Java-GI

Java-GI is a tool for generating GObject-Introspection bindings for Java. The generated bindings use the Panama Foreign Function & Memory API (JEP 442, currently in preview status) to directly access native resources from inside the JVM, with wrapper classes based on GObject-Introspection to offer an elegant API. Java-GI version 0.8.1 generates bindings to develop Java applications for libraries, based of the versions in GNOME Platform 45:

Library	Java-GI 0.7.x	Java-GI 0.8.x
OpenJDK	20	21
GLib	2.76	2.78
GTK	4.10	4.12
LibAdwaita	1.3	1.4
GStreamer	1.20	1.22
GtkSourceview	5.9	5.10
WebkitGtk	2.41	2.42

Please note that Java-GI is still under active development. The bindings should not be used in a production environment yet, and the API is subject to unannounced changes. However, feel free to try out the latest release; feedback is welcome.

For more information, visit the Java-GI website.

Quickstart

To use Java-GI in your app, add the dependency to your gradle.build or pom.xml as described here. Furthermore, you must set the Java language version to 21, and, while the Panama foreign function API is still in preview status, set the --enable-preview option to the compile and execution tasks. See this build.gradle file for a complete example.
Write, compile and run a GTK application:

import org.gnome.gtk.*;
import org.gnome.gio.ApplicationFlags;

public class HelloWorld {

    public static void main(String[] args) {
        new HelloWorld(args);
    }

    private final Application app;

    public HelloWorld(String[] args) {
        app = new Application("my.example.HelloApp", ApplicationFlags.DEFAULT_FLAGS);
        app.onActivate(this::activate);
        app.run(args);
    }

    public void activate() {
        var window = new ApplicationWindow(app);
        window.setTitle("GTK from Java");
        window.setDefaultSize(300, 200);

        var box = Box.builder()
                .setOrientation(Orientation.VERTICAL)
                .setHalign(Align.CENTER)
                .setValign(Align.CENTER)
                .build();

        var button = Button.withLabel("Hello world!");
        button.onClicked(window::close);

        box.append(button);
        window.setChild(box);
        window.present();
    }
}

The result:

Examples

You can find some examples here. Each example can be separately built and run with gradle run:


Web Browser	Peg Solitaire	Calculator	Notepad

Current features

Nearly all types, functions and parameters defined in the GIR files are supported by Java-GI. Even complex function signatures with combinations of arrays, callbacks, out-parameters and varargs are available in Java.

Some interesting features of the bindings that Java-GI generates:

Automatic memory management

Memory management of native resources is automatically taken care of. Java-GI uses GObject toggle references to dispose the native object when the Java instance is garbage-collected, and releases all other memory allocations (for strings, arrays and structs) after use.

Javadoc

All API docstrings are translated into Javadoc, so they are directly available in your IDE.

As an example, the generated documentation of gtk_button_get_icon_name contains links to other methods, and specifies the return value. This is all translated to valid Javadoc:

/**
 * Returns the icon name of the button.
 * <p>
 * If the icon name has not been set with {@link Button#setIconName}
 * the return value will be {@code null}. This will be the case if you create
 * an empty button with {@link Button#Button} to use as a container.
 * @return The icon name set via {@link Button#setIconName}
 */
public @Nullable java.lang.String getIconName() {
    ...

The Javadoc is published online.

GObject type system

GObject classes are available as Java classes (obviously). The GObject TypeClass definition is an inner class in the Java class.

Interfaces are mapped to Java interfaces, using default interface methods to call native methods.

Type aliases (typedefs in C) for classes, records and interfaces are represented in Java with a subclass of the original type. Aliases for primitive types such as int or float are represented by simple wrapper classes.

Enumeration types are represented as Java enum types.

Most classes have one or more constructors. However, constructors in GTK are often overloaded, and the name contains valuable information for the user. Java-GI therefore maps constructors named "new" to regular Java constructors, and generates static factory methods for all other constructors:

// gtk_button_new
var button1 = new Button();

// gtk_button_new_with_label
var button2 = Button.withLabel("Open...");

// gtk_button_new_from_icon_name
var button3 = Button.fromIconName("document-open");

Some struct types (called "records" in GObject-Introspection) don't have constructors, because in C these are meant to be stack-allocated. An example is Gdk.RGBA. Java-GI offers a static allocate method that will allocate a new struct in an Arena of your choice. You can either allocate an empty struct (var color = RGBA.allocate(arena);) and fill in the values later, or pass the values immediately: var purple = RGBA.allocate(arena, 0.9f, 0.1f, 0.9f, 1.0f);

Signals, callbacks and closures

Signals are mapped to type-safe methods and objects in Java. (Detailed signals like notify have an extra String parameter.) A signal can be connected to a lambda expression or method reference:

var button = Button.withLabel("Close");
button.onClicked(window::close);

For every signal, a method to connect (e.g. onClicked) and emit the signal (emitClicked) is included in the API. New signal connections return a SignalConnection object, that allows you to disconnect, block and unblock a signal, or check whether the signal is still connected.

Functions with callback parameters are supported too. The generated Java bindings contain @FunctionalInterface definitions for all callback functions to ensure type safety.

Closures are marshaled to Java methods using reflection.

Registering new types

You can easily register a Java class as a GType:

public class MyWidget extends Widget {

    public static Type gtype = Types.register(MyWidget.class);

    // Construct new instance
    public MyWidget newInstance() {
        return GObject.newInstance(gtype);
    }

    // Default constructor, used by Java-GI for marshaling
    public MyWidget(MemorySegment address) {
        super(address);
    }
}

You can define custom GObject properties and signals using annotations. The following example defines an int property named "lives" and a "game-over" signal with a String parameter:

    @Property
    public int getLives() {
        return lives;
    }
    
    @Property
    public void setLives(int value) {
        this.lives = value;
        if (value == 0)
            emit("game-over", player.name());
    }

    @Signal
    public interface GameOver {
        void apply(String playerName);
    }

Java classes can implement interfaces and override methods without any additional effort. When implementing methods from an interface (or overriding virtual methods from a parent class), Java-GI will register it in the GObject type system, so native code will call your Java method too. See for example this implementation of the ListModel interface, or read the Java-GI documentation for an overview of all the possibilities.

Composite template classes

A class with a @GtkTemplate annotation will be registered as a Gtk composite template class:

@GtkTemplate(name="HelloWindow", ui="/my/example/hello-window.ui")
public class HelloWindow extends ApplicationWindow {

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

    @GtkChild(name="header_bar")
    public HeaderBar header;

    @GtkChild
    public Label label;
    
    @GtkCallback
    public void buttonClicked() {
        ...
    }

    ...

In the above example, the header and label fields and the buttonClicked callback function are all declared the hello-window.ui file.

You can read more about template classes in the documentation.

Parameters

Java-GI takes care of marshaling Java values from and to native values. When working with arrays, Java-GI will automatically copy native array contents from and to a Java array, marshaling the contents to the correct types along the way. A null terminator is added where applicable. You also don't need to specify the array length as a separate parameter.

Nullability of parameters (as defined in the GObject-introspection attributes) is indicated with @Nullable and @NotNull attributes, and checked at runtime. The nullability attributes are imported from Jetbrains Annotations (as a compile-time-only dependency).

Variadic functions (varargs) are supported too:

Dialog d = Dialog.withButtons(
        "Test dialog",
        window,
        DialogFlags.MODAL,
        "Accept",
        ResponseType.ACCEPT,
        "Cancel",
        ResponseType.CANCEL,
        null
);
d.show();

Out-parameters are mapped to a simple Out<T> container-type in Java, that offers typesafe get() and set() methods to retrieve or modify the value.

File file = ...
Out<byte[]> contents = new Out<byte[]>();
file.loadContents(null, contents, null));
System.out.printf("Read %d bytes%n", contents.get().length);

Builder pattern

You can construct an object with properties using a Builder pattern. In the "Hello World" app above, it's used to create a Box. It can be used for any other type too:

var window = ApplicationWindow.builder()
    .setApplication(this)
    .setTitle("Window")
    .setDefaultWidth(300)
    .setDefaultHeight(200)
    .build();

Java-GI generates builders for all classes. In a builder, you can set the properties of the class, its parents, and all implemented interfaces.

Exceptions

GError parameters are mapped to Java GErrorExceptions.

try {
    file.replaceContents(contents, null, false, FileCreateFlags.NONE, null, null);
} catch (GErrorException e) {
    e.printStackTrace();
}

Portability

The Java-GI bindings are cross-platform: You can use the same jar on all supported operating systems (Linux, Windows and MacOS) provided that the native libraries are installed. Platform-specific types and methods (like Gtk.PrintUnixDialog) check the operating system at runtime and throw an UnsupportedPlatformException when necessary.

Building and Contributing

To build Java-GI for yourself, make changes, or use Java-GI to generate bindings for other (GObject-Introspection based) libraries, follow the instructions here.

Known issues

The bindings are still under active development and have not been thoroughly tested yet. The most notable issues and missing features are currently:

Java does not distinguish between signed and unsigned data types. Be extra careful when native code returns, for example, a guint.
Java-GI makes heavy use of Cleaners to free memory or decrease an object's refcount. Cleaners are triggered during garbage collection of a Java instance. However, Java doesn't guarantee when, and if, the GC will run, and what it will clean, leading to memory leaks.
Some functions (like Gio.DesktopAppInfo.search) work with nested arrays (gchar***). Marshalling these arrays from and to Java String[][] values isn't supported yet.

jwharm/java-gi