Developing GNOME Applications with Java

public class ExampleSWT {
 public static void main(String[] args) {
  Display display = new Display();
  Shell shell = new Shell(display);
  shell.setLayout(new FillLayout(SWT.VERTICAL));

  Composite msgbox = new Composite(shell,
                                   SWT.NO_TRIM);
  RowLayout msglayout = new RowLayout();
  msglayout.justify = true;
  msgbox.setLayout(msglayout);

  Label label = new Label(msgbox, SWT.NO_TRIM);
  label.setText("Quit?");

  Composite buttonbox = new Composite(shell,
                                      SWT.NO_TRIM);
  RowLayout buttonlayout = new RowLayout();
  buttonlayout.justify = true;
  buttonlayout.pack = true;
  buttonbox.setLayout(buttonlayout);

  Button yesButton = new Button(buttonbox,
                                SWT.PUSH);
  yesButton.setText("Yes");
  Button noButton = new Button(buttonbox,
                               SWT.PUSH);
  noButton.setText("No");

  yesButton.addSelectionListener(
                            new SelectionAdapter() {
   public void widgetSelected(
                             SelectionEvent event) {
    System.exit(0);
   }
  });

  noButton.addSelectionListener(
                            new SelectionAdapter() {
   public void widgetSelected(
                             SelectionEvent event) {
    System.exit(1);
   }
  });

  shell.pack();
  shell.open();

  while (! shell.isDisposed()) {
   if (! display.readAndDispatch()) display.sleep();
  }
 }
}

IBM sponsors the Eclipse Project, an effort to produce an open-source development environment. One of the fruits of this project is the Standard Widget Toolkit, an alternative to AWT and Swing. SWT is a peer-based, operating system-independent interface that uses the host operating system's interface for rendering common components. Components not supported by an operating system are implemented in Java. On Linux, the libswt-gtk2 package provides a GTK peer for SWT. Peers also exist for other platforms, including Solaris and Windows. SWT code can run on any platform that has an SWT peer. An example SWT application is shown in Listing 4, which can be compiled against the GTK SWT peer with a variation of the following:

gcj --CLASSPATH=/usr/lib/libswt-gtk2.jar -lswt-gtk2
-o ExampleSWT --main=ExampleSWT ExampleSWT.java

See Resources for more information about the Standard Widget Toolkit.

With three existing Java GUI toolkits, one might ask why another alternative is necessary. GNOME's Java bindings are unique because they are tied directly to GNOME. An application written with GNOME's Java offerings looks and behaves exactly as if it had been written using GNOME's C libraries. It integrates seamlessly into the GNOME desktop and provides the same capabilities as any other GNOME application. The reason for this is GNOME's Java bindings use the Java Native Interface to delegate work directly to GNOME's C libraries.

Currently, GNOME's Java bindings consist of four libraries—libgconf-java, libglade-java, libgnome-java and libgtk-java. libgtk-java and libgnome-java provide the GUI components of the bindings. libglade-java allows Java applications to read graphical user interface descriptions created by GLADE. Investigating libgconf-java, the Java interface to the GConf configuration system, is left as an exercise for the reader.

libgtk-java and libgnome-java are similar to SWT and AWT because host code implements their graphical components. However, the GNOME libraries are quite different from AWT, Swing and SWT—GNOME libraries make no claim of platform-independence. GNOME applications written in Java run only in a GNOME environment. Any platform independence is a result of the entire GNOME environment itself being platform-independent.

public class ExampleGNOME {
 private LibGlade libglade;
 private static final String GLADE_FILE =
                               "ExampleGNOME.glade";

 public ExampleGNOME () throws IOException {
  libglade = new LibGlade(GLADE_FILE, this);
 }

 public void on_noButton_released(GtkEvent event) {
  Gtk.mainQuit();
  System.exit(1);
 }

 public void on_yesButton_released(GtkEvent event) {
  Gtk.mainQuit();
  System.exit(0);
 }

 public static void main(String args[]) {
  ExampleGNOME gui;

  Gtk.init(args);
  try {
   gui = new ExampleGNOME();
  } catch (IOException e) {
   System.err.println(e);
   System.exit(1);
  }
 Gtk.main();
 }
}

A gcc-java-compiled GNOME application is captured in Figure 3. Listing 5 shows the GNOME application's source code and can be compiled with:

gcj --CLASSPATH=/usr/share/java/gtk2.4.jar:\
/usr/share/java/gnome2.8.jar:\
/usr/share/java/glade2.8.jar \
-lgtkjar2.4 -lgnomejar2.8 -lgladejar2.8 \
-o ExampleGNOME --main=ExampleGNOME \
ExampleGNOME.java

Figure 3. A Java GNOME Application

At first glance, Listing 5 may look a little sparse compared to the others. ExampleGNOME's user interface is defined in ExampleGNOME.glade; as a result, there is not much GUI code in the application itself. Instead, libglade-java reads ExampleGNOME.glade and creates the application's GUI components automatically. The GUI code is tied back to our code by event callback methods. Two of these callbacks, whose names and corresponding signals are defined in ExampleGNOME.glade, are on_noButton_released and on_yesButton_released. Listing 6 contains the contents of a portion of ExampleGNOME.glade.