The data-driven type pattern

Verified against Minecraft 26.2 · Part II · A data pack’s loot table says “function”: “minecraft:set_count”, and the game turns that string into an object it never named in code.

A data-pack author writes a chest loot table, gives one entry a function whose function field says minecraft:set_count and whose count is a range, and drops the file into data/mypack/loot_table/chests/. Nothing in the pack says SetItemCountFunction. Nothing in the jar says mypack. When the server reloads, ReloadableServerRegistries.reload scans the directory, hands the file to LootTable.DIRECT_CODEC, and somewhere inside that codec the string minecraft:set_count is looked up in BuiltInRegistries.LOOT_FUNCTION_TYPE — a registry that was filled by a static initialiser and frozen before any world existed — and the MapCodec it finds there reads the rest of the object. The result is a SetItemCountFunction, and the first time a player opens that chest, SetItemCountFunction.run calls ItemStack.setCount on every stack the pool emits. The same move is made in fifty-six places. That is why type is the most important key in a data pack: every file that has one is a lookup in a registry data packs cannot add to, so a pack can compose the game’s behaviours endlessly and never add a new one.

The cast

The idea, stated once

A codec built by Codec.dispatch reads one field of a JSON object — type unless the caller names another — decodes it with a codec for the kind, and asks that kind for a MapCodec to read the remaining fields. The class that does it is DataFixerUpper’s KeyDispatchCodec, which is why the MapCodec a kind supplies is a map codec: it reads a set of fields from the same object the type key came from, so the file looks flat. When the kind codec is Registry.byNameCodec over a registry in BuiltInRegistries, the set of kinds is whatever the jar registered at Bootstrap, and a pack can reach every one of them by name and none it did not ship. The element that comes out is then either registered — a RegistryDataLoader registry such as Registries.STRUCTURE, or one of the three ReloadableServerRegistries registries — or inline, a value inside a larger element that has no id of its own, such as the PlacementModifier list in a PlacedFeature. A registered element is referred to everywhere else by Holder: a RegistryFileCodec reads either an id or the inline object, and the identifiers page explains how the reference is handed out before the entry exists (identifiers and registries).

flowchart LR
    F["a data-pack file with a type key"] --> D["Codec.dispatch over Registry.byNameCodec"]
    D --> K["a built-in registry of kinds, frozen at Bootstrap"]
    K --> M["that kind's MapCodec reads the remaining fields"]
    M --> O["an object of a class the file never named"]
    O --> R["registered by RegistryDataLoader or ReloadableServerRegistries, or inline in a larger element"]
    R --> H["referred to by Holder from other files and from the wire"]

The pattern has two spellings, and they differ only in what the registry holds. In the bare spelling the element is the MapCodec: BuiltInRegistries.LOOT_FUNCTION_TYPE is a Registry of MapCodec, LootItemFunctions.bootstrap registers SetItemCountFunction.MAP_CODEC under set_count, and LootItemFunction.codec is how a live object names its own kind for encoding. In the type-object spelling the element is a small interface or record that wraps the codec: PlacementModifierType is an interface with one method, PlacementModifierType.codec, its constants such as PlacementModifierType.COUNT are registered into BuiltInRegistries.PLACEMENT_MODIFIER_TYPE, and PlacementModifier.CODEC dispatches on PlacementModifier.type. The type object exists so that a kind can carry something beside its codec — RecipeSerializer, ConsumeEffect.Type and RecipeDisplay.Type are records of a MapCodec and a StreamCodec, one for the file and one for the wire. There is a third spelling with two members, in which the type object is the behaviour itself: a Feature is registered into BuiltInRegistries.FEATURE, Feature.place is what it does, and what the file supplies is only a config — Feature.configuredCodec wraps the feature’s configuration codec under that key, and ConfiguredFeature.DIRECT_CODEC dispatches to it. WorldCarver and ConfiguredWorldCarver.DIRECT_CODEC are the same shape.

Seven of the instances accept a bare value in place of the object: IntProviders.CODEC, FloatProviders.CODEC and NumberProviders.CODEC read a plain number as a constant, DensityFunctions.DIRECT_CODEC reads a plain number as DensityFunctions.Constant, a height provider reads a bare anchor, and NbtProviders.CODEC and ScoreboardNameProviders.CODEC read a bare string as the context form. Three accept a bare list: LootItemFunctions.ROOT_CODEC tries LootItemFunctions.TYPED_CODEC and falls back to SequenceFunction.INLINE_CODEC, so a JSON array where one function was expected is a sequence of them; LootItemCondition does the same through AllOfCondition.INLINE_CODEC, and SlotSources through GroupSlotSource.INLINE_CODEC.

For a 1.21-era reader. The loot package has no type-object class any more: there is no LootItemFunctionType record wrapping a MapCodec, and BuiltInRegistries.LOOT_FUNCTION_TYPE, BuiltInRegistries.LOOT_CONDITION_TYPE and the provider registries hold the MapCodec itself. The worldgen registries kept their type objects. Both spellings dispatch identically.

Fifty-six of them

Fifty-six — registries in BuiltInRegistries that a codec dispatches on from the value of a field, counted at the dispatch sites: thirty-one bare, twenty-three type-object, two where the type is the behaviour. The dispatch key is type unless the row says otherwise. The criterion is the value in the data this book is about, not Registry.byNameCodec itself: four more registries dispatch through it and are not here. BuiltInRegistries.GAME_RULE and BuiltInRegistries.STAT_TYPE spell the registry name as the key of a map rather than the value of a field. BuiltInRegistries.ENVIRONMENT_ATTRIBUTE and BuiltInRegistries.DATA_COMPONENT_TYPE do both: they are keys everywhere a data pack meets them, and each also backs exactly one field dispatch — EnvironmentAttributeCheck.MAP_CODEC on attribute, and one client item-model property on component. All four are among the exceptions below.

The bare spelling: the registry holds a MapCodec

The type-object spelling: the registry holds a type that carries a MapCodec

The first nine rows are the sub-objects of a configured or placed feature, and the trace on features and placement walks a tree through all of them.

The type is the behaviour: the file supplies only a config

Three of the registered destinations are not in RegistryDataLoader at all. Registries.LOOT_TABLE, Registries.ITEM_MODIFIER and Registries.PREDICATE are built by ReloadableServerRegistries, which DatapackStructureReport calls stable dynamic registries, while Registries.RECIPE and Registries.ADVANCEMENT it calls pseudo-registries: keys exist for them, directories are named after them, and no Registry is ever constructed. The elements that reach the client are the ones in RegistryDataLoader.SYNCHRONIZED_REGISTRIES, re-encoded with the same direct codec, which is why BuiltInRegistries must be identical on both sides: the client runs the same dispatch on the same kinds (protocol phases).

One instance traced: set_count

sequenceDiagram
    participant RSReg as ReloadableServerRegistries
    participant LT as LootTable
    participant LIF as LootItemFunctions
    participant BIR as BuiltInRegistries
    participant SICF as SetItemCountFunction
    participant CBE as ChestBlockEntity

    Note over RSReg: a reload, on the background executor
    RSReg->>RSReg: reload builds a RegistryOps over JsonOps, then scanDirectory per LootDataType
    RSReg->>LT: DIRECT_CODEC parses data/mypack/loot_table/chests/mine.json
    LT->>LIF: a functions entry, ROOT_CODEC then TYPED_CODEC reads the function key
    LIF->>BIR: LOOT_FUNCTION_TYPE.byNameCodec looks up minecraft:set_count
    BIR-->>LIF: SetItemCountFunction.MAP_CODEC, out of a frozen registry
    LIF->>SICF: MAP_CODEC reads conditions, count and add, the object exists
    SICF-->>LT: compose folds the list into one BiFunction
    LT-->>RSReg: registered in a fresh MappedRegistry, validated, the RELOADABLE layer replaced
    Note over CBE: a later tick, on the server thread, a player opens the chest
    CBE->>RSReg: unpackLootTable asks reloadableRegistries for the key
    RSReg-->>CBE: the LootTable, or LootTable.EMPTY for an unknown key
    CBE->>LT: fill, then getRandomItems with a CHEST context
    LT->>SICF: decorate wraps the output, every emitted stack passes through apply
    SICF->>SICF: the conditions pass, run calls ItemStack.setCount with count.getInt
    SICF-->>CBE: the stack lands in a slot

The reload half. MinecraftServer.reloadResources — and WorldLoader.load on first start — calls ReloadableServerResources.loadResources, whose first act is ReloadableServerRegistries.reload on the background executor. It builds a RegistryOps over JsonOps.INSTANCE from a HolderLookup.Provider that already carries the updated tags, so a loot condition can name an item tag while it decodes. For each of the three LootDataTypes it creates a new MappedRegistry and calls SimpleJsonResourceReloadListener.scanDirectory, whose lister is FileToIdConverter.registry over Registries.elementsDirPath — the directory is the registry’s path, loot_table — and which parses every file with the type’s codec. A file that fails to parse is logged and skipped, and a duplicate id is an error, so one bad table costs one table.

Inside LootTable.DIRECT_CODEC the functions list is LootItemFunctions.ROOT_CODEC; the entry in question is an object, so LootItemFunctions.TYPED_CODEC runs: Registry.byNameCodec on BuiltInRegistries.LOOT_FUNCTION_TYPE reads minecraft:set_count, finds SetItemCountFunction.MAP_CODEC, and that codec reads conditions (the LootItemConditionalFunction.commonFields every conditional function shares, itself a list dispatched on BuiltInRegistries.LOOT_CONDITION_TYPE), count through NumberProviders.CODEC — a third dispatch, on BuiltInRegistries.LOOT_NUMBER_PROVIDER_TYPE — and the optional add. Three built-in registries were consulted to build one function, and the file named none of them. A misspelt kind fails at the first of them with an Unknown registry key error that names the registry of kinds, and the whole file is dropped. The LootTable constructor then calls LootItemFunctions.compose, which folds each list of functions into a single function; a list of one is the function itself.

After all three registries are built, ReloadableServerRegistries.createUpdatedRegistries replaces the RegistryLayer.RELOADABLE layer through LayeredRegistryAccess.replaceFrom and ReloadableServerRegistries.validateLootRegistries runs LootDataType.runValidation over every element: SetItemCountFunction.validate checks its count provider’s references against the finished lookup. Validation warns — problems are logged, the element stays registered. Every element in these three registries is Lifecycle.experimental.

The run half. RandomizableContainerBlockEntity.getItem — like the other container methods on that class — calls RandomizableContainer.unpackLootTable, which asks MinecraftServer.reloadableRegistries for the table by ResourceKey; an unknown key is LootTable.EMPTY, never an exception. LootTable.fill rolls LootTable.getRandomItems with a LootContextParamSets.CHEST context and LootTable.shuffleAndSplitItems spreads the result over the slots. On the way out, each level wraps the consumer: LootTable.getRandomItemsRaw decorates the output with the table’s composite function, LootPool.addRandomItems with the pool’s, and LootPoolSingletonContainer.EntryBase with the entry’s, each through LootItemFunction.decorate. LootItem.createItemStack makes a stack of one, and it passes through LootItemConditionalFunction.apply, which tests the conditions and, if they pass, calls SetItemCountFunction.run — ItemStack.setCount with NumberProvider.getInt, added to the current count if add was set. The object the pack described by a string is now a method call on a stack in a chest.

What does not follow the pattern

Not every registry in BuiltInRegistries whose name ends in type is a registry of kinds, and most of the ones that are not fall into three groups. A few fall outside them altogether — BuiltInRegistries.TICKET_TYPE, BuiltInRegistries.MAP_DECORATION_TYPE, BuiltInRegistries.POINT_OF_INTEREST_TYPE and BuiltInRegistries.VILLAGER_TYPE are registries of ordinary things whose names happen to end in type, dispatching nothing, and BuiltInRegistries.ATTRIBUTE_TYPE has a Registry.byNameCodec — AttributeTypes.CODEC — that nothing in the tree reads; the attribute name a file actually uses as a key belongs to BuiltInRegistries.ENVIRONMENT_ATTRIBUTE.

A key, not a kind. BuiltInRegistries.DATA_COMPONENT_TYPE, BuiltInRegistries.ENCHANTMENT_EFFECT_COMPONENT_TYPE, BuiltInRegistries.GAME_RULE and BuiltInRegistries.ENVIRONMENT_ATTRIBUTE each hold objects that carry a codec for their value, and a file uses them as JSON keys: GameRuleMap.CODEC and DataComponentPredicate.CODEC are Codec.dispatchedMap, a map whose key codec is Registry.byNameCodec and whose value codec depends on the key. There is no type field because the name of the field is the type. BuiltInRegistries.ENTITY_SUB_PREDICATE_TYPE is the same shape and holds a plain Codec rather than a MapCodec, and EntityPredicate reads it as a dispatched map too. BuiltInRegistries.STAT_TYPE is a key whose value codec is derived from StatType.getRegistry rather than stored, which is what PlayerPredicate.StatMatcher builds on. BuiltInRegistries.MEMORY_MODULE_TYPE is a key in a brain’s saved memories, MemoryMap.CODEC, the same way.

A type object with no codec. BuiltInRegistries.RECIPE_TYPE is the one a modder reaches for and the wrong one: RecipeType.CRAFTING groups recipes for lookup, and the kind a recipe file names — the field Recipe.CODEC dispatches on through Recipe.getSerializer — is a RecipeSerializer, in BuiltInRegistries.RECIPE_SERIALIZER. BuiltInRegistries.STRUCTURE_PIECE holds StructurePieceType, a loader from NBT with a StructurePieceSerializationContext, for the pieces of a started structure saved in the chunk — save data, not a pack. BuiltInRegistries.ENTITY_TYPE, BuiltInRegistries.BLOCK_ENTITY_TYPE, BuiltInRegistries.MENU, BuiltInRegistries.SENSOR_TYPE and BuiltInRegistries.COMMAND_ARGUMENT_TYPE are registries of type objects that no codec dispatches on: they are looked up by name and construct or describe things in Java.

A registry of kinds with nothing to load. BuiltInRegistries.BLOCK_TYPE is a complete instance of the bare spelling — BlockTypes.CODEC dispatches on it through Block.codec — that no data pack and no loader ever reads. Its one caller is the data generator’s BlockListReport, which encodes every block with it. It is the pattern applied for the sake of the report.

Questions players ask

Can a data pack add a new loot function, placement modifier or dialog kind? No. Every kind is an entry in a BuiltInRegistries registry, and those are frozen at Bootstrap. A pack composes kinds; only the jar adds one.

Why does one typo break the whole file and not the whole pack? SimpleJsonResourceReloadListener.scanDirectory parses each file on its own and logs the ones that fail, so a loot table that names minecraft:set_cuont is simply missing, and the chest that names the table gets LootTable.EMPTY. RegistryDataLoader is stricter: it collects every error by key and fails the whole load, which is why a broken biome stops the world from opening and a broken loot table does not.

Why is the same kind name accepted in a pool, an entry and a table? Because the field is decoded by the same LootItemFunctions.ROOT_CODEC in all three places, and the three composite functions wrap the output consumer one inside the other. A function on the table runs last.

Why do worldgen files sometimes take a number where an object was expected? Codec.either in front of the dispatch: a bare number is a constant for int, float and number providers and for density functions.

Where to look

Codec.dispatch · KeyDispatchCodec · Registry.byNameCodec · BuiltInRegistries · LootItemFunctions.TYPED_CODEC · LootItemFunctions.bootstrap · SetItemCountFunction.MAP_CODEC · PlacementModifier.CODEC · PlacementModifierType · ConfiguredFeature.DIRECT_CODEC · Feature.configuredCodec · LootDataType · ReloadableServerRegistries.reload · SimpleJsonResourceReloadListener.scanDirectory · RegistryFileCodec · RegistryDataLoader.WORLDGEN_REGISTRIES · LootTable.fill · LootItemFunction.decorate · SetItemCountFunction.run

Rules: names, never code · how the system works, not how the code reads · newest version only · every backticked name passes tools/verify_names.py.