What causes the Not on FX application thread error?

Question

Java

I am writing an mp3 player. I want to make it so that when the song ends, the next one on the list starts playing automatically. There is a "beginTimer()" method that counts the time while the song is playing and moves the slider according to it:

private void beginTimer() {
        timer = new Timer();

        mediaPlayer.setOnReady(() -> songDuration.setMax(media.getDuration().toSeconds()));

        timerTask = new TimerTask() {
            @Override
            public void run() {
                double current = mediaPlayer.getCurrentTime().toSeconds();
                double end = media.getDuration().toSeconds();

                running = true;
                songDuration.setValue((int) current);

                if (current / end == 1) {
                    cancelTimer();
                    nextMedia();
                }
            }
        };

        timer.scheduleAtFixedRate(timerTask, 0, 1000);
    }

The error itself:

Exception in thread "Timer-2" java.lang.IllegalStateException: Not on FX application thread; currentThread = Timer-2
  at javafx.graphics/com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:292)
  at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:446)
  at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Parent.java:474)
  at javafx.base/com.sun.javafx.collections.VetoableListDecorator.setAll(VetoableListDecorator.java:113)
  at javafx.base/com.sun.javafx.collections.VetoableListDecorator.setAll(VetoableListDecorator.java:108)
  at javafx.controls/javafx.scene.control.skin.LabeledSkinBase.updateChildren(LabeledSkinBase.java:283)
  at javafx.controls/javafx.scene.control.skin.LabeledSkinBase.lambda$new$11(LabeledSkinBase.java:220)
  at javafx.controls/com.sun.javafx.scene.control.LambdaMultiplePropertyChangeListenerHandler.lambda$new$1(LambdaMultiplePropertyChangeListenerHandler.java:49)
  at javafx.base/javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:86)
  at javafx.base/com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:181)
  at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
  at javafx.base/javafx.beans.property.StringPropertyBase.fireValueChangedEvent(StringPropertyBase.java:104)
  at javafx.base/javafx.beans.property.StringPropertyBase.markInvalid(StringPropertyBase.java:111)
  at javafx.base/javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:145)
  at javafx.base/javafx.beans.property.StringPropertyBase.set(StringPropertyBase.java:50)
  at javafx.base/javafx.beans.property.StringProperty.setValue(StringProperty.java:71)
  at javafx.controls/javafx.scene.control.Labeled.setText(Labeled.java:147)
  at main.Controller.setSongLabel(Controller.java:182)
  at main.Controller.nextMedia(Controller.java:105)
  at main.Controller$1.run(Controller.java:138)
  at java.base/java.util.TimerThread.mainLoop(Timer.java:556)
  at java.base/java.util.TimerThread.run(Timer.java:506)

"nextMedia()" method:

public void nextMedia() {
        if (songCounter < playList.size() - 1) songCounter++;
        else songCounter = 0;

        mediaPlayer.stop();

        if (running) cancelTimer();

        setMediaPlayer();
        setSongLabel();
        playMedia();
    }

The error occurs exactly on the line songLabel.setText(fileName.substring(0, fileName.length() - 4));
The "setSongLabel" method itself:

private void setSongLabel() {
        String fileName = playList.get(songCounter).getName();

        songLabel.setText(fileName.substring(0, fileName.length() - 4));
    }

Answer 1

Almost any UI system has its own thread, on which you need to access UI elements. Obviously, here you need to transfer the execution back to the FX application thread.