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. The registry name for an object may be accessed with #getRegistryName/#setRegistryName. The setter can only be called once; calling it twice results in an exception.

Every type of registrable object has its own registry. To see all registries supported 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 RegistryEvent$Register lifecycle event.

DeferredRegister

DeferredRegister is the newer and documented 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 the proper RegistryEvent$Register event.

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());
}

Register events

The RegistryEvents are the second and more flexible way to register objects. These events are fired after the mod constructors and before the loading of configs.

The event used to register objects is RegistryEvent$Register<T>. The type parameter T should be set to the type of the object being registered. Calling #getRegistry will return the registry, upon which objects are registered with #register or #registerAll.

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

@SubscribeEvent
public void registerBlocks(RegistryEvent.Register<Block> event) {
    event.getRegistry().registerAll(new Block(...), new Block(...), ...);
}

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 their respective RegistryEvent$Register event is fired. 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 their corresponding registry’s RegistryEvent$Register event is fired, along with the @ObjectHolder annotations.

To get a RegistryObject, call RegistryObject#of with a ResourceLocation and the IForgeRegistry of the registrable object. Custom registries can also be used by giving a supplier of the object’s class. 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.of(new ResourceLocation("minecraft:bow"), ForgeRegistries.ITEMS);

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

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;
  • one of the following conditions are true:
    • the enclosing class has an @ObjectHolder annotation, and the field is final, and:
      • the name value is the field’s name; and
      • the namespace value is the enclosing class’s namespace
      • An exception is thrown if the namespace value cannot be found and inherited
    • the field is annotated with @ObjectHolder, and:
      • the name value is explicitly defined; and
      • the namespace value is either explicitly defined or the enclosing class’s namespace
  • the field type or one of its supertypes corresponds to a valid registry (e.g. Item or ArrowItem for the Item registry);
  • An exception is thrown if a field does not have a corresponding registry.
• 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 their corresponding registry’s RegistryEvent$Register event is fired, along with the RegistryObjects.

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

@ObjectHolder("minecraft") // Inheritable resource namespace: "minecraft"
class AnnotatedHolder {
    public static final Block diamond_block = null;   // No annotation. [public static final] is required.
                                                      // Block has a corresponding registry: [Block]
                                                      // Name path is the name of the field: "diamond_block"
                                                      // Namespace is not explicitly defined.
                                                      // So, namespace is inherited from class annotation: "minecraft"
                                                      // To inject: "minecraft:diamond_block" from the [Block] registry

    @ObjectHolder("ambient.cave")
    public static SoundEvent ambient_sound = null;    // Annotation present. [public static] is required.
                                                      // SoundEvent has a corresponding registry: [SoundEvent]
                                                      // Name path is the value of the annotation: "ambient.cave"
                                                      // Namespace is not explicitly defined.
                                                      // So, namespace is inherited from class annotation: "minecraft"
                                                      // To inject: "minecraft:ambient.cave" from the [SoundEvent] registry

    // Assume for the next entry that [ManaType] is a valid registry.          
    @ObjectHolder("neomagicae:coffeinum")
    public static final ManaType coffeinum = null;    // Annotation present. [public static] is required. [final] is optional.
                                                      // ManaType has a corresponding registry: [ManaType] (custom registry)
                                                      // Resource location is explicitly defined: "neomagicae:coffeinum"
                                                      // To inject: "neomagicae:coffeinum" from the [ManaType] registry

    public static final Item ENDER_PEARL = null;      // No annotation. [public static final] is required.
                                                      // Item has a corresponding registry: [Item].
                                                      // Name path is the name of the field: "ENDER_PEARL" -> "ender_pearl"
                                                      // !! ^ Field name is valid, because they are
                                                      //      converted to lowercase automatically.
                                                      // Namespace is not explicitly defined.
                                                      // So, namespace is inherited from class annotation: "minecraft"
                                                      // To inject: "minecraft:ender_pearl" from the [Item] registry

    @ObjectHolder("minecraft:arrow")
    public static final ArrowItem arrow = null;       // Annotation present. [public static] is required. [final] is optional.
                                                      // ArrowItem does not have a corresponding registry.
                                                      // ArrowItem's supertype of Item has a corresponding registry: [Item]
                                                      // Resource location is explicitly defined: "minecraft:arrow"
                                                      // To inject: "minecraft:arrow" from the [Item] registry                                                    

    public static Block bedrock = null;               // No annotation, so [public static final] is required.
                                                      // Therefore, the field is ignored.

    public static final CreativeModeTab group = null; // No annotation. [public static final] is required.
                                                      // CreativeModeTab does not have a corresponding registry.
                                                      // No supertypes of CreativeModeTab has a corresponding registry.
                                                      // Therefore, THIS WILL PRODUCE AN EXCEPTION.
}

class UnannotatedHolder { // Note the lack of an @ObjectHolder annotation on this class.
    @ObjectHolder("minecraft:flame")
    public static final Enchantment flame = null;     // Annotation present. [public static] is required. [final] is optional.
                                                      // Enchantment has corresponding registry: [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 enclosing class.
                                                      // Therefore, the field is ignored.

    @ObjectHolder("minecraft:creeper")
    public static Entity creeper = null;              // Annotation present. [public static] is required.
                                                      // Entity does not have a corresponding registry.
                                                      // No supertypes of Entity has a corresponding registry.
                                                      // Therefore, THIS WILL PRODUCE AN EXCEPTION.

    @ObjectHolder("levitation")
    public static final Potion levitation = null;     // Annotation present. [public static] is required. [final] is optional.
                                                      // Potion has a corresponding registry: [Potion].
                                                      // Name path is the value of the annotation: "levitation"
                                                      // Namespace is not explicitly defined.
                                                      // No annotation in enclosing class.
                                                      // Therefore, THIS WILL PRODUCE AN EXCEPTION.
}

Creating Custom Registries

Custom registries are created by using RegistryBuilder during the RegistryEvent$NewRegistry event. The class RegistryBuilder takes certain parameters (such as the name, the Class of its values, and various callbacks for different events happening on the registry). Calling RegistryBuilder#create will result in the registry being built, registered to the RegistryManager, and returned to the caller for additional processing.

The Class of the value of the registry must implement IForgeRegistryEntry, which defines that #setRegistryName and #getRegistryName can be called on the objects of that class. It is recommended to extend ForgeRegistryEntry, the default implementation instead of implementing the interface directly. When #setRegistryName(String) is called with a string, and that string does not have an explicit namespace, its namespace will be set to the current modid.

The Forge registries can be accessed through the ForgeRegistries class. Any other registries can be stored and cached during their associated RegistryEvent$Register.