下面一些內容定義來自《Box2D中文手冊》

關於

關節用於把物體約束到世界，或約束到其它物體上。在遊戲中，典型例子有木偶，蹺蹺板和滑輪。用不同的方式將關節結合起來使用，可以創造出有趣的運動。
有些關節提供了限制(limit)，使可以控制運動的範圍。有些關節還提供了馬達(motor)，它可以以指定的速度驅動關節一直運動，直到你指定了更大的力或扭矩來抵消這種運動。
關節馬達有許多不同的用途。可以使用關節來控制位置，只要提供一個與目標之距離成正比例的關節速度即可。還可以模擬關節摩擦：將關節速度置零，並且提供一個小的、但有效的最大力或扭矩；那麼馬達就會努力保持關節不動，直到負載變得過大為止。

關節定義

每種關節型別都有各自的定義(definition)，它們都派生自 JointDef。所有的關節都連線兩個不同的物體，其中一個物體有可能是靜態的。關節也可以連線兩個 static 或者 kinematic 型別的物體，但這沒有任何實際用途，只會浪費處理器時間。
可以為任何一種關節型別指定使用者資料。還可以提供一個標記，用於防止用關節相連的物體之間發生碰撞。實際上, 這是預設行為。也可以通過設定 collideConnected，來允許相連的物體之間發生碰撞。
很多關節定義需要提供一些幾何資料。一個關節常常需要一個錨點(anchor point)來定義，這是固定於相接物體中的點。 Box2D 要求這些點在區域性座標系中指定，這樣，即便當前物體的變化違反了關節約束(joint constraint)，關節還是可以被指定–這通常會發生在遊戲儲存或載入進度時。
另外，有些關節定義需要知道物體之間預設的相對角度。這樣才能正確地約束旋轉。
初始化幾何資料可能有些乏味。所以很多關節提供了初始化函式，使用當前的物體的形狀，來消除大部分工作。然而，這些初始化函式通常只應用於原型，在產品程式碼中應該直接地定義幾何資料。這能使關節行為更具健壯性。

滑鼠關節

滑鼠關節用於通過滑鼠來操控物體，它試圖將物體託向當前滑鼠游標的位置，而在旋轉方面就沒有限制。
滑鼠關節的定義需要一個目標點(target point)，最大力(maximum force)，頻率(frequency)，阻尼率(damping ratio)。目標點最開始與物體的錨點重合，最大力用於防止再多個動態物體相互作用時的激烈反應。頻率和阻尼用於創造一種彈性效果。
1.com.badlogic.gdx.physics.box2d.JointDef

    /** The first attached body. **/
    public Body bodyA = null
 
;

    /** The second attached body **/
    public Body bodyB = null;

    /** Set this flag to true if the attached bodies should collide. **/
    public boolean collideConnected = false;

2.com.badlogic.gdx.physics.box2d.joints.MouseJointDef

    /** The initial world target point. This is assumed to coincide with the body anchor initially.   錨點  */
    public final Vector2 target = new Vector2();

    /** The maximum constraint force that can be exerted to move the candidate body. Usually you will express as some multiple of
     * the weight (multiplier * mass * gravity). */
    public float maxForce = 0;

    /** The response speed. */
    public float frequencyHz = 5.0f;

    /** The damping ratio. 0 = no damping, 1 = critical damping. */
    public float dampingRatio = 0.7f;

3.com.badlogic.gdx.physics.box2d.joints.MouseJoint extends Joint
/** A mouse joint is used to make a point on a body track a specified world point. This a soft constraint with a maximum force.
** This allows the constraint to stretch and without applying huge forces. NOTE: this joint is not documented in the manual
* because it was developed to be used in the testbed. If you want to learn how to use the mouse joint, look at the testbed. /

![這裡寫圖片描述](http://img.blog.csdn.net/20170105225558705?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvenFpYW5nXzU1/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast)
/**
 * 滑鼠關節測試
 */
public class MouseJointTest extends ApplicationAdapter {

    World world;
    Box2DDebugRenderer box2DDebugRenderer;
    Body mouseJointBody, groundBody;
    Fixture fixture;

    OrthographicCamera camera;
    Vector3 point = new Vector3();

    float scene_width = 12.8f;
    float scene_height = 7.2f;
    Vector2 target = new Vector2();

    @Override
    public void create() {
        world = new World(new Vector2(0.0f, -9.8f), true);
        box2DDebugRenderer = new Box2DDebugRenderer();

        camera = new OrthographicCamera(scene_width, scene_height);
        camera.position.set(scene_width / 2, scene_height / 2, 0);
        camera.update();

        groundBody = createGroundWall();

        Gdx.input.setInputProcessor(new HandA());

        BodyDef bodyDef = new BodyDef();
        bodyDef.position.set(3.0f, 4.8f);
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        mouseJointBody = world.createBody(bodyDef);

        PolygonShape polygonShape = new PolygonShape();
        polygonShape.setAsBox(0.5f, 0.5f);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = polygonShape;
        fixtureDef.density = 1.0f;
        fixtureDef.restitution = 0.6f;

        fixture = mouseJointBody.createFixture(fixtureDef);
    }

    @Override
    public void render() {
        world.step(1 / 60f, 6, 2);

        Gdx.gl.glClearColor(0.39f, 0.58f, 0.92f, 1.0f);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        box2DDebugRenderer.render(world, camera.combined);
    }

    @Override
    public void dispose() {
        world.dispose();
        box2DDebugRenderer.dispose();
    }

    public Body createGroundWall() {
        BodyDef bodyDef = new BodyDef();
        bodyDef.position.set(scene_width * 0.5f, 0.2f);
        Body body1 = world.createBody(bodyDef);

        PolygonShape polygonShape = new PolygonShape();
        polygonShape.setAsBox(scene_width * 0.5f, 0.2f);
        body1.createFixture(polygonShape, 0.0f);

        bodyDef.position.set(0.4f, scene_height * 0.5f);
        Body body2 = world.createBody(bodyDef);

        polygonShape.setAsBox(0.2f, scene_height * 0.5f);
        body2.createFixture(polygonShape, 0);

        bodyDef.position.set(12.4f, scene_height * 0.5f);
        Body body3 = world.createBody(bodyDef);

        polygonShape.setAsBox(0.2f, scene_height * 0.5f);
        body3.createFixture(polygonShape, 0);

        bodyDef.position.set(scene_width * 0.5f, 7.0f);
        Body body4 = world.createBody(bodyDef);

        polygonShape.setAsBox(scene_width * 0.5f, 0.2f);
        body4.createFixture(polygonShape, 0);
        polygonShape.dispose();

        return body1;
    }

    private void logInfo(String message) {
        Gdx.app.log("MouseJointTest", message);
    }

    class HandA extends InputAdapter {

        MouseJoint mouseJoint;

        @Override
        public boolean touchDragged(int screenX, int screenY, int pointer) {
            logInfo("touchDragged");
            if (mouseJoint != null) {
                camera.unproject(point.set(screenX, screenY, 0));
                mouseJoint.setTarget(target.set(point.x, point.y));
            }

            return false;
        }

        @Override
        public boolean touchDown(int screenX, int screenY, int pointer, int button) {
            logInfo("touchDown");
            camera.unproject(point.set(screenX, screenY, 0));
            //  當滑鼠點選到物體時才會建立滑鼠關節,並繫結
            if (fixture.testPoint(point.x, point.y)) {
                if (mouseJointBody == null) return false;

                MouseJointDef mouseJointDef = new MouseJointDef();
                mouseJointDef.bodyA = groundBody;
                mouseJointDef.bodyB = mouseJointBody;
                mouseJointDef.collideConnected = true;
                mouseJointDef.target.set(point.x, point.y);
                mouseJointDef.maxForce = 1000.0f * mouseJointBody.getMass();

                mouseJoint = (MouseJoint) world.createJoint(mouseJointDef);
                mouseJointBody.setAwake(true);

            }
            return false;
        }

        @Override
        public boolean touchUp(int screenX, int screenY, int pointer, int button) {
            logInfo("touchUp");
            // 滑鼠關節，不再使用時要銷燬
            if (mouseJoint != null) {
                world.destroyJoint(mouseJoint);
                mouseJoint = null;
            }
            return false;
        }
    }
}