Waiting for mouse input in Java Swing

Question

I'm working on a Java Swing application. I have a button whose action runs a query on a database and then plots the results. These commands are performed from the listener on the Run button. As I understand it, this means that the thread running at this point is from the EventQueue.

Given certain input, I need to halt processing and wait for the user to click on the plot area. I'm struggling with finding a way to do this. I've looked all over, but I can't seem to find a solution that works. I've tried to create a new thread that waits for the user input (I call join on this thread). The problem is that this makes the previous thread (from the EventQueue) wait. For some reason, the mouse clicks are not causing an interrupt so the mouse clicks are never captured.

Any help would be much appreciated. Thanks in advance.

Answer 1

Let the EventDispatcherThread handle input. Rather, your spawned thread should be the one doing calculations. I built an interactive UI that did something similar a year or two ago, and that's how I accomplished this behavior.

Essentially, you create a producer/consumer model between the GUI and the data processing thread. When an input is fired off, you feed to the consumer thread and resume activity.

Answer 2

Use the SwingWorker class.

Answer 3

It seems like you are doing roughly the right thing but you should not be calling join on the background thread from the UI thread as that will freeze the UI until the background thread has completed.

Here is a highly compressed example of the basic method I would use (if SwingWorker is not available):

class QueryPerformer implements Runnable {
     private volatile boolean plotAreaClicked;

     public void run() {
         // Perform query and process
         while (!plotAreaClicked) {
              try {
                   Thread.sleep(500);
              } catch (InterruptedException exception) {
              }
         }
         // Perform tasks following plot area click
     }

     public void setPlotAreaClicked(boolean plotAreaClicked) {
         this.plotAreaClicked = plotAreaClicked;
     }
}

And in your UI listeners:

private QueryPerformer queryPerformer;

public void actionPerformed(ActionEvent event) {
    // Run button pressed
    queryPerformer = new QueryPerformer();
    new Thread(queryPerformer).start();
}

public void mouseClicked(MouseEvent event) {
    // Plot area clicked
    if (queryPerformer != null) {
        queryPerformer.setPlotAreaClicked(true);
    }
}

Please note that the above is not optimal and some things have not been handled (such as multiple run button presses).