Taken from https://bukkit.org/threads/tutorial-metadata-what-it-is-and-how-to-use-it.276338/

MetadataTutorial.java

package com.valygard.aohruthless.metadatatutorial;
 
import org.bukkit.Bukkit;
import org.bukkit.entity.EntityType;
import org.bukkit.entity.Player;
import org.bukkit.entity.Zombie;
import org.bukkit.metadata.FixedMetadataValue;
 
/**
* To put some context into it, I use metadata in relation to zombies in my King
* of the Hill plugin. This has saved me lots of time and code from using NMS /
* Reflection where it was unnecessary.
*
* @author Anand
*
*/
public class MetadataTutorial {
 
    /**
    * This static method will spawn a zombie on the player and add metadata.
    * Note: Adding metadata does not make the zombie stop attacking the player,
    * you will need to listen for that in an EntityTargetEvent.
    *
    * @param plugin
    *            this is your main class. Replace "MainClass" with the class
    *            that extends JavaPlugin.
    * @param p
    *            this is the player we want to make the owner of the zombie.
    */
    public static void spawnZombie(MainClass plugin, Player p) {
        Zombie z = (Zombie) p.getWorld().spawnEntity(p.getLocation(),
                EntityType.ZOMBIE);
 
        /*
        * Setting the metadata.
        *
        * LivingEntity#setMetadata(String s, MetadataValue mv); The string is
        * the metadata tag that we will use to get the zombie anywhere else in
        * the plugin.
        *
        * There are two types of metadata: LazyMetadataValues, and
        * FixedMetadataValues which extend lazy ones. We will be working with
        * FixedMetadataValues.
        *
        * A FixedMetadataValue is, well, fixed. It is unchanging and therefore
        * very reliable.
        *
        * You will need to call a new FixedMetadataValue, which requires a
        * plugin argument and an arbitrary string. This string does not matter
        * and you can name it whatever you want. I recommend that your plugin
        * directly points your main class but Plugin, and JavaPlugin variable
        * types also work.
        */
        z.setMetadata(p.getName(), new FixedMetadataValue(plugin, "yes!"));
    }
 
    /**
    * Metadata is useless unless we can check if the zombie has metadata to
    * begin with.
    *
    * @param z
    *            the zombie to check
    * @return the player, or null if none was found online.
    */
    public static Player getPlayerWithZombie(Zombie z) {
        for (Player p : Bukkit.getOnlinePlayers()) {
            /*
            * Remember how we set metadata. We had a string argument, and a
            * metadata value. If the zombie has a metadata cache with this
            * string value, we have a match.
            *
            * So right away with a few lines of code we are able to retrieve a
            * player who the zombie belongs to.
            */
            if (z.hasMetadata(p.getName())) {
                return p;
            }
        }
        return null;
    }
 
    /**
    * Maybe we want to remove the player as a zombie owner. We can do that
    * because the Metadata API allows us to remove metadata with just one line
    * of code.
    *
    * @param z
    *            the zombie who has the metadata to be removed.
    * @param owner
    *            the player whose name is the metadata.
    * @param plugin
    *            we need a plugin argument to remove metadata.
    */
    public static void setOwnerless(Zombie z, Player owner, MainClass plugin) {
        if (z.hasMetadata(owner.getName())) {
            /*
            * Remember that string argument? We remove a zombie's metadata
            * based on this string argument, so we don't accidentally avoid all
            * metadata tags from other plugins or even elsewhere in this
            * plugin. (Yes! The plot thickens: You can have multiple metadata
            * tags)
            *
            * We also need the plugin argument from which we set metadata.
            */
            z.removeMetadata(owner.getName(), plugin);
        }
    }
 
}