Multithreading in JavaFX hangs the UI

Question

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?

Answer 1

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.