Java Clipboards and Data Transfer

(Ported from betaveros.stash. Wow, I get syntax highlighting and footnotes! Probably years out of date though. I probably wrote this somewhere in 2012–2014, but am editing this parenthetical in 2021.)

A quick brief guide. At least, that’s how I planned it.

A lot of stuff is in the package java.awt.datatransfer. Class Toolkit is in java.awt.

Some basic classes. The class Clipboard is a clipboard, obviously. Its content is/will be an instance of the class Transferable. Some content can be read as different types of objects depending on what you want; to choose which type you use an instance of DataFlavor. It provides three basic ones: DataFlavor.imageFlavor, DataFlavor.javaFileListFlavor, and DataFlavor.stringFlavor.

Okay, now step by step. This is the low-level method.

  1. Get the default clipboard with Clipboard clipboard = Toolkit.getDefaultToolkit().getSystemClipboard();
  2. Get a transferable with Transferable content = clipboard.getContents(null); 1
  3. Check if content can be read as the kind of object you want with (content != null) && content.isDataFlavorSupported(someFlavor)
  4. If it does, get the object with content.getTransferData(someFlavor) 2.

If you just want a quick-and-dirty function:

For designing a Swing component, however, there are more integrated ways. This also allows drag-and-drop as well as cut-copy-paste, the conventional UI ideas. For many components (ex. JTextField) the mechanisms are pretty much in place, and you can call setDragEnabled(true) on them and be done. Further customizations are possible, particularly for complex things JList, JTable, JTree; I’ll just skip them today. The first thing is defining/subclassing a TransferHandler, which will be asked to handle all of this stuff by Swing internals:

Note that you do not have to define all of the methods, only the ones you need. TransferHandler is not abstract, although directly instantiating one is probably not a good idea.

TransferHandler.TransferSupport is3 a wrapper around Transferable containing also where the transfer is coming from and of what type it is. The transferable is extracted with getTransferable(), but the wrapper also directly has data-flavor compatibility checking methods4. Go look its methods up.

Okay, now that you have a handler set up, how do you get it used? someJComponent.setTransferHandler(someTransferHandler) will allow the handler to handle the transfer events. For cc&p, you can get corresponding Actions from TransferHandler.get[Cut|Copy|Paste]Action() (yes, static method).

  1. The argument is a handler of type ClipboardOwner which needs one method lostOwnership(Clipboard c, Transferable t) that is called when the clipboard no longer contains your content. I guess you use it to free resources? I don’t need it though.

  2. It throws checked exceptions UnsupportedFlavorException and IOException. My former self would hope that you have an IDE that would tell you that and salutes you if you don’t. Now, I just think whatever, if you’re writing Scala like me then you don’t even care.

  3. At least I think of it as one, but I don’t know if everybody’s definition of “wrapper” matches mine. Oh well.

  4. isDataFlavorSupported(DataFlavor) and getDataFlavors(); the latter returns an array. The documentation says these are faster than extracting the Transferable and querying it.

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