调用btnLoginActionPerformed(evt)的结果集。代码如下
的 Login.java

private void btnLoginActionPerformed(java.awt.event.ActionEvent evt) {                                         

    this.txtUsernameActionPerformed(evt);
    this.txtPassActionPerformed(evt);
    txtUsername.setText("");
    txtPass.setText("");

    try {
        ResultSet rs = Auth.login(username, pass);
        if (!rs.isBeforeFirst()) {
            JOptionPane.showMessageDialog(null, "Wrong credentials");
        }else
        {
            do
                System.out.println(rs.getString("username") + "\t" + rs.getString("email"));
            while(rs.next());
        }
    } catch (SQLException ex) {
        Logger.getLogger(Login.class.getName()).log(Level.SEVERE, null, ex);
    }
}

Auth.java

public static ResultSet login(String username,String password) throws SQLException
{
    String sql = "Select * from users where username=?";
    PreparedStatement st = link.prepareStatement(sql);
    st.setString(1, username);
    ResultSet rs = st.executeQuery();
    return rs;
 }

这会在运行时抛出NullPointerException，我不知道为什么。请给出建议这是堆栈跟踪

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at core.Auth.login(Auth.java:50)
at views.Login.btnLoginActionPerformed(Login.java:245)
at views.Login.access$200(Login.java:25)
at views.Login$3.actionPerformed(Login.java:111)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

我有一个不会让步的持久性java.lang.NullPointerException

0 个答案: