Sample.java

import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.DefaultCaret;
import javax.swing.SwingUtilities;


public class Sample extends JFrame {
    private  JTextArea area;
    public Sample() {
        super();
        area = new JTextArea();
        area.getDocument().addDocumentListener(new DocumentListenerImpl());
        setLayout(new BorderLayout());
        add(area);
        setSize(400, 300);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
    public static void main(String[] args) {
        Sample sample = new Sample();
        sample.setVisible(true);
    }

    class DocumentListenerImpl implements DocumentListener {
        public void changedUpdate(DocumentEvent e) {
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    System.out.println(area.getCaret().getMagicCaretPosition());
                }
            });
        }
        public void insertUpdate(DocumentEvent e) {
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    System.out.println(area.getCaret().getMagicCaretPosition());
                }
            });
        }
        public void removeUpdate(DocumentEvent e) {
            SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    System.out.println(area.getCaret().getMagicCaretPosition());
                }
            });
        }
    }
}