Custom Item Data

A Fabric library mod for Minecraft 1.20.6 that lets you store persistent, typed, schema-validated data on any ItemStack.

Two ways to use it:

• As a mod developer, call the ItemDataAPI from your own mod to read/write values on items.
• As a server admin, use the /itemdata commands in-game to define and edit item data without writing code.

Features

• Store typed data (int, float, string, boolean + your own types for framework) on any ItemStack.
• Schema-based validation — the framework rejects values that don't match the registered type.
• Automatic persistence (survives save/load, gives/clones, etc.).
• Stacks with different data don't stack together.
• In-game admin commands (/itemdata […]) persisted in the world save.
• Pluggable custom types via a simple CustomDataType<T> interface.

Installation (end users)

1. Install Fabric Loader for Minecraft 1.20.6: https://fabricmc.net/use/installer/
2. Download Fabric API for 1.20.6 and drop the .jar into your mods/ folder.
3. Drop customitemdata-1.0.0.jar into the same mods/ folder.
4. Launch the game (or server). You should see [ItemDataFramework] Initialisé avec succès. in the log.

That's it — the mod is now active. By itself it does nothing visible; it becomes useful when another mod calls its API, or when an OP uses the /itemdata commands.

Admin commands tutorial

All commands require OP level 2. They operate on the item currently held in the main hand.

/itemdata define <param_name> <type>
/itemdata define <param_name> <type> <visible> <nullable> [default]
/itemdata remove <param_name>
/itemdata set <param_name> <value>
/itemdata get <param_name>
/itemdata list
/itemrename [name]

Example session — give a stick a "points" counter:

/itemdata define nb_point int
/itemdata set nb_point 42
/itemdata get nb_point → 42
/itemdata list → nb_point (int)

Schemas defined this way are saved in the world save, so they persist across restarts.

Framework tutorial (for mod developers)

This section walks you through using ItemDataFramework from your own Fabric mod — from zero to a working example in four steps.

Step 1 — Add the dependency

Until the mod is published to a Maven repository, the simplest approach is to drop its .jar into your project and reference it locally.

Option A — local jar (fastest):

1. Build this mod: ./gradlew build — the jar lands in build/libs/customitemdata-1.0.0.jar.
2. Copy that jar into a libs/ folder in your own mod project.
3. In your build.gradle:

dependencies {
    modImplementation files("libs/customitemdata-1.0.0.jar")
    // or: modImplementation fileTree(dir: "libs", include: ["*.jar"])
}

1. In your fabric.mod.json, declare it as a dependency so it loads first:

"depends": {
    "fabricloader": ">=0.18.4",
    "minecraft": "~1.20.6",
    "fabric-api": "*",
    "customitemdata": "*"
}

Step 2 — Register a schema

A schema tells the framework which parameters an item type is allowed to carry, and of what type. Register your schemas once, in your mod's onInitialize(), after Minecraft's registries are ready.

package com.example.mymod;

import fr.hdi.customitemdata.schema.SchemaEntry;
import fr.hdi.customitemdata.schema.SchemaManager;
import net.fabricmc.api.ModInitializer;

public class MyMod implements ModInitializer {

    @Override
    public void onInitialize() {
        SchemaManager schemas = SchemaManager.getInstance();

        // A simple integer counter on sticks, visible in the tooltip,
        // defaulting to "0" and required (not nullable).
        schemas.registerSchema("minecraft:stick",
            new SchemaEntry.Builder("nb_point", "int")
                .visible(true)
                .nullable(false)
                .defaultValue("0")
                .build());

        // A string "owner" field on diamond swords.
        schemas.registerSchema("minecraft:diamond_sword",
            new SchemaEntry.Builder("owner", "string")
                .visible(true)
                .build());
    }
}

Available primitive types: int, float, string, boolean.

Schemas registered via the API are re-applied every time the server starts — they are not persisted to the world. Only schemas created via /itemdata define are persisted.

Step 3 — Read and write with ItemDataAPI

Once a schema exists, use ItemDataAPI anywhere you have an ItemStack. The API validates against the schema automatically; if validation fails, set* returns false and the stack is left unchanged.

import fr.hdi.customitemdata.api.ItemDataAPI;
import net.minecraft.item.ItemStack;

ItemStack stack = player.getMainHandStack();

ItemDataAPI.setCustomName(stack, "My Item");

// Write
ItemDataAPI.setInt(stack, "nb_point", 42);
ItemDataAPI.setString(stack, "owner", "Steve");

// Read (with default fallback)
int points   = ItemDataAPI.getInt(stack, "nb_point", 0);
String owner = ItemDataAPI.getString(stack, "owner", "unknown");

// Read as Optional (absent if the key isn't present)
ItemDataAPI.getInt(stack, "nb_point").ifPresent(value -> ...);

// Check / delete
boolean has = ItemDataAPI.has(stack, "nb_point");
ItemDataAPI.remove(stack, "nb_point");

Under the hood, every value is serialized as a String inside a single ItemDataComponent attached to the stack. Two stacks with different data will not merge, because that's how vanilla Data Components work.

Step 4 — Create a custom data type

When int / float / string / boolean aren't enough, implement CustomDataType<T> to plug in your own type. You need to provide:

• a unique string ID (namespace:name),
• an encode that turns your object into a string,
• a decode that parses it back,
• a Codec<T> (used for Minecraft data integration, e.g. datapacks).

package com.example.mymod;

import com.mojang.serialization.Codec;
import com.mojang.serialization.codecs.RecordCodecBuilder;
import fr.hdi.customitemdata.api.CustomDataType;

public final class PointDataType implements CustomDataType<PointDataType.Point> {

    public static final String TYPE_ID = "mymod:point";
    public static final PointDataType INSTANCE = new PointDataType();

    private PointDataType() {}

    @Override public String getId() { return TYPE_ID; }

    @Override public Codec<Point> getCodec() { return Point.CODEC; }

    @Override
    public String encode(Point value) {
        return value.x() + "," + value.y();
    }

    @Override
    public Point decode(String stored) {
        String[] parts = stored.split(",", 2);
        return new Point(Integer.parseInt(parts[0]), Integer.parseInt(parts[1]));
    }

    // Nicer tooltip display: "(3, 7)" instead of "Point[x=3, y=7]"
    @Override
    public String display(String stored) {
        Point p = decode(stored);
        return "(" + p.x() + ", " + p.y() + ")";
    }

    public record Point(int x, int y) {
        public static final Codec<Point> CODEC = RecordCodecBuilder.create(i -> i.group(
            Codec.INT.fieldOf("x").forGetter(Point::x),
            Codec.INT.fieldOf("y").forGetter(Point::y)
        ).apply(i, Point::new));
    }
}

Register it before any schema that uses it:

SchemaManager schemas = SchemaManager.getInstance();

schemas.registerCustomType(PointDataType.INSTANCE);

schemas.registerSchema("minecraft:compass",
    new SchemaEntry.Builder("target", PointDataType.TYPE_ID).build());

Then read/write through the generic get/set overloads:

ItemDataAPI.set(stack, "target", new PointDataType.Point(3, 7), PointDataType.INSTANCE);

Optional<PointDataType.Point> target =
    ItemDataAPI.get(stack, "target", PointDataType.INSTANCE);

Full working example

A minimal mod that defines two parameters on a stick, sets them on the first tick, and reads them back:

package com.example.mymod;

import fr.hdi.customitemdata.api.ItemDataAPI;
import fr.hdi.customitemdata.schema.SchemaEntry;
import fr.hdi.customitemdata.schema.SchemaManager;
import net.fabricmc.api.ModInitializer;
import net.fabricmc.fabric.api.event.player.UseItemCallback;
import net.minecraft.item.ItemStack;
import net.minecraft.item.Items;
import net.minecraft.util.ActionResult;
import net.minecraft.util.TypedActionResult;

public class MyMod implements ModInitializer {

    @Override
    public void onInitialize() {
        SchemaManager schemas = SchemaManager.getInstance();

        schemas.registerSchema("minecraft:stick",
            new SchemaEntry.Builder("nb_point", "int")
                .nullable(false).defaultValue("0").build());

        schemas.registerSchema("minecraft:stick",
            new SchemaEntry.Builder("owner", "string").build());

        // Right-click a stick to increment its counter.
        UseItemCallback.EVENT.register((player, world, hand) -> {
            ItemStack stack = player.getStackInHand(hand);
            if (stack.getItem() != Items.STICK) {
                return TypedActionResult.pass(stack);
            }

            int current = ItemDataAPI.getInt(stack, "nb_point", 0);
            ItemDataAPI.setInt(stack, "nb_point", current + 1);
            ItemDataAPI.setString(stack, "owner", player.getName().getString());

            player.sendMessage(
                net.minecraft.text.Text.literal("Points: " + (current + 1)), true);
            return TypedActionResult.success(stack);
        });
    }
}

Reference

ItemDataAPI (static methods)

SchemaEntry.Builder

SchemaManager (singleton via getInstance())