Java定时器不工作

Question

由于某种原因，我的java定时器不能在我的一个程序中工作。每当我编译我的代码，它给我以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: 
    Task already scheduled or cancelled 
    at java.util.Timer.sched(Timer.java:401) 
    at java.util.Timer.scheduleAtFixedRate(Timer.java:328) 

这是为什么？ （注：我在的Java定时器新手）

//Timer Prerequisites 
Timer timer = new Timer(); 
TimerTask task = new TimerTask() 
{ 
    public void run() 
    { 
     System.out.println("We have waited one second."); 
    } 
}; 

//Check to see if user has enetered anything 
while(!answered) 
{ 
timer.scheduleAtFixedRate(task, 0, duration); 
afk = true; 
incorrect += 1; 
answered = true; 
}   

Answer 1

一个Timer只能安排一次，但调度本身是在一个固定的速度。只需拨打timer.scheduleAtFixedRate方法一次。如果您需要稍后启动相同的计时器任务，则需要构建新的java.util.Timer。

Answer 2

Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException: Task already scheduled or cancelled at java.util.Timer.sched(Timer.java:401) at java.util.Timer.scheduleAtFixedRate(Timer.java:328)

因为你正试图在这里再次增加相同的任务：

timer.scheduleAtFixedRate(task, 0, duration); 

Answer 3

由于在标签如前所述，你应该使用SwingTimer，而不是使用java.util.Timer。

请看看这个样本例如：

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

public class SwingTimerExample { 

    private String[] questions = { 
     "How are you?", 
     "How is your day?", 
     "Will you work tonight?" 
    }; 

    private JLabel questionLabel; 
    private ButtonGroup radioGroup; 
    private JRadioButton yesRadioButton; 
    private JRadioButton noRadioButton; 

    private int counter; 

    private static final int GAP = 5; 

    private Timer timer; 

    private ActionListener timerActions = new ActionListener() { 
     @Override 
     public void actionPerformed (ActionEvent ae) { 
      ++counter; 
      counter %= questions.length; 
      questionLabel.setText (questions [ counter ]); 
      if (counter == questions.length - 1) { 
       ((Timer) ae.getSource()).stop(); 
      } 
     } 
    }; 

    public SwingTimerExample() { 
     counter = 0; 
    } 

    private void displayGUI() { 
     JFrame frame = new JFrame ("Swing Timer Example"); 
     frame.setDefaultCloseOperation (JFrame.DISPOSE_ON_CLOSE); 

     JPanel contentPane = new JPanel(); 
     contentPane.setBorder (BorderFactory.createEmptyBorder (
      GAP, GAP, GAP, GAP)); 
     contentPane.setLayout (new BorderLayout (GAP, GAP)); 

     JPanel labelPanel = new JPanel(); 
     questionLabel = new JLabel ( 
          questions [ counter ], JLabel.CENTER); 
     labelPanel.add (questionLabel); 

     JPanel radioPanel = new JPanel(); 
     yesRadioButton = new JRadioButton ("YES"); 
     noRadioButton = new JRadioButton ("NO"); 
     radioGroup = new ButtonGroup(); 
     radioGroup.add (yesRadioButton); 
     radioGroup.add (noRadioButton); 
     radioPanel.add (yesRadioButton); 
     radioPanel.add (noRadioButton); 

     contentPane.add (labelPanel, BorderLayout.CENTER); 
     contentPane.add (radioPanel, BorderLayout.PAGE_END); 

     frame.setContentPane (contentPane); 
     frame.pack(); 
     frame.setLocationByPlatform (true); 
     frame.setVisible (true); 

     timer = new Timer (5000, timerActions); 
     timer.start(); 
    } 

    public static void main (String[] args) { 
     Runnable runnable = new Runnable() { 
      @Override 
      public void run() { 
       new SwingTimerExample().displayGUI(); 
      } 
     }; 
     EventQueue.invokeLater (runnable); 
    } 
}