JComposition

JComposition is a lightweight Java API based on annotations for creating compositions at compile-time. Composition over inheritance.

Introduction

Say you have 2 interfaces:

public interface IDrawable {
    void draw();
}
public interface IUpdatable {
    void update();
}

And 2 classes that implements them:

public class Drawable implements IDrawable {
    @Override
    public void draw() {
        System.out.println("I'm drawing");
    }
}
public class Updatable implements IUpdatable {
    @Override
    public void update() {
        System.out.println("I'm updating");
    }
}

And perhaps you want a GameObject class that have Drawable and Updatable behaviour:

public class GameObject extends Drawable, Updatable {
}
GameObject gameObject = new GameObject();
gameObject.update();
gameObject.draw();

But Java does not allow you to have more than one superclass. So the code above will not compile.

JComposition is tool that you can use to mix such logic in one class without duplicated code. Also it support dependency injection and generics.

Using JComposition

JComposition uses interfaces as base for code generation.

Basic usage

Define your interfaces for each module you want to extend:

@Bind(Drawable.class)
public interface IDrawable {
    void draw();
}
@Bind(Updatable.class)
public interface IUpdatable {
    void update();
}

@Bind annotation binds your interface to class that implements logic. Implementation of IDrawable and IUpdatable you can find in Introduction section

Then create a interface for GameObject class:

@Composition(name = "GameObjectBase")
@Bind(GameObject.class)
public interface IGameObject extends IUpdatable, IDrawable {
}

@Composition annotation marks this interface for annotation processor to generate composite class with name = "GameObjectBase".

Then just extend GameObject from GameObjectBase:

public class GameObject extends GameObjectBase {
}
// Now this will work
GameObject gameObject = new GameObject();
gameObject.update();
gameObject.draw();

Overriding

By default gameObject.draw() will call getComposition().composition_Drawable.draw(), but you can override behaviour this way:

public class GameObject extends GameObjectBase {
    @Override
    public void draw() {
        super.draw(); // will call getComposition().composition_Drawable.draw()
        
        // Some custom action
        getComposition().composition_Updatable.update();
    }
}
gameObject.draw();
// Output
I'm drawing
I'm updating

Dependency injection

Use @UseInjection annotation to let processor mark composition's fields @Inject annotation.

@Bind(Movable.class)
@UseInjection
public interface IMovable {
    boolean moveTo(int x, int y);
}
public class Movable implements IMovable {
    @Override
    public boolean moveTo(int x, int y) {
        System.out.println("I'm moving to (" + x + ", " + y + ")");
        return false;
    }

    protected abstract void onMove();
}

// And finally @Module declaration
@Module
public final class MovableModule {
    private GameObjectWithInjection.Composition composition;

    public MovableModule(GameObjectWithInjection.Composition composition) {
        this.composition = composition;
    }

    @Provides
    public GameObjectWithInjectionBase.Composition.Composition_Movable provideMovable()  {
        return composition.new Composition_Movable();
    }
}

When Movable composition will ready for injection abstract method onInject(Composition) will be called:

public class GameObjectWithInjection extends GameObjectWithInjectionBase {
    private InjectionComponent injectionComponent;

    @Override
    protected void onMove() {
        System.out.println("OnMove()");
    }

    @Override
    protected void onInject(Composition composition) {
        injectionComponent = DaggerInjectionComponent
                .builder()
                .movableModule(new MovableModule(composition))
                .build();

        injectionComponent.inject(composition);
    }
}

Diamond Problem

JComposition allow you to inherit functionality from many instances, and you could get into a situation that two or more components of composition has some equal methods. Here and below I will call such situation as 'merge conflict'. To solve merge conflict you could use special option in annotation @Composition onConflict, which accept one of classes below or you own, that implements IMergeConflictPolicy:

MakeAbstractPolicy.class
MixVoidPolicy.class
UseFirstPolicy.class

Constraints

If you are not using dependency injection, binded class must have an empty argument constructor.

Examples

You can find more examples here

Download

We are using JitPack for publishing our libraries. Add jitpack.io to your repositories first to build.gradle:

allprojects {
	repositories {
		...
		maven { url 'https://jitpack.io' }
	}
}

And add the dependency:

dependencies {
    // Use compile for processor instead of apt if you haven't apt dependency.
    apt 'com.github.trollsoftware.jcomposition:processor:1.2.1'
    compile 'com.github.trollsoftware.jcomposition:api:1.2.1'
}

Ideas

Custom constructor support
Check how jcomposition works on java 8-9
Inherit java docs in generated files
Add more documentation and examples

License

This library is distributed under the Apache 2.0 license found in the LICENSE file.

trollsoftware/jcomposition