<AWT Dev> [9] Review Request for JDK-8024061: Exception thrown when drag and drop between two components is executed quickly

[Alexey Ivanov]

Hi Petr,

If the requested data flavor is DataFlavor.javaJVMLocalObjectMimeType, 
then transferable is local to the JVM. It is stored to a static variable 
in SunDropTargetContextPeer.setCurrentJVMLocalSourceTransferable(). When 
processEnterMessage() is handled, its value is copied to instance 
variable "local". The instance variable is cleaned by 
processExitMessage(), the static variable is cleared by 
SunDragSourceContextPeer.cleanup().

If drag-and-drop operation is performed quickly, 
SunDragSourceContextPeer.cleanup() is called when mouse button is 
released and *before* processEnterMessage() is called. So when EDT 
finally handles MOUSE_ENTERED event and DropTargetListener.dragEnter() 
gets called, "local" transferable is already null. The listener tries to 
get the data from the transferable:
DropTargetDragEvent.getTransferable(), and then
Transferable.getTransferData(dataFlavor).

The dataFlavor has MIME type of DataFlavor.javaJVMLocalObjectMimeType, 
and SunDropTargetContextPeer.currentJVMLocalSourceTransferable is 
already cleared. The representationClass field of the DataFlavor object 
is set to java.io.InputStream.class. So when 
SunDropTargetContextPeer.getNativeData() executes it tries to cast the 
data object to java.io.InputStream to deserialize the data but fails 
with the exception:
java.lang.ClassCastException: DnD$DropObject cannot be cast to 
java.io.InputStream.

This exception is caught internally, and IOException is thrown as it 
failed to get the data. At the same time, no IO should have occurred 
because a local source was dragged, so this exception is rather unexpected.


What actually happens is that native drag-and-drop operation completed 
already whereas this drag-and-drop operation on EDT didn't even started 
yet. Does throwing InvalidDnDOperationException look more appropriate 
than returning null in this case?


Sure, I'll submit a new bug for 9 to implement the proper fix.


Regards,
Alexey.

On 10.04.2014 12:52, Petr Pchelko wrote:
> Hello, Dmitry.
>
> Thank you for the detailed explanation of the issue.
>
> I’m OK with this workaround as this should be back ported to 8u and 7u, but I think we could try to provide a real fix in 9.
> Could you please add some comment which shortly explains why the if clause was added?
> And could you also file a new bug for 9 so that we try to replace the workaround with a real fix?
>
> Thank you.
> With best regards. Petr.
>
> 10 апр. 2014 г., в 12:18 после полудня, Alexey Ivanov <alexey.ivanov at oracle.com> написал(а):
>
>> Hello AWT team,
>>
>> Could you please review the fix for jdk9:
>>     bug: https://bugs.openjdk.java.net/browse/JDK-8024061
>>     webrev: http://cr.openjdk.java.net/~dmarkov/8024061/jdk9/webrev.00/
>>
>> Problem description:
>> Dragging and dropping an object from one panel to another very quickly causes an exception.
>>
>> Fix:
>> The fix addresses only the thrown exception.
>> The dragged object would not be moved.
>>
>>
>> Thanks,
>> Alexey.