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

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.