Swing Hacks

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Chapter 9, Drag-and-Drop

#66 Handle Dropped URLs

HACK

H A C K

#66

Handle Dropped URLs

Hack #66

Drag-and-drop is like a box of chocolates; you never know what you’re going

to get....

Bookmark menus are so 1995. Today, you should expect to be able to drag

URLs to other applications and have those applications open web pages,

store the address in a bookmark database, start an email in response to a

mailto: URL, etc. Java’s networking chops are well-established and aren’t

the problem here. The issue is actually getting the URL itself from the drop.

To accept drops of native objects, your GUI needs to designate some

Component as the onscreen drop target. Your code then implements the

DropTarget

interface, which means your implementation will get callbacks

when the user drags the mouse into your component, over it, out of it, etc.

Most of

DropTarget’s methods can be left as no-ops; for now, only the drop( )

method matters.

What’s interesting about native drag-and-drop (and copy-and-paste, for that

matter) is that there’s not necessarily one way to represent the data being

transferred. Instead, you do a sort of negotiation with the

Transferable

passed to you by the DropTargetDropEvent: it specifies, in order of robust-

ness, which

DataFlavors it can deliver, or you ask whether specific

DataFlavors are supported.

In the demo code in Example 9-3, I have a method called

dumpDataFlavors( ),

which shows the

DataFlavors offered to you by the drop. You can use

System.out.println( ) on a particular flavor to get its MIME type, which

describes the contents of the drop, and a representation class, which indi-

cates how those contents will be provided to you by

Transferable.

getTransferData( )

. For example, some browsers will give you a java.net.

URL

, whose DataFlavor looks like this:

java.awt.datatransfer.DataFlavor[mimetype=application/x-java-url;

representationclass=java.net.URL]

One thing that’s surprising is the number of DataFlavors offered by popular

web browsers. Table 9-1 is a short listing of some of the browsers I tested.

Table 9-1. Browsers and their supported DataFlavors

Browser Supported DataFlavors

Firefox 1.0 / Windows XP 78

MSIE 6.0 / Windows XP 53

Safari 1.3 / Mac OS X 53

Firefox 1.0 / Mac OS X 30

MSIE 5.2.3 / Mac OS X 53

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Fortunately, a few DataFlavors appear on the lists for each browser, so your

drop can just handle these common cases.

The

URLDropTargetDemo

class in Example 9-3 presents a large Drop Here

label and a

JTextField

to show the dropped URL. I could have opened a

stream from that URL and rendered its contents in Swing’s

HTMLEditorKit

,

but the result looks so bad with real-world web pages that it wasn’t worth it.

Besides, your app might be doing something other than opening web pages,

such as storing bookmarks or starting emails.

Example 9-3. A drop target for URLs

public class URLDropTargetDemo extends JPanel

implements DropTargetListener {

DropTarget dropTarget;

JLabel dropHereLabel;

JTextField statusField;

static DataFlavor urlFlavor;

static {

try {

urlFlavor =

new DataFlavor ("application/x-java-url; class=java.net.URL");

} catch (ClassNotFoundException cnfe) {

cnfe.printStackTrace( );

}

}

public URLDropTargetDemo( ) {

super(new BorderLayout( ));

dropHereLabel = new JLabel ("Drop here",

SwingConstants.CENTER);

dropHereLabel.setFont (getFont( ).deriveFont (Font.BOLD, 24.0f));

add (dropHereLabel, BorderLayout.CENTER);

// set up drop target stuff

dropTarget = new DropTarget (dropHereLabel, this);

statusField = new JTextField (30);

statusField.setEditable(false);

add (statusField, BorderLayout.SOUTH);

}

public static void main (String[] args) {

JFrame frame = new JFrame ("URL DropTarget Demo");

URLDropTargetDemo demoPanel = new URLDropTargetDemo( );

frame.getContentPane( ).add (demoPanel);

frame.pack( );

frame.setVisible(true);

}

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#66 Handle Dropped URLs

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A static initializer sets up a flavor for getting real java.net.URLs (MIME type

application/x-java-url) from applications that provide them. The only

other

DataFlavor you’ll need is a string flavor that exists as a constant in the

DataFlavor class itself.

// drop target listener events

public void dragEnter (DropTargetDragEvent dtde) {}

public void dragExit (DropTargetEvent dte) {}

public void dragOver (DropTargetDragEvent dtde) {}

public void drop (DropTargetDropEvent dtde) {

System.out.println ("drop");

dtde.acceptDrop (DnDConstants.ACTION_COPY_OR_MOVE);

Transferable trans = dtde.getTransferable( );

dumpDataFlavors (trans);

boolean gotData = false;

try {

// try for application/x-java-url flavor

if (trans.isDataFlavorSupported (urlFlavor)) {

URL url = (URL) trans.getTransferData (urlFlavor);

statusField.setText (url.toString( ));

statusField.setCaretPosition (0);

gotData = true;

} else if (trans.isDataFlavorSupported (DataFlavor.stringFlavor)) {

// try for string flavor

String s =

(String) trans.getTransferData (DataFlavor.stringFlavor);

statusField.setText (s);

gotData = true;

}

} catch (Exception e) {

e.printStackTrace( );

} finally {

System.out.println ("gotData is " + gotData);

dtde.dropComplete (gotData);

}

}

public void dropActionChanged (DropTargetDragEvent dtde) {}

private void dumpDataFlavors (Transferable trans) {

System.out.println ("Flavors:");

DataFlavor[] flavors = trans.getTransferDataFlavors( );

for (int i=0; i<flavors.length; i++) {

System.out.println ("*** " + i + ": " + flavors[i]);

}

}

}

Example 9-3. A drop target for URLs (continued)