Why canImport () is no longer called after setDropTarget ()?

Question

I am experimenting with the drag & drop JList functions in Java SE 7. The problem is that the canImport () method in my TransferHandler implementation ceases to be called as soon as I set my DropTarget to JList. Here's a simplified code to demonstrate the problem:

main class:

import javax.swing.ListSelectionModel;

public class TestDragDrop extends javax.swing.JFrame {

    public TestDragDrop() {
        initComponents();
        lstTest.setDragEnabled(true);
        lstTest.setTransferHandler(new MyTransferHandler());
        // try to comment and uncomment the following line:
        lstTest.setDropTarget(new MyDropTarget());
        // - commented => canImport() is called during drag gesture
        // - uncommented => canImport() is not called during drag gesture
        lstTest.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        scroll1 = new javax.swing.JScrollPane();
        lstTest = new javax.swing.JList();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        lstTest.setModel(new javax.swing.AbstractListModel() {
            String[] strings = { "Item 1", "Item 2", "Item 3", "Item 4", "Item 5" };
            public int getSize() { return strings.length; }
            public Object getElementAt(int i) { return strings[i]; }
        });
        scroll1.setViewportView(lstTest);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(scroll1, javax.swing.GroupLayout.DEFAULT_SIZE, 249, Short.MAX_VALUE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(scroll1, javax.swing.GroupLayout.DEFAULT_SIZE, 174, Short.MAX_VALUE)
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    public static void main(String args[]) {
        new TestDragDrop().setVisible(true);
    }

    // Variables declaration - do not modify                     
    private javax.swing.JList lstTest;
    private javax.swing.JScrollPane scroll1;
    // End of variables declaration                   
}

my migration handler implementation:

import java.awt.datatransfer.Transferable;
import javax.swing.JComponent;
import javax.swing.JList;
import javax.swing.TransferHandler;

public class MyTransferHandler extends TransferHandler {

    @Override
    public int getSourceActions(JComponent component) {
        return COPY_OR_MOVE;
    }

    @Override
    protected Transferable createTransferable(JComponent component) {
        if(component instanceof JList) {
            return new MyTransferable(new String("TestObject"));
        }
        return null;
    }

    @Override
    public boolean canImport(TransferHandler.TransferSupport support) {
        System.out.println("canImport()");
        return true;
    }

    @Override
    public boolean importData(TransferHandler.TransferSupport support) {
        System.out.println("importData()");
        return true;
    }

}

my portable implementation:

import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.io.IOException;

public class MyTransferable implements Transferable {

    private final Object object;

    public MyTransferable(Object object) {
        this.object = object;
    }

    @Override
    public DataFlavor[] getTransferDataFlavors() {
        return new DataFlavor[] { DataFlavor.javaFileListFlavor };
    }

    @Override
    public boolean isDataFlavorSupported(DataFlavor flavor) {
        return flavor.equals(DataFlavor.javaFileListFlavor);
    }

    @Override
    public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
        if(!isDataFlavorSupported(flavor)) {
            throw new UnsupportedFlavorException(flavor);
        }
        return object;
    }

}

my drag and drop implementation:

import java.awt.dnd.DropTarget;
import java.awt.dnd.DropTargetDropEvent;

public class MyDropTarget extends DropTarget {

    @Override
    public synchronized void drop(DropTargetDropEvent evt) {
        evt.acceptDrop(evt.getDropAction());
        if(evt.isLocalTransfer()) {
            System.out.println("local transfer");
        } else {
            System.out.println("extern transfer");
        }
        evt.dropComplete(true);
    }

}

If the string is lstTest.setDropTarget (new MyDropTarget ()); in the main class is disabled / commented out, the canImport () method in MyTransferHandler is called during drag and drop as expected. But as soon as I uncomment this line, the canImport () method is not called during drag and drop ...

- , canImport() ?

! .

Answer 1

setTransferHandler DropTarget, , TransferHandler. DropTarget . DropTarget TransferHandler, Swing, . :

jComponent.setTransferHandler(new MyTransferHandler());
DropTarget original = jComponent.getDropTarget();// the Swing DropTarget
MyDropTarget myDropTarget = new MyDropTarget();
myDropTarget.addDropTargetListener(original);// delegate for original behavior
jComponent.setDropTarget(myDropTarget);