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Desktop Integration

Sun’s Java desktop team has introduced several new features that are designed to increase Java’s acceptance on the desktop. Three of these features are splash-screen support (which lets applications display splash screens while initializing), system tray support (which lets applications add icons, tooltips, and pop-up menus to the system tray), and a new Desktop API.

Consider the Desktop API, which seamlessly integrates Java applications with the desktop. This API enables Java applications to launch the operating system’s default browser with a specific Uniform Resource Identifier (URI); launch the OS’s default email client; and launch applications that open, edit, or print files associated with these applications.

The Desktop API uses the OS’s file associations to launch applications associated with specific file types. For example, the .doc file extension is often associated with Microsoft Word. Via the Desktop API, a Java application could launch Word to open, print, or edit files associated with this extension—or even a different application with each of open, print, or edit (assuming that the OS supports this feature).

Before launching the browser, email client, or any application, your Java application must determine whether your OS supports the API. This determination is made by invoking the java.awt.Desktop class’s public static boolean isDesktopSupported() method. This method returns true if the OS supports the API; otherwise, it returns false.

After invoking isDesktopSupported(), the application proceeds to retrieve the Desktop object by invoking Desktop’s public static Desktop getDesktop() method. This method throws a java.awt.Headless exception if the OS does not support a keyboard, display, or mouse. An UnsupportedOperationException is thrown if the Desktop API isn’t supported by the OS.

Now that the Java application has a Desktop instance, it can call various methods to browse, mail, open, edit, or print. Before executing any of these actions, the application can call Desktop’s public boolean isSupported(Desktop.Action action) method, which returns true if the action, as represented by the appropriate Desktop.Action enumeration instance, is supported:

  • BROWSE: This enumeration instance represents a browse action performed by the OS’s default browser.
  • MAIL: This enumeration instance represents a mail action performed by the OS’s default email client.
  • OPEN: This enumeration instance represents an open action performed by an application associated with opening a specific file type.
  • EDIT: This enumeration instance represents an edit action performed by an application associated with editing a specific file type.
  • PRINT: This enumeration instance represents a print action performed by an application associated with printing a specific file type.

You don’t have to invoke isSupported(Desktop.Action action) to determine if an action is supported before invoking the action’s Desktop method: You could invoke the method directly, but then you would have to deal with the method potentially throwing an UnsupportedOperationException. The following action methods are accessible from Desktop:

  • public void browse(URI uri) launches the user’s default browser to display a URI if the browser can handle this URI; otherwise, it launches the default handler application for the URI depending on the protocol and path (as defined in the java.net.URI class).

    A NullPointerException is thrown if uri contains null. A java.io.IOException is thrown if the user’s default browser is not found, or if it fails to be launched—or if the default handler application failed to be launched.

  • public void edit(File file) launches the associated editor application and opens a file for editing.

    A NullPointerException is thrown if file contains null. An IllegalArgumentException is thrown if the specified file doesn’t exist. Finally, an IOException is thrown if the specified file has no associated editor or if the associated application fails to be launched.

  • public void mail() launches the mail composing window of the user’s default email client.

    If the user’s default email client is not found or if it fails to be launched, an IOException is thrown.

  • public void mail(URI mailtoURI) launches the mail composing window of the user’s default email client, filling the message fields specified by a mailto: URI. This URI can specify various message fields, including "cc", "subject", and "body".

    A NullPointerException is thrown if mailtoURI contains null. An IllegalArgumentException is thrown if the URI’s scheme is not mailto. Finally, an IOException is thrown if the user’s default email client is not found or fails to be launched.

  • public void open(File file) launches the associated application to open the file. If the specified file is a directory, the OS’s file manager is launched to open it.

    A NullPointerException is thrown if file contains null. An IllegalArgumentException is thrown if the specified file doesn’t exist. Finally, an IOException is thrown if the specified file has no associated application, or if this application fails to be launched.

  • public void print(File file) prints a file with the native desktop printing facility, using the associated application’s print command.

    A NullPointerException is thrown if file contains null. An IllegalArgumentException is thrown if the specified file doesn’t exist. Finally, an IOException is thrown if the specified file has no associated application that can be used to print its contents.

I created a DesktopDemo application that demonstrates the Desktop API. This application creates a GUI consisting of radio buttons for the browse, mail, open, edit, and print actions; and a text field for entering a URI or filename. After entering a URI or filename, click the appropriate button to launch an application from the desktop. Listing 2 presents DesktopDemo’s source code.

Listing 2 DesktopDemo.java

// DesktopDemo.java

import java.awt.*;
import java.awt.event.*;

import java.io.*;

import java.net.*;

import javax.swing.*;

class DesktopDemo extends JFrame
{
   Desktop desktop;

   DesktopDemo (String title)
   {
      // Assign title to the frame window's title bar.

      super (title);

      // Tell application to automatically exit when the user selects the Close
      // menu item from the frame window's system menu.

      setDefaultCloseOperation (EXIT_ON_CLOSE);

      // Obtain the desktop.

      desktop = Desktop.getDesktop ();

      // Build top panel, consisting of a text-entry field for specifying a
      // URI, a mail recipient, or a filename.

      JPanel pnlTop = new JPanel ();

      pnlTop.add (new JLabel ("Enter URI or filename (as appropriate):"));

      final JTextField txtText = new JTextField (30);
      pnlTop.add (txtText);

      getContentPane ().add (pnlTop, BorderLayout.NORTH);

      // Build bottom panel, consisting of five mutually-exclusive radio
      // buttons for selecting an action.

      JPanel pnlBottom = new JPanel ();

      final ButtonGroup bg = new ButtonGroup ();

      JRadioButton rbBrowse = new JRadioButton ("Browse");
      rbBrowse.setMnemonic (KeyEvent.VK_B);

      ActionListener al;
      al = new ActionListener ()
           {
               public void actionPerformed (ActionEvent e)
               {
                  try
                  {
                      String uri = txtText.getText ();
                      desktop.browse (new URI (uri));
                  }
                  catch (IOException ioe)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "I/O problem");
                  }
                  catch (URISyntaxException use)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "Bad URI syntax");
                  }
               }
           };
      rbBrowse.addActionListener (al);

      if (!desktop.isSupported (Desktop.Action.BROWSE))
          rbBrowse.setEnabled (false);

      bg.add (rbBrowse);
      pnlBottom.add (rbBrowse);

      JRadioButton rbMail = new JRadioButton ("Mail");
      rbMail.setMnemonic (KeyEvent.VK_M);

      al = new ActionListener ()
           {
               public void actionPerformed (ActionEvent e)
               {
                  try
                  {
                      String mailTo = txtText.getText ();
                      if (mailTo.length () > 0)
                          desktop.mail (new URI ("mailto", mailTo, null));
                      else
                          desktop.mail ();
                  }
                  catch (IOException ioe)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "I/O problem");
                  }
                  catch (URISyntaxException use)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "Bad URI syntax");
                  }
               }
           };
      rbMail.addActionListener (al);

      if (!desktop.isSupported (Desktop.Action.MAIL))
          rbMail.setEnabled (false);

      bg.add (rbMail);
      pnlBottom.add (rbMail);

      JRadioButton rbOpen = new JRadioButton ("Open");
      rbOpen.setMnemonic (KeyEvent.VK_O);

      al = new ActionListener ()
           {
               public void actionPerformed (ActionEvent e)
               {
                  try
                  {
                      String filename = txtText.getText ();
                      desktop.open (new File (filename));
                  }
                  catch (IOException ioe)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "I/O problem");
                  }
                  catch (IllegalArgumentException iae)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "File doesn't exist");
                  }
               }
           };
      rbOpen.addActionListener (al);

      if (!desktop.isSupported (Desktop.Action.OPEN))
          rbOpen.setEnabled (false);

      bg.add (rbOpen);
      pnlBottom.add (rbOpen);

      JRadioButton rbEdit = new JRadioButton ("Edit");
      rbEdit.setMnemonic (KeyEvent.VK_E);

      al = new ActionListener ()
           {
               public void actionPerformed (ActionEvent e)
               {
                  try
                  {
                      String filename = txtText.getText ();
                      desktop.edit (new File (filename));
                  }
                  catch (IOException ioe)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "I/O problem");
                  }
                  catch (IllegalArgumentException iae)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "File doesn't exist");
                  }
               }
           };
      rbEdit.addActionListener (al);

      if (!desktop.isSupported (Desktop.Action.EDIT))
          rbEdit.setEnabled (false);

      bg.add (rbEdit);
      pnlBottom.add (rbEdit);

      JRadioButton rbPrint = new JRadioButton ("Print");
      rbPrint.setMnemonic (KeyEvent.VK_P);

      al = new ActionListener ()
           {
               public void actionPerformed (ActionEvent e)
               {
                  try
                  {
                      String filename = txtText.getText ();
                      desktop.print (new File (filename));
                  }
                  catch (IOException ioe)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "I/O problem");
                  }
                  catch (IllegalArgumentException iae)
                  {
                      JOptionPane.showMessageDialog (DesktopDemo.this,
                                                     "File doesn't exist");
                  }
               }
           };
      rbPrint.addActionListener (al);

      if (!desktop.isSupported (Desktop.Action.PRINT))
          rbPrint.setEnabled (false);

      bg.add (rbPrint);
      pnlBottom.add (rbPrint);

      getContentPane ().add (pnlBottom, BorderLayout.CENTER);

      // Resize the GUI to its preferred size.

      pack ();

      // Display GUI and start the AWT's event-dispatching thread.

      setVisible (true);
   }

   void showError (String msg)
   {
      JOptionPane.showMessageDialog (DesktopDemo.this, msg);
   }

   public static void main (String [] args)
   {
      Runnable r = new Runnable ()
                   {
                       public void run ()
                       {
                          // Determine whether or not the desktop is
                          // available. Under the Solaris and Linux operating
                          // systems, for example, the Desktop API depends on
                          // the Gnome libraries. If those libraries are not
                          // available, Desktop's isDesktopSupported() method
                          // returns false.

                          if (!Desktop.isDesktopSupported ())
                          {
                              JOptionPane.showMessageDialog (null,
                                                 "Desktop is not supported!");
                              System.exit (-1);
                          }

                          new DesktopDemo ("Desktop Demo");
                       }
                   };
      EventQueue.invokeLater (r);
   }
}

After compiling and running DesktopDemo, enter a filename into the text field and click one of the Open, Edit, or Print radio buttons to launch the file’s associated application (for this action). Or enter a URI and click Browse or Mail. Figure 1 reveals my having entered a mail URI into the text field.

Figure 1

Figure 1 I don’t specify the mailto: prefix when entering a mail URI because my email client prepends this prefix to the recipient’s email address in the mail composing window.

The mail URI consists of just the recipient’s email address and a subject. I could have included body text by appending &BODY= followed by some text to the URI, but I chose not to do this. After clicking the Mail radio button, my email client’s mail composing window presented the recipient’s email address and the subject in the correct places, as shown in Figure 2.

Figure 2

Figure 2 The mail URI can specify more than just the primary recipient’s email address.

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