CustomTagType

package de.craftstuebchen.minez3.weaponry.persistence;

import org.bukkit.NamespacedKey;
import org.bukkit.persistence.PersistentDataAdapterContext;
import org.bukkit.persistence.PersistentDataContainer;
import org.bukkit.persistence.PersistentDataType;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class WeaponDataTagType implements PersistentDataType<PersistentDataContainer, WeaponData> {

    // thats the one from the JavaDoc example 
    private static final UUIDTagType UUID_TAG_TYPE = new UUIDTagType();

    private JavaPlugin javaPlugin;

    public WeaponDataTagType(JavaPlugin javaPlugin) {
        this.javaPlugin = javaPlugin;
    }

    @Override
    public Class<PersistentDataContainer> getPrimitiveType() {
        return PersistentDataContainer.class;
    }

    @Override
    public Class<WeaponData> getComplexType() {
        return WeaponData.class;
    }

    @Override
    public PersistentDataContainer toPrimitive(WeaponData weaponData, PersistentDataAdapterContext persistentDataAdapterContext) {
        PersistentDataContainer persistentDataContainer = persistentDataAdapterContext.newPersistentDataContainer();
        persistentDataContainer.set(key("weapon"), PersistentDataType.STRING, weaponData.getId());
        persistentDataContainer.set(key("customtrail"), PersistentDataType.STRING, weaponData.getCustomTrails() == null ? "undefined" : weaponData.getCustomTrails());
        persistentDataContainer.set(key("rounds"), PersistentDataType.INTEGER, weaponData.getRounds());
        persistentDataContainer.set(key("uuid"), UUID_TAG_TYPE, weaponData.getUuid());
        persistentDataContainer.set(key("boltpulled"), PersistentDataType.BYTE, weaponData.isBoltpulled() ? (byte) 1 : 0);
        persistentDataContainer.set(key("firemode"), PersistentDataType.INTEGER, weaponData.getFireMode());
        writeAttachments(persistentDataContainer, weaponData.getAttachments());
        return persistentDataContainer;
    }

    @Override
    public WeaponData fromPrimitive(PersistentDataContainer persistentDataContainer, PersistentDataAdapterContext persistentDataAdapterContext) {
        WeaponData weaponData = new WeaponData();
        weaponData.setId(persistentDataContainer.get(key("weapon"), PersistentDataType.STRING));
        weaponData.setCustomTrails(persistentDataContainer.get(key("customtrail"), PersistentDataType.STRING));
        weaponData.setRounds(getOrDefault(persistentDataContainer.get(key("rounds"), PersistentDataType.INTEGER), 0));
        weaponData.setUuid(persistentDataContainer.get(key("uuid"), UUID_TAG_TYPE));
        weaponData.setBoltpulled(fromByte(persistentDataContainer.get(key("boltpulled"), PersistentDataType.BYTE), false));
        weaponData.setFireMode(getOrDefault(persistentDataContainer.get(key("firemode"), PersistentDataType.INTEGER), 0));
        weaponData.setAttachments(readAttachements(persistentDataContainer));
        return weaponData;
    }


    private void writeAttachments(PersistentDataContainer customItemTagContainer, Map<String, String> attachments) {
        List<String> concatedAttachements = new ArrayList<>();
        for (Map.Entry<String, String> attachment : attachments.entrySet()) {
            concatedAttachements.add(attachment.getKey() + "," + attachment.getValue());
        }
        customItemTagContainer.set(key("weaponattachments"), PersistentDataType.STRING, String.join(";", concatedAttachements));
    }

    private Map<String, String> readAttachements(PersistentDataContainer persistentDataContainer) {
        Map<String, String> list = new HashMap<>();
        if (persistentDataContainer.has(key("weaponattachments"), PersistentDataType.STRING)) {
            String attachments = persistentDataContainer.get(key("weaponattachments"), PersistentDataType.STRING);
            String[] attachmentList = attachments.split(";");
            for (String singleAttachement : attachmentList) {
                String[] slit = singleAttachement.split(",");
                if (slit.length == 2) {
                    list.put(slit[0], slit[1]);
                }
            }
        }
        return list;
    }


    private NamespacedKey key(String key) {
        return new NamespacedKey(javaPlugin, key);
    }

    private int getOrDefault(Integer integer, int defaultValue) {
        return integer != null ? integer : defaultValue;
    }

    private boolean fromByte(Byte byteValue, boolean defaultValue) {
        return byteValue != null ? byteValue == 1 : defaultValue;
    }
}

How_to_set_or_get

public class DataPersistence {

    private final NamespacedKey weaponKey;
    private final WeaponDataTagType weaponDataTagType;

    public DataPersistence(JavaPlugin javaPlugin){
        this.weaponKey = new NamespacedKey(plugin, "weapon");
        this.weaponDataTagType = new WeaponDataTagType(plugin);
    }

    public ItemStack set(ItemStack itemStack, WeaponData weaponData) {
        ItemMeta itemMeta = itemStack.getItemMeta();
        PersistentDataContainer customItemTagContainer = itemMeta.getPersistentDataContainer();
        customItemTagContainer.set(weaponKey, weaponDataTagType, weaponData);
        itemStack.setItemMeta(itemMeta);
        return itemStack;
    }
    
    public WeaponData get(ItemStack itemStack) {
        ItemMeta itemMeta = itemStack.getItemMeta();
        PersistentDataContainer customItemTagContainer = itemMeta.getPersistentDataContainer();
        return customItemTagContainer.get(weaponKey, weaponDataTagType);
    }
    
}