Ender Bar ProtocolLib example (irrelevant without code from this project, but it's still pretty cool)

EnderBar.java

package net.cogzmc.core.enderBar;

import com.comphenix.packetwrapper.WrapperPlayServerEntityDestroy;
import com.comphenix.packetwrapper.WrapperPlayServerEntityMetadata;
import com.comphenix.packetwrapper.WrapperPlayServerEntityTeleport;
import com.comphenix.packetwrapper.WrapperPlayServerSpawnEntityLiving;
import com.comphenix.protocol.wrappers.WrappedDataWatcher;
import lombok.AccessLevel;
import lombok.Data;
import lombok.Setter;
import net.cogzmc.core.player.CPlayer;
import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.entity.EntityType;

@Data
final class EnderBar {
    private final CPlayer player;
    private final Integer id;

    private String text;
    private Float health = 1f;
    @Setter(AccessLevel.NONE) private boolean spawned;
    @Setter(AccessLevel.NONE) private Location currentLocation;
    private final WrappedDataWatcher dataWatcher = new WrappedDataWatcher();

    void spawn() {
        if (spawned) throw new IllegalStateException("You cannot spawn this entity twice!");
        WrapperPlayServerSpawnEntityLiving packet = new WrapperPlayServerSpawnEntityLiving();
        packet.setHeadYaw(0f);
        packet.setHeadPitch(0f);
        packet.setEntityID(id);
        updateDataWatcher();
        packet.setMetadata(dataWatcher);
        packet.setType(EntityType.ENDER_DRAGON);
        currentLocation = getBestLocation();
        packet.setX(currentLocation.getX());
        packet.setY(currentLocation.getY());
        packet.setZ(currentLocation.getZ());
        packet.sendPacket(player.getBukkitPlayer());
        spawned = true;
    }

    void updateLocation() {
        if (!spawned) return;
        WrapperPlayServerEntityTeleport entityTeleport = new WrapperPlayServerEntityTeleport();
        entityTeleport.setEntityID(id);
        currentLocation = getBestLocation();
        entityTeleport.setX(currentLocation.getX());
        entityTeleport.setY(currentLocation.getY());
        entityTeleport.setZ(currentLocation.getZ());
        entityTeleport.setPitch(0f);
        entityTeleport.setYaw(0f);
        entityTeleport.sendPacket(player.getBukkitPlayer());
    }

    void remove() {
        if (!spawned) throw new IllegalStateException("You cannot remove an entity that has not spawned!");
        WrapperPlayServerEntityDestroy destroyPacket = new WrapperPlayServerEntityDestroy();
        destroyPacket.setEntities(new int[]{id});
        destroyPacket.sendPacket(player.getBukkitPlayer());
        spawned = false;
    }

    void update() {
        if (!spawned) return;
        updateDataWatcher();
        WrapperPlayServerEntityMetadata metaPacket = new WrapperPlayServerEntityMetadata();
        metaPacket.setEntityId(id);
        metaPacket.setEntityMetadata(dataWatcher.getWatchableObjects());
        metaPacket.sendPacket(player.getBukkitPlayer());
    }

    void newWorld() {
        if (!spawned) return;
        spawned = false;
        spawn();
    }

    void setText(String text) {
        this.text = ChatColor.translateAlternateColorCodes('&',text.substring(0, Math.min(64, text.length())));
        update();
    }

    void setHealth(Float health) {
        this.health = health;
        update();
    }

    Location getBestLocation() {
        return player.getBukkitPlayer().getLocation().clone().add(0, -300, 0);
    }

    private void updateDataWatcher() {
        dataWatcher.setObject(6, health*200f); //Set the health (health is a %, 200 is the actual health)
        if (text != null) dataWatcher.setObject(10, text); //Set the display name text now.
    }
}

EnderBarListener.java

package net.cogzmc.core.enderBar;

import lombok.Data;
import net.cogzmc.core.Core;
import net.cogzmc.core.player.CPlayer;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerChangedWorldEvent;
import org.bukkit.event.player.PlayerMoveEvent;

@Data
final class EnderBarListener implements Listener {
    private final EnderBarManager manager;

    @EventHandler(priority = EventPriority.MONITOR, ignoreCancelled = true)
    public void onPlayerMove(PlayerMoveEvent event) {
        CPlayer onlinePlayer = Core.getOnlinePlayer(event.getPlayer());
        EnderBar enderBarFor = manager.getEnderBarFor(onlinePlayer);
        if (enderBarFor != null && enderBarFor.isSpawned() && event.getTo().distanceSquared(enderBarFor.getCurrentLocation()) > 25) enderBarFor.updateLocation();
    }

    @EventHandler(priority = EventPriority.MONITOR, ignoreCancelled = true)
    public void onPlayerChangeWorld(PlayerChangedWorldEvent event) {
        CPlayer player = Core.getOnlinePlayer(event.getPlayer());
        EnderBar enderBarFor = manager.getEnderBarFor(player);
        enderBarFor.newWorld();
    }
}

EnderBarManager.java

package net.cogzmc.core.enderBar;

import net.cogzmc.core.Core;
import net.cogzmc.core.player.CPlayer;
import net.cogzmc.core.player.CPlayerConnectionListener;
import net.cogzmc.core.player.CPlayerJoinException;

import java.net.InetAddress;
import java.util.HashMap;
import java.util.Map;

/**
 *
 */
@SuppressWarnings("UnusedDeclaration")
public final class EnderBarManager implements CPlayerConnectionListener {
    private final Map<CPlayer, EnderBar> enderBars = new HashMap<>();
    private int lastId = 3000;

    /**
     * Creates a manager for the EnderBar system.
     */
    public EnderBarManager() {
        Core.getPlayerManager().registerCPlayerConnectionListener(this); //Registers us as a listener so we can remove our CPlayer references when they disconnect.
        Core.getInstance().registerListener(new EnderBarListener(this)); //Also need to move the ender dragon around.
    }

    /**
     * Creates an Ender Dragon for the player if it does not exist, then updates the text for the dragon.
     * @param player The {@link net.cogzmc.core.player.CPlayer} to set ender bar text for.
     * @param text The text to set the bar to.
     */
    public void setTextFor(CPlayer player, String text) {
        createIfDoesNotExist(player);
        enderBars.get(player).setText(text);
    }

    /**
     * Creates an Ender Dragon for the player if it does not exist, then updates the health percentage for that dragon.
     * @param player The {@link net.cogzmc.core.player.CPlayer} to set ender bar health for.
     * @param health The health to set the ender dragon's bar to.
     */
    public void setHealthPercentageFor(CPlayer player, Float health) {
        createIfDoesNotExist(player);
        enderBars.get(player).setHealth(health);
    }

    /**
     * Hides the bar for the specified player if it is currently visible.
     * @param player The {@link net.cogzmc.core.player.CPlayer} to hide the bar for.
     */
    public void hideBarFor(CPlayer player) {
        if (!enderBars.containsKey(player)) return;
        EnderBar enderBar = enderBars.get(player);
        if (enderBar.isSpawned()) enderBar.remove();
    }

    /**
     * Creates an ender dragon if the player does not already have one, and then shows it if it is hidden.
     * @param player The {@link net.cogzmc.core.player.CPlayer} to show the ender bar for.
     */
    public void showBarFor(CPlayer player) {
        EnderBar enderBar = enderBars.get(player);
        if (!createIfDoesNotExist(player) && !enderBar.isSpawned()) enderBar.spawn();
    }

    private boolean createIfDoesNotExist(CPlayer player) {
        if (enderBars.containsKey(player)) return false;
        EnderBar enderBar = new EnderBar(player, getNextId());
        enderBar.spawn();
        enderBars.put(player, enderBar);
        return true;
    }

    Integer getNextId() {
        lastId++;
        return lastId;
    }

    @Override
    public void onPlayerLogin(CPlayer player, InetAddress address) throws CPlayerJoinException {
    }

    @Override
    public void onPlayerDisconnect(CPlayer player) {
        this.enderBars.remove(player);
    }

    EnderBar getEnderBarFor(CPlayer player) {
        return this.enderBars.get(player);
    }
}