Building applications

An application consists of a number of files:

The binary

This gets installed in /usr/bin.

A desktop file

The desktop file provides important information about the application to the desktop shell, such as its name, icon, D-Bus name, commandline to launch it, etc. It is installed in /usr/share/applications.

An icon

The icon gets installed in /usr/share/icons/hicolor/48x48/apps, where it will be found regardless of the current theme.

A settings schema

If the application uses GSettings, it will install its schema in /usr/share/glib-2.0/schemas, so that tools like dconf-editor can find it.

Other resources

Other files, such as GtkBuilder ui files, are best loaded from resources stored in the application binary itself (in case of a native application) or, for a Java application, as a resource in the jar file. This eliminates the need for most of the files that would traditionally be installed in an application-specific location in /usr/share.

GTK includes application support that is built on top of GApplication. In this tutorial we'll build a simple application by starting from scratch, adding more and more pieces over time. Along the way, we'll learn about Gtk.Application, templates, resources, application menus, settings, Gtk.HeaderBar, Gtk.Stack, Gtk.SearchBar, Gtk.ListBox, and more.

The full, buildable sources for these examples can be found online on GitHub. You can build each example separately by using Gradle with the build.gradle file. For more information, see the README.md file included in the examples repository.

A trivial application

When using GtkApplication, the main() method can be very simple. We just call Application.run() on an instance of our application class.

public class ExampleMainClass {

  public static void main(String[] args) {
    ExampleApp.create().run(args);
  }
}

All the application logic is in the application class, which is a subclass of the GTK Application class. Our example does not yet have any interesting functionality. All it does is open a window when it is activated without arguments, and open the files it is given, if it is started with arguments.

To handle these two cases, we override the activate() method, which gets called when the application is launched without commandline arguments, and the open() method, which gets called when the application is launched with commandline arguments.

To learn more about GApplication entry points, consult the GIO documentation.

import io.github.jwharm.javagi.gobject.types.Types;
import org.gnome.gio.ApplicationFlags;
import org.gnome.gio.File;
import org.gnome.glib.List;
import org.gnome.glib.Type;
import org.gnome.gobject.GObject;
import org.gnome.gtk.Application;
import org.gnome.gtk.Window;
import java.lang.foreign.MemorySegment;

public class ExampleApp extends Application {

  // Register this Java class with the GObject type system
  private static final Type gtype = Types.register(ExampleApp.class);

  public static Type getType() {
    return gtype;
  }

  public ExampleApp(MemorySegment address) {
    super(address);
  }

  @Override
  public void activate() {
    ExampleAppWindow win = ExampleAppWindow.create(this);
    win.present();
  }

  @Override
  public void open(File[] files, String hint) {
    ExampleAppWindow win;
    List<Window> windows = super.getWindows();
    if (!windows.isEmpty())
      win = (ExampleAppWindow) windows.getFirst();
    else
      win = ExampleAppWindow.create(this);

    for (File file : files)
      win.open(file);

    win.present();
  }

  public static ExampleApp create() {
    return GObject.newInstance(getType(),
        "application-id", "org.gtk.exampleapp",
        "flags", ApplicationFlags.HANDLES_OPEN,
        null);
  }
}

Another important class that is part of the application support in GTK is Gtk.ApplicationWindow. It is typically subclassed as well. Our subclass does not do anything yet, so we will just get an empty window.

import io.github.jwharm.javagi.gobject.types.Types;
import org.gnome.gio.File;
import org.gnome.glib.Type;
import org.gnome.gobject.GObject;
import org.gnome.gtk.ApplicationWindow;
import java.lang.foreign.MemorySegment;

public class ExampleAppWindow extends ApplicationWindow {

  private static final Type gtype = Types.register(ExampleAppWindow.class);

  public static Type getType() {
    return gtype;
  }

  public ExampleAppWindow(MemorySegment address) {
    super(address);
  }

  public static ExampleAppWindow create(ExampleApp app) {
    return GObject.newInstance(getType(), "application", app, null);
  }

  public void open(File file) {
  }
}

As part of the initial setup of our application, we also create an icon and a desktop file.

[Desktop Entry]
Type=Application
Name=Example
Icon=exampleapp
StartupNotify=true
Exec=@bindir@/exampleapp

Note that @bindir@ needs to be replaced with the actual path to the binary before this desktop file can be used.

Full source

Here is what we've achieved so far:

This does not look very impressive yet, but our application is already presenting itself on the session bus, it has single-instance semantics, and it accepts files as commandline arguments.

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