JProgressBar does not update within the loop

问题

I am currently self-studying Java. I've been trying different things like JRadioButtons, JcomboBoxes etc. Now, I'm trying to use JProgressBars but it doesn't seem to work properly.

Relevant piece of code:

JProgressBar progress;
JButton button;     //Fields of the class

progress=new JProgressBar(JProgressBar.HORIZONTAL,0,100);
button=new JButton("Done");    //Done from methods

progress.setValue(0);
progress.setStringPainted(true);
progress.setBorderPainted(true);  //Also done from methods

button.addActionListener(this);   //Also done from methods

When button is clicked, I want to show the JProgressBar go from 0% to 100%. Here is the relevant portion of the actionPerformed method:

public void actionPerformed(ActionEvent e)
{

        for(int i=0;i<=progress.getMaximum();i++)
        {
            progress.setValue(i);
            /*try{
                Thread.sleep(10);
            }catch(InterruptedException ex)
            {
                System.err.println("An error occured:"+ex);
                ex.printStackTrace();
            }*/
        }


        progress.setValue(progress.getMinimum());
}

I've added both progress and button into a JPanel and the panel into a JFrame on which I use setVisible(true);.

The problem here is, whenever I press the JButton button, the JProgressBar progress does not go from 0% to 100%. Instead, nothing happens. If I uncomment the try...catch block, and then press button, the program "freezes" for a moment and then continues. This time too, The JProgressBar stays at 0% and I never see it moving. I've also tried a repaint(); just after the try...catch block but the same thing happened.

I've tried adding

System.out.println(i+"");

inside the for loop in actionPerformed and this printed numbers 0 to 100 in the terminal. So, I'm sure that the loop runs.

How can I solve this problem?

回答1:

Swing is single threaded,
all updates must be done on EDT,
all events in current EDT are painted in GUI in one moment,
Thread.sleep to lock execution of event in EDT, repaint can be painted at the end of for_loop, after all lock(s) from Thread.sleep are executed, more in Oracle trail - Concurency in Swing and EventDispatchThread

output from AWT (Swing) Listener - ActionListener, should be done if all events inside actionPerformed are executed, then there is executed only progress.setValue(progress.getMinimum());, doesn't matter if is there Thread.sleep or not

public void actionPerformed(ActionEvent e) -> progress.setValue(progress.getMinimum());

use SwingWorker or Runnable#Thread with progress.setValue(i); wrapped into invokeLater
for better help sooner post an SSCCE/MCVE, short, runnable, compilable

回答2:

You have to put following piece of code to new thread

public void actionPerformed(ActionEvent e)
{

    for(int i=0;i<=progress.getMaximum();i++)
    {
        progress.setValue(i);
        /*try{
            Thread.sleep(10);
        }catch(InterruptedException ex)
        {
            System.err.println("An error occured:"+ex);
            ex.printStackTrace();
        }*/
    }


    progress.setValue(progress.getMinimum());
}

Something like this

public void actionPerformed(ActionEvent e)
{
    new Thread(){
        public void run(){
            for(int i=0;i<=progress.getMaximum();i++)
            {
                progress.setValue(i);
                try{
                    sleep(10);
                }catch(InterruptedException ex)
                {
                    System.err.println("An error occured:"+ex);
                    ex.printStackTrace();
                }
            }
        }
    }.start();
    progress.setValue(progress.getMinimum());
}

Not tested, progress instance should be final maybe

回答3:

As far as your code goes, it all looks legit. Doesn't seem to give any issue regardless of not initiating a thread.

What I suggest you do is remove this:

//for testing purposes
progress.setValue(progress.getMinimum());

And then make the value of the progress bar 50. Remove the thread code, and use the for loop with the condition "< 50".

If all else fails, this will surely shed some light: JProgress Bar: Working!

Let me know of the outcome.

来源：https://stackoverflow.com/questions/29727991/jprogressbar-does-not-update-within-the-loop

标签

java

swing

jprogressbar