Repeat the execution of a SwingWorker in Java

Question

I was reading through multiple posts and I wasn't able to find a clear answer to my question.

I have an application (Main Frame) that has a start and a stop button and just a jTextArea. When a user presses the "start" button, a SwingWorker class is instantiated and runs (execute()) a task. That task is to connect to a database and do some checks. For every check, it updates the jTextArea correctly and everything is fine. When the stop button is pressed, i set the isCancelled() to true and thus the background process stops (I have the appropriate checks inside the SwingWorker to stop execution etc).

What I want to do is when the user presses "Start", the SwingWorker starts execution, sleeps for a few minutes and then re-runs the same task. When the "Stop" button is pressed, everything stops.

How can I achieve that? I understand that a SwingWorker background process is supposed to be run once. What would other ways of doing this be? Note that the reason I picked a SwingWorker was because I wanted to update the jTextArea while the background process runs.

Thank you in advance.

SwingWorker isn't designated for that, to use standard Workers Thread, but output to the Swing GUI must be wrapped into invokeLater — mKorbel, Sep 09 '13 at 21:55

Hovercraft Full Of Eels · Answer 1 · 2013-09-10T21:15:54.413

You can run a SwingWorker once and only once, without exception. Possible solutions include:

Create a new SwingWorker object each time one is needed, run it, and stop and discard it when done.
Run a single SwingWorker throughout the run of your program, do not cancel it when stop is pressed, but rather pause it (how will depend on the non-Swing portion of the code that we don't know about), and awaken it when play is pressed. A state design pattern could work well for this.

Regarding your statement,

What I want to do is when the user presses "Start", the SwingWorker starts execution, sleeps for a few minutes and then re-runs the same task. When the "Stop" button is pressed, everything stops.

Consider using a ScheduledExecutorService for this inside of the SwingWorker. The ScheduledExecutorService creates a ScheduledFuture<V> which can be canceled if the need arises.

Edit
Your comments and my responses:

Wouldn't a ScheduledExecutorService cause the background process to run multiple times?

No. The background process would be run once. The Futures created by the ScheduledExecutorService would also be created and then run at their scheduled times.

What would happen, for example, when the delay is less than the time it took the previous iteration to run?

Multiple Futures would be created and would start running simultaneously.

Should I just go with a while loop inside the doInBackground() with a Thread.Sleep()?

It's so very hard to tell based on what we know about your current project. This could potentially work, yes, but for more complex needs, then no, you would want to use the ScheduledExecutorService.

@nachokk: inside of the SwingWorker's `doInBackground()`, you wouldn't use a Swing Timer, but rather a java.util.Timer since it is called off of the EDT. I have read many times here that you're better off using a ScheduledExecutorService in lieu of a java.util.Timer. — Hovercraft Full Of Eels, Sep 10 '13 at 01:47
@nachokk: for instance, please check out [Java Timer vs ExecutorService?](http://stackoverflow.com/questions/409932/java-timer-vs-executorservice) — Hovercraft Full Of Eels, Sep 10 '13 at 01:50
Wouldn't a ScheduledExecutorService cause the background process to run multiple times? What would happen, for example, when the delay is less than the time it took the previous iteration to run? Should I just go with a while loop inside the doInBackground() with a Thread.Sleep()? — achilles, Sep 10 '13 at 12:06
Thank you sir. What I ended up doing is use a while loop with while (!isCancelled()) { ... }. This "background" task is short and even though it is not the best way to do it, it works for my case. I appreciate your help. I do understand a lot more now about the SwingWorker and in the future I'll be using this closer to how it's supposed to be used. — achilles, Sep 12 '13 at 14:56

score 0 · Answer 2 · edited Nov 16 '13 at 05:48

I normally use a loop inside the swingworker, works fine.

Global Variables

private boolean monitor = true;

Swingworker Class

private class MonitorQueue extends SwingWorker<Void, Void> {

    @Override
    protected Void doInBackground() throws Exception {
        while (monitor) {
            System.out.println("monitor");
            Thread.sleep(Constants.SECOND * 5);
        }
        return null;
    }
}

Added advantage that I have is instead of stopping or pausing the SwingWorker itself, I just set monitor = false. This will let the SwingWorker finish all tasks before stopping.

Repeat the execution of a SwingWorker in Java

2 Answers2