import java.awt.Color;
import java.awt.Dimension;
import java.awt.dnd.DnDConstants;
import java.awt.dnd.DropTarget;
import java.awt.dnd.DropTargetDragEvent;
import java.awt.dnd.DropTargetDropEvent;
import java.awt.dnd.DropTargetEvent;
import java.awt.dnd.DropTargetListener;

import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;

public final class test {

    volatile static boolean inside = false;

    public static void main(final String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.setSize(500, 500);
                frame.setLocationRelativeTo(null);
                JTextArea jta = new JTextArea() {
                };
                jta.setPreferredSize(new Dimension(100, 100));
                jta.setBackground(Color.RED);
                frame.add(jta);
                jta.setDragEnabled(true);
                jta.setDropTarget(new DropTarget(jta, DnDConstants.ACTION_COPY,
                                                 new TestdropTargetListener()));
                frame.setVisible(true);
            }
        });
    }

    private static class TestdropTargetListener implements DropTargetListener {

        @Override
        public void dragEnter(DropTargetDragEvent dtde) {
            if (inside) {
                throw new RuntimeException();
            }
            inside = true;
            System.out.println("test.dragEnter");
            try {
                Thread.sleep(100);
            } catch (Exception e) {
            }
        }

        @Override
        public void dragOver(DropTargetDragEvent dtde) {
            if (!inside) {
                throw new RuntimeException();
            }
            System.out.println("test.dragOver");
        }

        @Override
        public void dropActionChanged(DropTargetDragEvent dtde) {
            System.out.println("test.dropActionChanged");
        }

        @Override
        public void dragExit(DropTargetEvent dte) {
            if (!inside) {
                throw new RuntimeException();
            }
            System.out.println("test.dragExit");
            inside = false;
        }

        @Override
        public void drop(DropTargetDropEvent dtde) {
            System.out.println("test.drop");
        }
    }
}