I have a mob. I would like it to have a boss bar when it spawns.
I also have a structure, I would like that when I enter it, this boss will spawn in it at a certain point.
And if possible, how to make the boss not go beyond the structure?

public class Anubis extends Monster implements IAnimatable {
    private AnimationFactory factory = new AnimationFactory(this);

    public Anubis(EntityType<? extends Monster> entityType, Level level) {
        super(entityType, level);
    }

    public static AttributeSupplier setAttributes() {
        return Monster.createMobAttributes()
                .add(Attributes.MAX_HEALTH, 400D)
                .add(Attributes.ARMOR, 10D)
                .add(Attributes.ATTACK_DAMAGE, 20D)
                .add(Attributes.ATTACK_SPEED, 5f)
                .add(Attributes.MOVEMENT_SPEED, 0.2f).build();
    }

    @Override
    protected void registerGoals() {
        this.goalSelector.addGoal(1, new MeleeAttackGoal(this, 2.2D, false));

        this.targetSelector.addGoal(2, new NearestAttackableTargetGoal<>(this, Player.class, true));
        this.targetSelector.addGoal(3, new NearestAttackableTargetGoal<>(this, IronGolem.class, true));
    }

    private <E extends IAnimatable> PlayState predicate(AnimationEvent<E> event) {
        if (event.isMoving()) {
            event.getController().setAnimation(new AnimationBuilder().addAnimation("animation.anubis.walk", true));
            return PlayState.CONTINUE;
        }

        event.getController().setAnimation(new AnimationBuilder().addAnimation("animation.anubis.idle", true));
        return PlayState.CONTINUE;
    }

    private PlayState attackPredicate(AnimationEvent event) {
        if(this.swinging && event.getController().getAnimationState().equals(AnimationState.Stopped)) {
            event.getController().markNeedsReload();
            event.getController().setAnimation(new AnimationBuilder().addAnimation("animation.anubis.attack", false));
            this.swinging = false;
        }

        return PlayState.CONTINUE;
    }

    @Override
    public void registerControllers(AnimationData data) {
        data.addAnimationController(new AnimationController(this, "controller", 0, this::predicate));
        data.addAnimationController(new AnimationController(this, "attackController", 0, this::attackPredicate));
    }

    @Override
    public AnimationFactory getFactory() {
        return factory;
    }
}

 

First, this isn't a teaching forum. Nor is it a way for you use other people as a search engine.

You need to do your own research (look at how vanilla does it or other mods) and try to implement it yourself.

When you get stuck feel free to ask here.

By that point you will have a better understanding and be able to ask more focused questions. And also better understand the answers.

 

Anyway, obvious answers:

Boss bar -> Look at the WitherBoss.bossEvent and how it gets used. Alternatives in vanilla are raids and the dragon.

Structure -> An obvious vanilla example is OceanMonumentPiece.spawnElder() - the mob at the centre of ocean monuments.

 

As for the staying in the structure, that is something you will have to code into the entity's AI.

I am not aware of a mob that does that. There are plenty of AI goals that move towards different targets you might adapt?

You would also need to consider how that works if some user decides to spawn your entity far away from the structure, e.g. using the /spawn command