Forge Datastorage Saveddata

Saved Data

The Saved Data (SD) system is an alternative to level capabilities for attaching data per level.

Declaration

Each SD implementation must subtype the SavedData class:

• save: writes NBT data to the level.
• setDirty: must be called after changing data to mark it for writing; without it, #save is not called.

Attaching to a level

SavedDatas are loaded/attached dynamically — if never created on a level, they don't exist. Create or load one via DimensionDataStorage#computeIfAbsent, obtained from ServerChunkCache#getDataStorage or ServerLevel#getDataStorage. computeIfAbsent takes a load function (NBT → SD), a supplier (new instance), and the .dat file name in the level's data folder.

Example (SD named "example" in the Nether creates ./<level_folder>/DIM-1/data/example.dat):

public ExampleSavedData create() {
  return new ExampleSavedData();
}

public ExampleSavedData load(CompoundTag tag) {
  ExampleSavedData data = this.create();
  // Load saved data
  return data;
}

netherDataStorage.computeIfAbsent(this::load, this::create, "example");

To persist across levels, attach the SD to the Overworld (MinecraftServer#overworld) — the only dimension never fully unloaded.

1.21.11+ SavedData current API (verified from mc_java_sources)

In Forge 1.21.11, define a Codec<MyData> and a static SavedDataType<MyData>, then load with computeIfAbsent(TYPE):

public static final Codec<MyData> CODEC = RecordCodecBuilder.create(instance -> instance.group(
    Codec.BOOL.optionalFieldOf("active", false).forGetter(d -> d.active)
).apply(instance, MyData::new));

public static final SavedDataType<MyData> TYPE = new SavedDataType<>(
    "mydata", MyData::new, CODEC, DataFixTypes.SAVED_DATA_MAP_DATA);

public static MyData get(ServerLevel level) {
    return level.getDataStorage().computeIfAbsent(TYPE);
}

The old DimensionDataStorage#computeIfAbsent(String, Supplier, DataFixTypes) still exists for compatibility, but the codec-based SavedDataType is the current pattern used by vanilla.