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Box2D-JavaFFM

This project provides Java bindings for Box2D using the Java FFM API.

Supported Platforms

Windows and Linux are directly supported. The Linux .so file was built on Linux Mint 22.2.

Nevertheless, you should be able to use these bindings for Mac if you provide your own .dylib file and load it.

Building The Project

  1. Download Java 26
  2. Download the sources or clone the repo

Using The Terminal

  1. Install Maven
  2. Open the terminal in the directory of the downloaded project
  3. Run the goal mvn package.
  4. The output is in the target/ folder

Using An IDE

  1. Import the project in an IDE of your choice as a maven project
  2. Create a new maven run configuration and add the goal mvn package
  3. Run the created configuration
  4. The output is also in the target/ folder

If you are using the Eclipse IDE, you do not need to build the project explicitly. You can import this project into a workspace and just add the Maven coordinates into the pom.xml file of any other Maven project in the same workspace. This works as Eclipse can resolve Maven dependencies directly from the workspace, without requiring the project to be installed into the local Maven repository.

Usage

This project requires Java 26 and preview features enabled.

Before calling any method you need to load the native library. For Windows and Linux you can call Box2D.loadNativeLibrary().

To set the extract directory of the native library, change NativeLibraryLoaderConfig.SHARED_LIBRARY_EXTRACT_PATH.

Due to the introduction of restricted methods, it is recommended (and in later versions required) to run the application with the VM argument --enable-native-access=ALL-UNNAMED.

Memory Management

When creating an object, native memory will be allocated.

Structs are always allocated with an arena. By default an automatic arena is used, but you can use any arena you want (even a global arena, but this may not be desirable) by using the appropriate constructor.

When a struct is allocated through a native function call, the first parameter is always a SegmentAllocator (a parent type of an arena). This parameter is not part of the original C function signature and is added automatically in Java.

Example

This is a port of the Hello Box2D introduction.

public class HelloBox2D {

	public static void main(String[] args) {

		HelloBox2D helloBox2D = new HelloBox2D();
		helloBox2D.run();
		helloBox2D.dispose();

	}

	private WorldId worldId;
	
	private BodyId groundBodyId;
	private BodyId dynamicBodyId;
	
	public HelloBox2D() {
		Box2D.loadNativeLibrary();
		
		try (Arena arena = Arena.ofConfined()) {
			WorldDef worldDef = WorldDef.defaultWorldDef(arena)
					.gravity(g -> g.x(0).y(-10));
			worldId = World.createWorld(Arena.ofAuto(), worldDef);
			
			BodyDef groundBodyDef = BodyDef.defaultBodyDef(arena)
					.position(p -> p.x(0).y(-10));
			groundBodyId = Body.createBody(Arena.ofAuto(), worldId, groundBodyDef);
			
			Polygon groundBox = Polygon.makeBox(arena, 50, 10);
			ShapeDef groundShapeDef = ShapeDef.defaultShapeDef(arena);	
			Shape.createPolygonShape(arena, groundBodyId, groundShapeDef, groundBox);
			
			BodyDef dynamicBodyDef = BodyDef.defaultBodyDef(arena)
					.type(BodyType.DYNAMIC_BODY)
					.position(p -> p.x(0).y(4));
			dynamicBodyId = Body.createBody(Arena.ofAuto(), worldId, dynamicBodyDef);
			
			Polygon dynamicBox = Polygon.makeBox(arena, 1, 1);
			ShapeDef dynamicShapeDef = ShapeDef.defaultShapeDef(arena)
					.density(1)
					.material(m -> m.friction(0.4f));
			Shape.createPolygonShape(arena, dynamicBodyId, dynamicShapeDef, dynamicBox);
		}
	}
	
	public void run() {
		float timeStep = 1 / 60f;
		int subStepCount = 4;

		for (int i = 0; i < 90; i++) {
			World.step(worldId, timeStep, subStepCount);
			
			try (Arena arena = Arena.ofConfined()) {
				Vec2 position = Body.getPosition(arena, dynamicBodyId);
				Rot rotation = Body.getRotation(arena, dynamicBodyId);
				
				double angle = Box2DMath.atan2(rotation.s(), rotation.c());
				System.out.println(position.x() + ", " + position.y() + ", " + angle);
			}
		}
	}
	
	public void dispose() {
		World.destroyWorld(worldId);
	}
	
}