我正在尝试为Java创建一个登录系统,并且我的代码出现问题。我总是得到这个错误而且我不知道为什么。
这是我的代码线程中的异常" AWT-EventQueue-0"显示java.lang.NullPointerException
package SADlife;
import SADlife.Connection.*;
import java.sql.*;
public class loginSample extends javax.swing.JFrame {
public loginSample() {
initComponents();
}
Connection connection = null;
Statement st;
public boolean checkLogin(){
connection = DBConnection.getDBConnection();
boolean check = false;
try{
String username = userField.getText();
String password = passField.getText();
String searchUser = "select username,password from userAccountsTable where username='"+username+"' and password = '"+password+"'";
ResultSet rs = st.executeQuery(searchUser);
while(rs.next()){
check = true;
}
if (check==false){
System.out.println("No login found!");
}
}catch(SQLException e){
System.out.println(e.toString());
}
return check;
}
/**
* This method is called from within the constructor to initialize the form.
* WARNING: Do NOT modify this code. The content of this method is always
* regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
userField = new javax.swing.JTextField();
jLabel1 = new javax.swing.JLabel();
jLabel2 = new javax.swing.JLabel();
passField = new javax.swing.JPasswordField();
loginButton = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
userField.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
userFieldActionPerformed(evt);
}
});
jLabel1.setText("Username");
jLabel2.setText("Password");
loginButton.setText("Submit");
loginButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
loginButtonActionPerformed(evt);
}
});
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(114, 114, 114)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addComponent(jLabel2)
.addGap(45, 45, 45))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addComponent(jLabel1)
.addGap(43, 43, 43)))
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
.addComponent(userField, javax.swing.GroupLayout.DEFAULT_SIZE, 84, Short.MAX_VALUE)
.addComponent(passField))
.addContainerGap(111, Short.MAX_VALUE))
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addComponent(loginButton)
.addGap(44, 44, 44))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addContainerGap(112, Short.MAX_VALUE)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel1)
.addComponent(userField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(29, 29, 29)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jLabel2)
.addComponent(passField, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(76, 76, 76)
.addComponent(loginButton)
.addGap(20, 20, 20))
);
loginButton.getAccessibleContext().setAccessibleDescription("");
pack();
}// </editor-fold>
private void userFieldActionPerformed(java.awt.event.ActionEvent evt) {
}
private void loginButtonActionPerformed(java.awt.event.ActionEvent evt) {
if(checkLogin()){
System.out.println("Login is correct!");
}
else{
System.out.println("Wrong login!");
}
}
/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
try {
for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
javax.swing.UIManager.setLookAndFeel(info.getClassName());
break;
}
}
} catch (ClassNotFoundException ex) {
java.util.logging.Logger.getLogger(loginSample.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(loginSample.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(loginSample.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(loginSample.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new loginSample().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JButton loginButton;
private javax.swing.JPasswordField passField;
private javax.swing.JTextField userField;
// End of variables declaration
}
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at SADlife.loginSample.checkLogin(loginSample.java:21)
at SADlife.loginSample.loginButtonActionPerformed(loginSample.java:118)
at SADlife.loginSample.access$100(loginSample.java:6)
at SADlife.loginSample$2.actionPerformed(loginSample.java:66)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
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:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4703)
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$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
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$1.doIntersectionPrivilege(ProtectionDomain.java:75)
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)
答案 0 :(得分:0)
首先,您应该尝试实现Java命名标准。
ClassName
variableName
CONSTANT
AWT-EventQueue是一个处理GUI(图形用户界面)上的各种动作的过程。在你的情况下按一个按钮。
因为在该线程中出现错误,所以在线程“AWT-EventQueue-0”java.lang.NullPointerException 中的堆栈跟踪 Exception的开头处指出。
这意味着在 AWT-EventQueue-0 线程中抛出了 NullPointerException 。
第二行将显示您的程序中存在的问题。 在SADlife.loginSample.checkLogin(loginSample.java:21)
粗体路径具有以下模式
package.Class.method。(文件:线)
在本例中,您的问题位于loginSample.java的第21行。
(希望我没有弄乱我的计算:P)
第21行 - &gt; ResultSet rs = st.executeQuery(searchUser);
如果您尝试在NullPointerException
引用上应用方法,则会调用null
。这意味着在这种情况下st
为null
。所以基本上你从来没有创建过`Statement并将它保存到变量st中。
这tutorial应该会有所帮助。
祝你好运。祝你好运。