JavaFX TextArea onChange event

Using Lambda expressions

textArea.textProperty().addListener((obs,old,niu)->{
    // TODO here
});

As with all of JavaFX, just add a listener to the TextArea textProperty().

import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;

    textArea.textProperty().addListener(new ChangeListener<String>() {
        @Override
        public void changed(final ObservableValue<? extends String> observable, final String oldValue, final String newValue) {
            // this will run whenever text is changed
        }
    });