@Override
@SuppressWarnings("deprecation") // We do a version check ;)
public SkeletonType getSkeletonType() {
    if (Versioning.NMS_VERSION.compareTo(NmsVersion.v1_11_R1) >= 0) {
        if (getBukkitEntity() instanceof WitherSkeleton) {
            return SkeletonType.WITHER;
        } else if (getBukkitEntity() instanceof Stray) {
            return SkeletonType.STRAY;
        } else {
            return SkeletonType.NORMAL;
        }
    } else {
        switch (getBukkitEntity().getSkeletonType()) {
            case WITHER:
                return SkeletonType.WITHER;
            case STRAY:
                return SkeletonType.STRAY;
            case NORMAL:
                return SkeletonType.NORMAL;
            default:
                throw new AssertionError("Unknown skeleton type!");
        }
    }
}
 

/**
 * Comes AFTER the SpawnEvents{@link #onCreatureSpawn(CreatureSpawnEvent)} - so cancelled will have effect
 * @param e
 */
@EventHandler(priority = EventPriority.HIGH, ignoreCancelled = true)
public void onCreatureSpawn(CreatureSpawnEvent e) {
    if (!spawnEnabled || e.getSpawnReason() != CreatureSpawnEvent.SpawnReason.NATURAL) {
        return;
    }
    if (!plugin.getWorldManager().isSkyNether(e.getLocation().getWorld())) {
        return;
    }
    if (e.getLocation().getBlockY() > plugin.getWorldManager().getNetherWorld().getMaxHeight()) {
        // Block spawning above nether...
        e.setCancelled(true);
        return;
    }
    if (e.getEntity() instanceof PigZombie) {
        Block block = e.getLocation().getBlock().getRelative(BlockFace.DOWN);
        if (isNetherFortressWalkway(block)) {
            e.setCancelled(true);
            double p = RND.nextDouble();
            if (p <= chanceWither && block.getRelative(BlockFace.UP, 3).getType() == Material.AIR) {
                WitherSkeleton mob = (WitherSkeleton) e.getLocation().getWorld().spawnEntity(
                    e.getLocation(), EntityType.WITHER_SKELETON);
                mob.getEquipment().setItemInMainHand(new ItemStack(Material.STONE_SWORD, 1));
            } else if (p <= chanceWither+chanceBlaze) {
                e.getLocation().getWorld().spawnEntity(e.getLocation(), EntityType.BLAZE);
            } else if (p <= chanceWither+chanceBlaze+chanceSkeleton) {
                e.getLocation().getWorld().spawnEntity(e.getLocation(), EntityType.SKELETON);
            } else {
                e.setCancelled(false); // Spawn PigZombie
            }
        }
    }
}
 

@Override
public EntityKillHandler getItemHandler() {
    return (e, entity, killer, item) -> {
        Random random = ThreadLocalRandom.current();

        if (e.getEntity() instanceof Zombie) {
            if (random.nextInt(100) < chanceZombie.getValue()) {
                e.getDrops().add(new ItemStack(Material.ZOMBIE_HEAD));
            }
        }
        else if (e.getEntity() instanceof WitherSkeleton) {
            if (random.nextInt(100) < chanceWitherSkeleton.getValue()) {
                e.getDrops().add(new ItemStack(Material.WITHER_SKELETON_SKULL));
            }
        }
        else if (e.getEntity() instanceof Skeleton) {
            if (random.nextInt(100) < chanceSkeleton.getValue()) {
                e.getDrops().add(new ItemStack(Material.SKELETON_SKULL));
            }
        }
        else if (e.getEntity() instanceof Creeper) {
            if (random.nextInt(100) < chanceCreeper.getValue()) {
                e.getDrops().add(new ItemStack(Material.CREEPER_HEAD));
            }
        }
        else if (e.getEntity() instanceof Player && random.nextInt(100) < chancePlayer.getValue()) {
            ItemStack skull = new ItemStack(Material.PLAYER_HEAD);
            ItemMeta meta = skull.getItemMeta();
            ((SkullMeta) meta).setOwningPlayer((Player) e.getEntity());
            skull.setItemMeta(meta);

            e.getDrops().add(skull);
        }
    };
}