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Java AWT Reference by John Zukowski

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16

Data Transfer

In this chapter:

  • DataFlavor
  • Transferable Interface
  • ClipboardOwner Interface
  • Clipboard
  • StringSelection
  • UnsupportedFlavorException
  • Reading and Writing the Clipboard

One feature that was missing from Java 1.0 was the ability to access the system clipboard. It was impossible to cut and paste data from one program into another. Java 1.1 includes a package called java.awt.datatransfer that supports clipboard operations. Using this package, you can cut an arbitrary object from one program and paste it into another. In theory, you can cut and paste almost anything; in practice, you usually want to cut and paste text strings, so the package provides special support for string operations. The current version allows only one object to be on the clipboard at a time.

java.awt.datatransfer consists of three classes, two interfaces, and one exception. Objects that can be transferred implement the Transferable interface. The Transferable interface defines methods for working with different flavors of an object. The concept of flavors is basic to Java's clipboard model. Essentially, a flavor is a MIME content type. Any object can be represented in several different ways, each corresponding to a different MIME type. For example, a text string could be represented by a Java String object, an array of Unicode character data, or some kind of rich text that contains font information. The object putting the string on the clipboard provides whatever flavors it is capable of; an object pasting the string from the clipboard takes whatever flavor it can handle. Flavors are represented by the DataFlavor class, and the UnsupportedFlavorException is used when an object asks for a DataFlavor that is not available.

The Clipboard class represents the clipboard itself. There is a single system clipboard, but you can create as many private clipboards as you want. The system clipboard lets you cut and paste between arbitrary applications (for example, Microsoft Word and some Java programs). Private clipboards are useful within a single application, though you could probably figure out some way to export a clipboard to another application using RMI.

To put data on the clipboard, you must implement the ClipboardOwner interface, which provides a means for you to be notified when the data you write is removed from the clipboard. (There isn't any ClipboardReader interface; any object can read from the clipboard.) The final component of the datatransfer package is a special class called StringSelection that facilitates cutting and pasting text strings.

Cutting and pasting isn't the whole story; JavaSoft has also promised drag-and-drop capabilities, but this won't be in the initial release of Java 1.1.

16.1 DataFlavor

A DataFlavor represents a format in which data can be transferred. The DataFlavor class includes two common data flavors; you can create other flavors by extending this class. Flavors are essentially MIME content types and are represented by the standard MIME type strings. An additional content subtype has been added to represent Java classes; the content type of a Java object is:*

application/x-java-serialized-object
<classname>

For example, the content type of a Vector object would be:

application/x-java-serialized-object java.util.Vector

In addition to the content type, a DataFlavor also contains a presentable name. The presentable name is intended to be more comprehensible to humans than the MIME type. For example, the presentable name of a VectorFlavor object might just be “Vector”, rather than the complex and lengthy MIME type given previously. Presentable names are useful when a program needs to ask the user which data flavor to use.

16.1.1 DataFlavor Methods

Variables

The DataFlavor class includes two public variables that hold “prebuilt” flavors representing different kinds of text objects. These flavors are used in conjunction with the StringSelection class. Although these flavors are variables for all practical purposes, they are used as constants.

public static DataFlavor stringFlavor images

The stringFlavor variable is the data flavor for textual data represented as a Java String object. Its MIME type is application/x-javaserializedobjectString.

public static DataFlavor plainTextFlavor images

The plainTextFlavor variable is the data flavor for standard, Unicode-encoded text. Its MIME type is text/plain; charset=unicode.

Constructors

The DataFlavor class has two constructors. One creates a DataFlavor given a MIME content type; the other creates a DataFlavor given a Java class and builds the MIME type from the class name.

public DataFlavor(String mimeType, String humanPresentableName) images

The first constructor creates an instance of DataFlavor for the mimeType flavor of data. The humanPresentableName parameter should be a more user-friendly name. It might be used in a menu to let the user select a flavor from several possibilities. It might also be used to generate an error message when the UnsupportedFlavorException occurs. The plainTextFlavor uses “Plain Text” as its presentable name.

To read data from the clipboard, a program calls the Transferable.getTransferData() method. If the data is represented by a DataFlavor that doesn't correspond to a Java class (for example, plainTextFlavor), getTransferData() returns an InputStream for you to read the data from.

public DataFlavor(Class representationClass, String humanPresentableName) images

The other constructor creates an instance of DataFlavor for the specific Java class representationClass. Again, the humanPresentableName provides a more user-friendly name for use in menus, error messages, or other interactions with users. The stringFlavor uses “Unicode String” as its presentable name.

A program calls Transferable.getTransferData() to read data from the clipboard. If the data is represented by a Java class, getTransferData() returns an instance of the representation class itself. It does not return a Class object. For example, if the data flavor is stringFlavor, getTransferData() returns a String.

Presentations

public String getHumanPresentableName() images

The getHumanPresentableName() method returns the data flavor's presentable name; for example, stringFlavor.getHumanPresentableName() returns the string “Unicode String”.

public void setHumanPresentableName(String humanPresentableName) images

The setHumanPresentableName() method changes the data flavor's presentable name to a new humanPresentableName. It is hard to imagine why you would want to change a flavor's name.

public String getMimeType() images

The getMimeType() method gets the MIME content type for the DataFlavor as a String.

public Class getRepresentationClass() images

The getRepresentationClass() method returns the Java type that is used to represent data of this flavor (i.e., the type that would be returned by the getTransferData() method). It returns the type as a Class object, not an instance of the class itself. Note that all data flavors have a representation class, not just those for which the class is specified explicitly in the constructor. For example, the plainTextFlavor.getRepresentationClass() method returns the class java.io.StringReader.

public boolean isMimeTypeEqual(String mimeType) images

The isMimeTypeEqual() method checks for string equality between mimeType and the data flavor's MIME type string. For some MIME types, this comparison may be too simplistic because character sets may not be present on types like text/plain. Therefore, this method would tell you that the MIME type text/plain; charset=unicode is different from text/plain.

public final boolean isMimeTypeEqual(DataFlavor dataFlavor) images

The isMimeTypeEqual() method checks whether the MIME type of the dataFlavor parameter equals the current data flavor's MIME type. It calls the previous method, and therefore has the same weaknesses.

Protected methods

protected String normalizeMimeType(String mimeType) images

The normalizeMimeType() method is used to convert a MIME type string into a standard form. Its argument is a MIME type, as a String; it returns the new normalized MIME type. You would never call normalizeMimeType() directly, but you might want to override this method if you are creating a subclass of DataFlavor and want to change the default normalization process. For example, one thing you might do with this is add the string charset=US-ASCII to the text/plain MIME type if it appears without a character set.

protected String normalizeMimeTypeParameter(String parameterName, String parameterValue) images

The normalizeMimeTypeParameter() method is used to convert any parameters associated with MIME types into a standard form. Its arguments are a parameter name (for example, charset) and the parameter's value (for example, unicode). It returns parameterValue normalized. You would never call normalizeMimeTypeParameter() directly, but you might want to override this method if you are creating a subclass of DataFlavor and want to change the default normalization process. For example, parameter values may be case sensitive. You could write a method that would convert the value Unicode to the more appropriate form unicode.

While it may be more trouble than it's worth, carefully overriding these normalization methods might help you to get more predictable results from methods like isMimeTypeEqual().

Miscellaneous methods

public boolean equals(DataFlavor dataFlavor) images

The equals() method defines equality for flavors. Two DataFlavor objects are equal if their MIME type and representation class are equal.

16.2 Transferable Interface

Objects that can be placed on a clipboard must implement the Transferable interface. This interface defines a number of methods that let an object describe how it presents itself to clipboard readers. That sounds complex, but it isn't really; these methods let a clipboard reader find out what data flavors are available and what Java types they represent.

The significance of the Transferable interface is that it provides a way to get information about the object on the clipboard without knowing what the object actually is. When you read the clipboard, you don't necessarily know what kind of object is there. It might be some kind of text string, but it could just as likely be something bizarre. However, you shouldn't have to care. If you're looking for a String, you care only that the object exists in a stringFlavor representation. These methods let you ask the object what flavors it supports.

For text strings, the data transfer package provides a StringSelection class that implements Transferable. At this point, if you want to transfer other kinds of objects, you'll have to create a class that implements Transferable yourself. It wouldn't be unreasonable for JavaSoft to provide other “selection” classes (for example, ImageSelection) in the future.

Methods

public abstract DataFlavor[] getTransferDataFlavors() images

The getTransferDataFlavors() method should return a sorted array of DataFlavors that you support. The most descriptive flavor should be the first element in the array and the least descriptive, last. For example, a textual object would place DataFlavor.plainTextFlavor last, because it has less information than DataFlavor.stringFlavor (which includes information like the length of the string) and much less information than a hypothetical flavor like DataFlavor.richTextFlavor.

public abstract boolean isDataFlavorSupported(DataFlavor flavor) images

The isDataFlavorSupported() method should return true if the object supports the given flavor and false otherwise.

public abstract Object getTransferData(DataFlavor flavor)

throws UnsupportedFlavorException, IOException images

The getTransferData() method is the most complicated to implement. It should return an instance of the class representing the data in the given flavor. If flavor is not supported by this object, getTransferData() must throw the UnsupportedFlavorException. However, this method must be able to return a class for each flavor the object supports (i.e., each data flavor listed by getTransferDataFlavors()). The method could throw an IOException when returning with a Reader as the representation class. For example, if some data flavor required you to return a FileReader and the file doesn't exist, this method might throw an IOException.

16.3 ClipboardOwner Interface

Classes that need to place objects on a clipboard must implement the ClipboardOwner interface. An object becomes the clipboard owner by placing something on a Clipboard and remains owner as long as that object stays on the clipboard; it loses ownership when someone else writes to the clipboard. The ClipboardOwner interface provides a way to receive notification when you lose ownership—that is, when the object you placed on the clipboard is replaced by something else.

Methods

public abstract void lostOwnership(Clipboard clipboard, Transferable contents) images

The lostOwnership() method tells the owner of contents that it is no longer on the given clipboard. It is usually implemented as an empty stub but is available for situations in which you have to know.

16.4 Clipboard

The Clipboard class is a repository for a Transferable object and can be used for cut, copy, and paste operations. You can work with a private clipboard by creating your own instance of Clipboard, or you can work with the system clipboard by asking the Toolkit for it:

Toolkit.getDefaultToolkit().getSystemClipboard()

When working with the system clipboard, native applications have access to information created within Java programs and vice versa. Access to the system clipboard is controlled by the SecurityManager and is restricted within applets.

16.4.1 Clipboard Methods

Variables

protected ClipboardOwner owner images

The owner instance variable represents the current owner of contents. When something new is placed on the clipboard, the previous owner is notified by a call to the lostOwnership() method. The owner usually ignores this notification. However, the clipboard's contents are passed back to owner in case some special processing or comparison needs to be done.

protected Transferable contents images

The contents instance variable is the object currently on the clipboard; it was placed on the clipboard by owner. To retrieve the current contents, use the getContents() method.

Constructors

public Clipboard(String name) images

The constructor for Clipboard allows you to create a private clipboard named name. This clipboard is not accessible outside of your program and has no security constraints placed upon it.

Miscellaneous methods

public String getName() images

The getName() method fetches the clipboard's name. For private clipboards, this is the name given in the constructor. The name of the system clipboard is “System”.

public synchronized Transferable getContents(Object requester) images

The getContents() method allows you to retrieve the current contents of the clipboard. This is the method you would call when the user selects Paste from a menu.

Once you have the Transferable data, you try to get the data in whatever flavor you want by calling the Transferable.getTransferData() method, possibly after calling Transferable.isDataFlavorSupported(). The requester represents the object that is requesting the clipboard's contents; it is usually just this, since the current object is making the request.

public synchronized void setContents(Transferable contents, ClipboardOwner owner) images

The setContents() method changes the contents of the clipboard to contents and changes the clipboard's owner to owner. You would call this method when the user selects Cut or Copy from a menu. The owner parameter represents the object that owns contents. This object must implement the ClipboardOwner interface; it will be notified by a call to lostOwnership() when something else is placed on the clipboard.

16.5 StringSelection

StringSelection is a convenience class that can be used for copy and paste operations on Unicode text strings (String). It implements both the ClipboardOwner and Transferable interfaces, so it can be used both as the contents of the clipboard and as its owner. For example, if s is a StringSelection, you can call Clipboard. setContents(s,s). StringSelection supports both stringFlavor and plainTextFlavor and doesn't do anything when it loses clipboard ownership.

16.5.1 StringSelection Methods

Constructors

public StringSelection(String data) images

The constructor creates an instance of StringSelection containing data. You can use this object to place the data on a clipboard.

Miscellaneous methods

public DataFlavor[] getTransferDataFlavors() images

The getTransferDataFlavors() method returns a two-element DataFlavor array consisting of DataFlavor.stringFlavor and DataFlavor.plainTextFlavor. This means that you can paste a StringSelection as either a Java String or as plain text (i.e., the MIME type plain/text).

public boolean isDataFlavorSupported(DataFlavor flavor) images

The isDataFlavorSupported() method is returns true if flavor is either DataFlavor.stringFlavor or DataFlavor.plainTextFlavor; it returns false for any other flavor.

public Object getTransferData(DataFlavor flavor)

throws UnsupportedFlavorException, IOException images

The getTransferData() method returns an object from which you can get the data on the clipboard; the object's type is determined by the flavor parameter. This method returns a String containing the data on the clipboard if flavor is DataFlavor.stringFlavor; it returns a StringBufferInputStream from which you can read the data on the clipboard if you ask for DataFlavor. plainTextFlavor. Other wise, getTransferData() throws an UnsupportedFlavorException.

public void lostOwnership(Clipboard clipboard, Transferable contents) images

The lostOwnership() method of StringSelection is an empty stub; it does nothing when you lose ownership. If you want to know when you've lost ownership of string data placed on the clipboard, write a subclass of StringSelection and override this method.

16.6 UnsupportedFlavorException

The UnsupportedFlavorException exception is thrown when you ask Transferable. getTransferData() to give you data in a flavor that isn't supported by the object on the clipboard. For example, if the clipboard currently holds an image and you ask for the data in the stringFlavor, you will almost certainly get an UnsupportedFlavorException because it is unlikely that an image object will be able to give you its data as a String. You can either ignore the exception or display an appropriate message to the user.

16.6.1 UnsupportedFlavorException Method

Constructor

public UnsupportedFlavorException (DataFlavor flavor)

The sole constructor creates an UnsupportedFlavorException with a detail message containing the human presentable name of flavor. To retrieve this message, call getMessage(), which this exception inherits from the Exception superclass (and which is required by the Throwable interface).

16.7 Reading and Writing the Clipboard

Now that you know about the different java.awt.datatransfer classes required to use the clipboard, let's put them all together in an example. Example 16-1 creates a TextField for input (copying), a read-only TextArea for output (pasting), and a couple of buttons to control its operation. Figure 16-1 shows the program's user interface. When the user clicks on the Copy button or presses Return in the TextField, the text in the TextField is copied to the Clipboard. When the user clicks on the Paste button, the contents of the clipboard are drawn in the TextArea. Since the clipboard is not private, you can copy or paste from anywhere on your desktop, not just this program.

Example 16–1: Using the System Clipboard

// Java 1.1 only
import java.io.*;
import java.awt.*;
import java.awt.datatransfer.*;

public class ClipMe extends Frame {
    TextField tf;
    TextArea ta;
    Button copy, paste;
    Clipboard clipboard = null;
    ClipMe() {
        super ("Clipping Example");
        add (tf = new TextField("Welcome"), "North");
        add (ta = new TextArea(), "Center");
        ta.setEditable(false);
        Panel p = new Panel();
        p.add (copy = new Button ("Copy"));
        p.add (paste = new Button ("Paste"));
        add (p, "South");
        setSize (250, 250);
    }
    public static void main (String args[]) {
        new ClipMe().show();
    }
    public boolean handleEvent (Event e) {
        if (e.id == Event.WINDOW_DESTROY) {
            System.exit(0);
            return true;   // never gets here
        }
        return super.handleEvent (e);
    }
    public boolean action (Event e, Object o) {
        if (clipboard == null)
            clipboard = getToolkit().getSystemClipboard();
        if ((e.target == tf) || (e.target == copy)) {
            StringSelection data;
            data = new StringSelection (tf.getText());
clipboard.setContents (data, data);
        } else if (e.target == paste) {
            Transferable clipData = clipboard.getContents(this);
            String s;
            try {
                s = (String)(clipData.getTransferData(
                       DataFlavor.stringFlavor));
            } catch (Exception ee) {
                s = ee.toString();
            }
            ta.setText(s);
        }
        return true;
    }
}

images

Figure 16–1: Using the system clipboard

We won't say anything about how the display is set up; that should be familiar. All the interesting stuff happens in the action method, which is called in response to a button click. We check which button the user clicked; if the user clicked the Copy button, we read the text field tf and use it to create a new StringSelection named data. If the user clicked the Paste button, we retrieve the data from the clipboard by calling getContents(). This gives us an object about which (strictly speaking) we know nothing, except that it implements Transferable. In this case, we're pretty sure that we're getting text from the clipboard, so we call getTransferData() and ask for the data in the stringFlavor form. We catch the exception that might occur if we're wrong about the data flavor. This program has no way of placing anything but text on the clipboard, but there's no guarantee that the user didn't cut some other kind of object from a native application.

Once we have our String, we call the setText() method of the TextArea to tell it about the new string, and we are finished.

* The type name changed to x-java-serialized-object in the 1.1.1 release.

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