java,multithreading,swing,javafx , Why JavaFX requires Platform.runLater to change anything in it's UI inside a thread?


Why JavaFX requires Platform.runLater to change anything in it's UI inside a thread?

Question:

Tag: java,multithreading,swing,javafx

This code of a simple swing application:

final JFrame jFrame = new JFrame();
final JLabel jLabel = new JLabel("Test");
jFrame.add(jLabel);
jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
jFrame.pack();
jFrame.setLocationRelativeTo(null);
jFrame.setVisible(true);

Thread thread = new Thread(){
    @Override
    public void run() {
        while(true) {
            jLabel.setText("Time: " + System.currentTimeMillis());
            try { Thread.sleep(1000);} catch(Exception ex) {}
        }
    }
};
thread.setDaemon(true);
thread.start();

runs just like expected. If we do the same with JavaFX, we get an error java.lang.IllegalStateException: Not on FX application thread when setting the text to the label.

I just want to know, why it has to be like this? Why can't we just have the freedom to do what we whant, when we want?


Answer:

Because JavaFX is single threaded, like almost all UI toolkits.

Your Swing code is broken - Swing is also single threaded, and you should execute jLabel.setText(...) on the AWT event handling thread, either by wrapping it in SwingUtilities.runLater(...);, or by using a higher-level API such as a Timer. It may happen to run just like expected on your particular system, with your particular JDK implementation, but you are coding contrary to the contract of the toolkit you are using, and there is no guarantee it is going to work on other JDK implementations or other platforms.

One major improvement of JavaFX over Swing is that JavaFX enforces the threading rules as much as possible by throwing an IllegalStateException when you get the threading wrong. In Swing, your code is just prone to random, unpredictable failure at some point in the future, with no warning at all.

There are some cases where JavaFX does not enforce the rules, presumably because it would adversely affect performance to do the relevant checking, but you should still follow the contract: all changes to the UI must be performed on the UI thread.

As to why JavaFX, Swing, and other UI toolkits are written like this: that is a very complex question. For the most part, UI programming is event driven; so most of the time the code you write that modifies the scene graph is in response to a user event; i.e. the UI thread is the natural place to be modifying the UI anyway. Because of this, the extra effort and performance hit that come with making the UI toolkit thread safe are probably not worthwhile. Nevertheless, some attempts have been made to write thread-safe UI toolkits, but they've never worked well: the added burden of synchronization adversely affects performance to a degree that renders them unusable. This is outside my scope of understanding, but from what I've read the problem is that layout is typically done in the opposite direction to event handling (i.e. from the window down recursively down to child components, versus from the "leaf" components up via parents to the window). So while it's feasible to handle synchronization for either layout or for event handling, it becomes prohibitively expensive to do it for both. See this for a (probably much more accurate) discussion.


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