Registries

Registration is the process of taking the objects of a mod (such as items, blocks, sounds, etc.) and making them known to the game. Registering things is important, as without registration the game will simply not know about these objects, which will cause unexplainable behaviors and crashes.

Most things that require registration in the game are handled by the Forge registries. A registry is an object similar to a map that assigns values to keys. Forge uses registries with ResourceLocation keys to register objects. This allows the ResourceLocation to act as the "registry name" for objects.

Every type of registrable object has its own registry. To see all registries wrapped by Forge, see the ForgeRegistries class. All registry names within a registry must be unique. However, names in different registries will not collide. For example, there's a Block registry, and an Item registry. A Block and an Item may be registered with the same name example:thing without colliding; however, if two different Blocks or Items were registered with the same exact name, the second object will override the first.

Methods for Registering

There are two proper ways to register objects: the DeferredRegister class, and the RegisterEvent lifecycle event.

DeferredRegister

DeferredRegister is the recommended way to register objects. It allows the use and convenience of static initializers while avoiding the issues associated with it. It simply maintains a list of suppliers for entries and registers the objects from those suppliers during RegisterEvent.

An example of a mod registering a custom block:

private static final DeferredRegister<Block> BLOCKS = DeferredRegister.create(ForgeRegistries.BLOCKS, MODID);

public static final RegistryObject<Block> ROCK_BLOCK = BLOCKS.register("rock", () -> new Block(BlockBehaviour.Properties.of(Material.STONE)));

public ExampleMod() {
  BLOCKS.register(FMLJavaModLoadingContext.get().getModEventBus());
}

RegisterEvent

RegisterEvent is the second way to register objects. This event is fired for each registry after the mod constructors and before the loading of configs. Objects are registered using #register by passing in the registry key, the name of the registry object, and the object itself. There is an additional #register overload which takes in a consumed helper to register an object with a given name. It is recommended to use this method to avoid unnecessary object creation.

Here is an example: (the event handler is registered on the mod event bus)

@SubscribeEvent
public void register(RegisterEvent event) {
  event.register(ForgeRegistries.Keys.BLOCKS,
    helper -> {
      helper.register(new ResourceLocation(MODID, "example_block_1"), new Block(...));
      helper.register(new ResourceLocation(MODID, "example_block_2"), new Block(...));
      helper.register(new ResourceLocation(MODID, "example_block_3"), new Block(...));
      // ...
    }
  );
}

Registries that aren't Forge Registries

Not all registries are wrapped by Forge. These can be static registries, like LootItemConditionType, which are safe to use. There are also dynamic registries, like ConfiguredFeature and some other worldgen registries, which are typically represented in JSON. DeferredRegister#create has an overload which allows modders to specify the registry key of which vanilla registry to create a RegistryObject for. The registry method and attaching to the mod event bus is the same as other DeferredRegisters.

danger

Dynamic registry objects can only be registered through data files (e.g. JSON). They cannot be registered in-code.

private static final DeferredRegister<LootItemConditionType> REGISTER = DeferredRegister.create(Registries.LOOT_CONDITION_TYPE, "examplemod");

public static final RegistryObject<LootItemConditionType> EXAMPLE_LOOT_ITEM_CONDITION_TYPE = REGISTER.register("example_loot_item_condition_type", () -> new LootItemConditionType(...));

note

Some classes cannot by themselves be registered. Instead, *Type classes are registered, and used in the formers' constructors. For example, BlockEntity has BlockEntityType, and Entity has EntityType. These *Type classes are factories that simply create the containing type on demand.

These factories are created through the use of their *Type$Builder classes. An example: (REGISTER refers to a DeferredRegister<BlockEntityType>)

public static final RegistryObject<BlockEntityType<ExampleBlockEntity>> EXAMPLE_BLOCK_ENTITY = REGISTER.register(
  "example_block_entity", () -> BlockEntityType.Builder.of(ExampleBlockEntity::new, EXAMPLE_BLOCK.get()).build(null)
);

Referencing Registered Objects

Registered objects should not be stored in fields when they are created and registered. They are to be always newly created and registered whenever RegisterEvent is fired for that registry. This is to allow dynamic loading and unloading of mods in a future version of Forge.

Registered objects must always be referenced through a RegistryObject or a field with @ObjectHolder.

Using RegistryObjects

RegistryObjects can be used to retrieve references to registered objects once they are available. These are used by DeferredRegister to return a reference to the registered objects. Their references are updated after RegisterEvent is called for their registry, along with the @ObjectHolder annotations.

To get a RegistryObject, call RegistryObject#create with a ResourceLocation and the IForgeRegistry of the registrable object. Custom registries can also be used by supplying the registry name instead. Store the RegistryObject in a public static final field, and call #get whenever you need the registered object.

An example of using RegistryObject:

public static final RegistryObject<Item> BOW = RegistryObject.create(new ResourceLocation("minecraft:bow"), ForgeRegistries.ITEMS);

// assume that 'neomagicae:mana_type' is a valid registry, and 'neomagicae:coffeinum' is a valid object within that registry
public static final RegistryObject<ManaType> COFFEINUM = RegistryObject.create(new ResourceLocation("neomagicae", "coffeinum"), new ResourceLocation("neomagicae", "mana_type"), "neomagicae"); 

Using @ObjectHolder

Registered objects from registries can be injected into the public static fields by annotating classes or fields with @ObjectHolder and supplying enough information to construct a ResourceLocation to identify a specific object in a specific registry.

The rules for @ObjectHolder are as follows:

• If the class is annotated with @ObjectHolder, its value will be the default namespace for all fields within if not explicitly defined
• If the class is annotated with @Mod, the modid will be the default namespace for all annotated fields within if not explicitly defined
• A field is considered for injection if:
  • it has at least the modifiers public static;
  • the field is annotated with @ObjectHolder, and:
    • the name value is explicitly defined; and
    • the registry name value is explicitly defined
  • A compile-time exception is thrown if a field does not have a corresponding registry or name.
• An exception is thrown if the resulting ResourceLocation is incomplete or invalid (non-valid characters in path)
• If no other errors or exceptions occur, the field will be injected
• If all of the above rules do not apply, no action will be taken (and a message may be logged)

@ObjectHolder-annotated fields are injected with their values after RegisterEvent is fired for their registry, along with the RegistryObjects.

note

If the object does not exist in the registry when it is to be injected, a debug message will be logged and no value will be injected.

As these rules are rather complicated, here are some examples:

class Holder {
  @ObjectHolder(registryName = "minecraft:enchantment", value = "minecraft:flame")
  public static final Enchantment flame = null;     // Annotation present. [public static] is required. [final] is optional.
                                                    // Registry name is explicitly defined: "minecraft:enchantment"
                                                    // Resource location is explicitly defined: "minecraft:flame"
                                                    // To inject: "minecraft:flame" from the [Enchantment] registry

  public static final Biome ice_flat = null;        // No annotation on the field.
                                                    // Therefore, the field is ignored.

  @ObjectHolder("minecraft:creeper")
  public static Entity creeper = null;              // Annotation present. [public static] is required.
                                                    // The registry has not been specified on the field.
                                                    // Therefore, THIS WILL PRODUCE A COMPILE-TIME EXCEPTION.

  @ObjectHolder(registryName = "potion")
  public static final Potion levitation = null;     // Annotation present. [public static] is required. [final] is optional.
                                                    // Registry name is explicitly defined: "minecraft:potion"
                                                    // Resource location is not specified on the field
                                                    // Therefore, THIS WILL PRODUCE A COMPILE-TIME EXCEPTION.
}

Creating Custom Forge Registries

Custom registries can usually just be a simple map of key to value. This is a common style; however, it forces a hard dependency on the registry being present. It also requires that any data that needs to be synced between sides must be done manually. Custom Forge Registries provide a simple alternative for creating soft dependents along with better management and automatic syncing between sides (unless told otherwise). Since the objects also use a Forge registry, registration becomes standardized in the same way.

Custom Forge Registries are created with the help of a RegistryBuilder, through either NewRegistryEvent or the DeferredRegister. The RegistryBuilder class takes various parameters (such as the registry's name, id range, and various callbacks for different events happening on the registry). New registries are registered to the RegistryManager after NewRegistryEvent finishes firing.

Any newly created registry should use its associated registration method to register the associated objects.

Using NewRegistryEvent

When using NewRegistryEvent, calling #create with a RegistryBuilder will return a supplier-wrapped registry. The supplied registry can be accessed after NewRegistryEvent has finished posting to the mod event bus. Getting the custom registry from the supplier before NewRegistryEvent finishes firing will result in a null value.

With DeferredRegister

The DeferredRegister method is once again another wrapper around the above event. Once a DeferredRegister is created in a constant field using the #create overload which takes in the registry name and the mod id, the registry can be constructed via DeferredRegister#makeRegistry. This takes in a supplied RegistryBuilder containing any additional configurations. The method already populates #setName by default. Since this method can be returned at any time, a supplied version of an IForgeRegistry is returned instead. Getting the custom registry from the supplier before NewRegistryEvent is fired will result in a null value.

caution

DeferredRegister#makeRegistry must be called before the DeferredRegister is added to the mod event bus via #register. #makeRegistry also uses the #register method to create the registry during NewRegistryEvent.

Handling Missing Entries

There are cases where certain registry objects will cease to exist whenever a mod is updated or, more likely, removed. It is possible to specify actions to handle the missing mapping through the third of the registry events: MissingMappingsEvent. Within this event, a list of missing mappings can be obtained either by #getMappings given a registry key and mod id or all mappings via #getAllMappings given a registry key.

caution

MissingMappingsEvent is fired on the Forge event bus.

For each Mapping, one of four mapping types can be selected to handle the missing entry:

Action	Description
IGNORE	Ignores the missing entry and abandons the mapping.
WARN	Generates a warning in the log.
FAIL	Prevents the world from loading.
REMAP	Remaps the entry to an already registered, non-null object.

If no action is specified, then the default action will occur by notifying the user about the missing entry and whether they still would like to load the world. All actions besides remapping will prevent any other registry object from taking the place of the existing id in case the associated entry ever gets added back into the game.