Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

前幾天做一個學生選課系統時遇到一個空指針異常具體異常信息如下:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at org.yfl.service.LoginService.loginValidate(LoginService.java:17)
	at org.yfl.view.Login.actionPerformed(Login.java:125)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
	at java.awt.Component.processMouseEvent(Component.java:6297)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3275)
	at java.awt.Component.processEvent(Component.java:6062)
	at java.awt.Container.processEvent(Container.java:2039)
	at java.awt.Component.dispatchEventImpl(Component.java:4660)
	at java.awt.Container.dispatchEventImpl(Container.java:2097)
	at java.awt.Component.dispatchEvent(Component.java:4488)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4575)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4236)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4166)
	at java.awt.Container.dispatchEventImpl(Container.java:2083)
	at java.awt.Window.dispatchEventImpl(Window.java:2489)
	at java.awt.Component.dispatchEvent(Component.java:4488)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:674)
	at java.awt.EventQueue.access$400(EventQueue.java:81)
	at java.awt.EventQueue$2.run(EventQueue.java:633)
	at java.awt.EventQueue$2.run(EventQueue.java:631)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$3.run(EventQueue.java:647)
	at java.awt.EventQueue$3.run(EventQueue.java:645)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:644)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

 

當時這個問題我查了大半天 後來終於被我查到了 下面是導致這個異常發生的代碼

if (e.getSource() == login) {
	String userIdStr = inputUserId.getText().trim();
	//這裏如果是用的JPasswordField這個控件輸入密碼不應該寫成這樣 因爲這樣密碼就會是一個很奇怪的字符串,這是導致空指針異常的最終原因
	String passwdStr = inputPassword.getPassword().toString().trim();
			
	if(0 == userIdStr.length() || 0 == passwdStr.length()){
		JOptionPane.showMessageDialog(this, "用戶ID或密碼爲空!!!");
	}else{
		org.yfl.domain.Login login = new org.yfl.domain.Login();
		login.setLogin_id(userIdStr);
		login.setPasswd(passwdStr);
		new LoginService().loginValidate(login);
		System.out.println("**********"+login.getGrade());
		}
			
	} else if (e.getSource() == cancel) {
		System.exit(0);
	}

 

下面是修正後的代碼 (ok問題解決)

if (e.getSource() == login) {
	String userIdStr = inputUserId.getText().trim();
	//這裏如果是用的JPasswordField這個控件輸入密碼就應該這樣寫就沒有問題了
	String passwdStr = new String(inputPassword.getPassword()).trim();
			
	if(0 == userIdStr.length() || 0 == passwdStr.length()){
		JOptionPane.showMessageDialog(this, "用戶ID或密碼爲空!!!");
	}else{
		org.yfl.domain.Login login = new org.yfl.domain.Login();
		login.setLogin_id(userIdStr);
		login.setPasswd(passwdStr);
		new LoginService().loginValidate(login);
		System.out.println("**********"+login.getGrade());
		}
			
	} else if (e.getSource() == cancel) {
		System.exit(0);
	}

還有要注意的地方就是查詢數據庫是即使沒有一條結果也不會返回一個空的ResultSet 所以在使用resultSet.getMetaData().getColumnCount()時也會返回這個表的列數,使用對象數組作爲返回值的同學要小心,,,

 

resultSet = statement.executeQuery();
int columns = resultSet.getMetaData().getColumnCount();
//這裏的columns肯定大於0
if(columns > 0){
	objects = new Object[columns];
}
while (resultSet.next()) {
	
	for (int i = 0; i < objects.length; i++) {
		objects[i] = resultSet.getObject(i + 1);
	}
}