Link19 Link19 - 4 years ago 124
Java Question

Why can't I drop files on to Eclipse / SWT Drop Target from a CD in windows

In my Eclipse RCP application I have a TreeViewer that is a drop target for files, it works fine in most circumstances, however when I try and drag files stored on a CD ROM from windows explorer to the node the icon that indicates drops are allowed doesn't change and dropping does nothing.

Since people seemed confused about my question here's a more detailed explanation:

When executing the code below (supplied by Baz), I am able to drag files and drop them on to the text box, when I drag a file from most locations on my machine, the window appears like this ->

Working Drag

The cursor indicates that I can drop, and when I release the mouse the

drop(DropTargetEvent event)
method is invoked.

Now when I do the same thing, but take a file from explorer that is on a DVD in my optical drive then it looks like this:

enter image description here

Indicating that the drop is not allowed, and when I release the mouse the
drop(DropTargetEvent event)
method is NOT invoked.

It's also worth noting that I am able to drop the same DVD files in to a folder in the eclipse navigator, indicating that this isn't a machine specific issue, there must be something different in the eclipse trees that allows it but I can't see it.

package widgets;

import java.util.Arrays;

import org.eclipse.swt.SWT;
import org.eclipse.swt.dnd.DND;
import org.eclipse.swt.dnd.DropTarget;
import org.eclipse.swt.dnd.DropTargetAdapter;
import org.eclipse.swt.dnd.DropTargetEvent;
import org.eclipse.swt.dnd.FileTransfer;
import org.eclipse.swt.dnd.Transfer;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Text;
import org.eclipse.ui.part.PluginTransfer;

public class SourceTest {
public static void main(String[] args) {
final Display display = new Display();
Shell shell = new Shell(display);
shell.setLayout(new GridLayout(1,false));

final Text text = new Text(shell, SWT.BORDER);
text.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, true, true));

DropTarget dt = new DropTarget(text, DND.DROP_COPY | DND.DROP_MOVE | DND.DROP_LINK);
dt.setTransfer(new Transfer[] { FileTransfer.getInstance(), PluginTransfer.getInstance() });
dt.addDropListener(new DropTargetAdapter() {
public void drop(DropTargetEvent event) {
String fileList[] = null;
FileTransfer ft = FileTransfer.getInstance();
if (ft.isSupportedType(event.currentDataType)) {
fileList = (String[]);

shell.setSize(300, 100);;

while (!shell.isDisposed()) {
if (!display.readAndDispatch())


Answer Source

It appears that not all of the DND operations possible are supported when coming from a CDROM device. Therefore you need to implement a couple of more methods on the DropTargetAdapter in order to modify the Drop operation while in progress to narrow down the operations that will actually be performed so that the OS doesn't prevent the drop.

I took your example and just made the small change to the DropTargetAdapter

    public void dropAccept( DropTargetEvent event )
        Object object = FileTransfer.getInstance().nativeToJava( event.currentDataType );
        if( object instanceof String[] )
            String[] strs = (String[]) object;
            System.out.println("dropAccept " + strs[0]);
            event.detail = DND.DROP_COPY;
        event.detail = DND.DROP_COPY;
    public void dragEnter( DropTargetEvent event )
        event.detail = DND.DROP_COPY;
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