What could be the reason my Java Swing Timer isn't firing, and how can I ensure it executes once as expected?

General Discussions
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1

I’m using a Java Swing Timer with setRepeats(false) because I need the timer to fire only once. According to the documentation, this should work, but the timer never fires. Here’s the code snippet:

ActionListener taskPerformer = new ActionListener() {
    public void actionPerformed(ActionEvent evt) {
        //...Perform a task...
        logger.finest("Reading SMTP Info.");
    }
};
Timer timer = new Timer(100, taskPerformer);
timer.setRepeats(false);
timer.start();

What might be going wrong, and how can I make sure the timer fires once as intended?

2

Ah, I see what you’re trying to do! From my experience with Java Swing Timer, the issue could be related to the Event Dispatch Thread (EDT). Since Swing components and timers need to run on the EDT, if your timer isn’t started on that thread, it might not fire at all. This is a common pitfall.

:hammer_and_wrench: Fix: You can use SwingUtilities.invokeLater to ensure that the timer runs on the EDT like this:

SwingUtilities.invokeLater(() -> {
    Timer timer = new Timer(100, taskPerformer);
    timer.setRepeats(false);
    timer.start();
});

This should get your Java Swing Timer working as expected. If that doesn’t solve it, we can dig deeper, but this is the first thing I’d check.

3

That’s a good catch! I’ve seen issues where the main thread exits too soon, especially if your program doesn’t have a running GUI or a non-daemon thread keeping it alive. The JVM might shut down before your Java Swing Timer even gets a chance to fire.

:hammer_and_wrench: Fix: If you’re not displaying a GUI, you can create a simple JFrame to ensure the app stays alive long enough for the timer to trigger:

JFrame frame = new JFrame();
frame.setSize(200, 200);
frame.setVisible(true);

This might just be the thing keeping your Java Swing Timer from firing. If your app quits too early, the timer won’t get a chance to run.

4

You know, another reason this might be happening is related to your logging level. Sometimes the timer might fire as expected, but you just can’t see it because your logger’s level (like logger.finest()) is set too high. It might not be logging anything at all if the log level is too restrictive.

:hammer_and_wrench: Fix: A quick test would be to replace the logging call with a simple System.out.println() to verify if the timer is firing:

ActionListener taskPerformer = evt -> {
    System.out.println("Timer fired!");
};

This will bypass any logger configuration issues and give you a clearer indication that your Java Swing Timer is indeed firing. Once verified, you can dive back into the logger setup if needed.