Multithreading in JavaFX hangs the UI

I have a simple JavaFX 2 app, with 2 buttons, saying Start and Stop. When the start button is clicked, I want to create a background thread which will do some processing and update the UI (e.g a progress bar) as it goes along. If the stop button is clicked, I want the thread to terminate.

I've tried to do this using the javafx.concurrent.Task class which I gathered from the documentation would work fine for this. But whenever I click Start, the UI freezes/hangs rather than staying normal.

Her's the code from the main Myprogram extends Application class for showing the buttons:

public void start(Stage primaryStage)
{               
    final Button btn = new Button();
    btn.setText("Begin");

    //This is the thread, extending javafx.concurrent.Task :
    final MyProcessor handler = new MyProcessor();
    btn.setOnAction(new EventHandler<ActionEvent>()
    {
        public void handle(ActionEvent event)
        {                
           handler.run(); 
        }
    });

    Button stop = new Button();
    stop.setText("Stop");
    stop.setOnAction(new EventHandler<ActionEvent>()
        {
             public void handle(ActionEvent event)
             {
                handler.cancel();
             }
        }

    );
    // Code for adding the UI controls to the stage here.
}

Here's the code of MyProcessor class:

import javafx.concurrent.Task;
public class MyProcessor extends Task
{   
    @Override
    protected Integer call()
    {
        int i = 0;
        for (String symbol : feed.getSymbols() )
        {
            if ( isCancelled() )
            {
                Logger.log("Stopping!");
                return i;
            }
            i++;
            Logger.log("Doing # " + i);
            //Processing code here which takes 2-3 seconds per iteration to execute
            Logger.log("# " + i + ", DONE! ");            
        }
        return i;
    }
}

Pretty simple, but the UI hangs whenever I click the Start button, though the console messages continue to get displayed (Logger.log simply does System.out.println )

What am I doing wrong?

Task implements Runnable, so when you call handler.run(); you actually run the call method in the UI Thread. That will hang the UI.

You should start the task in a background thread, either via an executor or simply by calling new Thread(handler).start();.

This is explained (maybe not very clearly) in the javadoc or in the JavaFX concurrency tutorial.