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Registration is the process of making an object (such as an item or block) known to the game during runtime. If some objects are not registered, this could cause crashes even before the game is fully loaded or arbitrary behaviors such as bottlenecking mod compatibility for world generation.

Most objects that are known within the game are handled by a <code>Registry</code>. Each registry uniquely defines each object through a "registry name" via a [[Using Resources#ResourceLocation|ResourceLocation]].

{{Tip|In a global context, each object is universally unique through its <code>ResourceKey</code>: a concatenation of its registry's id and the object's registry name.}}

Due to the inconsistent ordering and registration process vanilla uses, Forge wraps most vanilla registries using <code>IForgeRegistry</code>. This guarantees that the loading order for these wrapped registries will be <code>Block</code>, <code>Item</code>, and then the rest of the wrapped registries in alphabetical order. All registries supported by Forge can be found within the <code>ForgeRegistries</code> class. Since all registry names are unique to a specific registry, different registry objects within different registries can have the same name (e.g. a <code>Block</code> and an <code>Item</code> each hold a registry object named <code>examplemod:object</code>.

{{Tip/Warning|If two registry objects within the same registry have the same name, the second object will override the first. The only registry that will throw an <code>IllegalArgumentException</code> is the <code>DataSerializerEntry</code> registry.}}

== Methods for Registering ==

There are two proper ways to register objects within an associated wrapped Forge registry: the <code>DeferredRegister</code> class, and the <code>RegisterEvent</code> lifecycle event.

=== DeferredRegister ===

<code>DeferredRegister</code> is an abstraction layer over the registry event used to register objects. It maintains a map of "registry name" to their associated suppliers and resolves those suppliers during <code>RegisterEvent</code> for the associated registry. This method is the currently recommended, and documented, way to handle these objects as it provides convenience and safety for those who want to statically initialize objects while avoiding some issues associated with it. 

An example of a mod registering a custom block:

{{Template:Tabs/Code_Snippets
|java=private static final DeferredRegister<Block> BLOCKS = DeferredRegister.create(ForgeRegistries.BLOCKS, MODID);

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

public ExampleMod() {
    BLOCKS.register(FMLJavaModLoadingContext.get().getModEventBus());
}
|kotlin=private val BLOCKS = DeferredRegister.create(ForgeRegistries.BLOCKS, MODID)

val EXAMPLE_BLOCK: RegistryObject<Block> = BLOCKS.register("example_block") { Block(BlockBehaviour.Properties.of(Material.ROCK)) }

internal class ExampleMod {
    init {
        BLOCKS.register(FMLJavaModLoadingContext.get().modEventBus)
    }
}
|scala=object ExampleMod {
    private final val BLOCKS = DeferredRegister.create(ForgeRegistries.BLOCKS, MODID)

    final val EXAMPLE_BLOCK = registerBlock("example_block", () => new Block(BlockBehaviour.Properties.of(Material.ROCK)))
}
class ExampleMod {
    BLOCKS.register(FMLJavaModLoadingContext.get.getModEventBus)
}
|groovy=private static final DeferredRegister<Block> BLOCKS = DeferredRegister.create(ForgeRegistries.BLOCKS, MODID);

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

ExampleMod() {
    BLOCKS.register(FMLJavaModLoadingContext.get().modEventBus);
}
|}}

{{Tip|When using a <code>DeferredRegister</code> to register any object, the name inputted will be automatically prefixed with the mod id passed in, giving the above object a "registry name" of <code>examplemod:example_block</code>.}}

=== RegisterEvent ===

The <code>RegisterEvent</code> is the second way register objects. This [[Events|event]] is fired for each registry synchronously in vanilla registry order after <code>FMLConstructModEvent</code> and before the configs are loaded. Objects are registered using <code>#register</code> by passing in the registry key, the name of the registry object, and the object itself. There is an additional <code>#register</code> 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''')

{{Template:Tabs/Code_Snippets
|java=@SubscribeEvent
public void register(RegisterEvent event) {
    event.register(ForgeRegistries.Keys.BLOCKS,
        helper -> helper.register(new ResourceLocation(MODID, "example_block"), new Block(...))
    );
}
|kotlin=@JvmStatic
@SubscribeEvent
private fun register(event: RegisterEvent) =
    event.register(ForgeRegistries.Keys.BLOCKS) {
        helper -> helper.register(ResourceLocation(MODID, "example_block"), Block(...))
    }
|scala=@SubscribeEvent
def register(event: RegisterEvent): Unit =
    event.register(ForgeRegistries.Keys.BLOCKS,
        helper => helper.register(new ResourceLocation(MODID, "example_block"), new Block(...))
    )
|}}

{{Tip/Important|Since all objects registered must be singleton, some classes cannot by themselves be registered. Instead, <code>*Type</code> classes are registered and used in the formers' constructors to wrap the flyweight objects. For example, a [[Basics_of_Block_Entities|<code>BlockEntity</code>]] is wrapped via <code>BlockEntityType</code>, and <code>Entity</code> is wrapped via <code>EntityType</code>. These <code>*Type</code> classes hold factories that simply create the containing type on demand.

These factory holders are created through the use of their <code>*Type$Builder</code> classes. An example: (<code>REGISTER</code> here refers to a <code>DeferredRegister<BlockEntityType<?>></code>)

{{Template:Tabs/Code_Snippets
|java=public static final RegistryObject<BlockEntityType<ExampleBlockEntity>> EXAMPLE_BLOCK_ENTITY = REGISTER.register(
    "example_block_entity", () -> BlockEntityType.Builder.of(ExampleBlockEntity::new, EXAMPLE_BLOCK.get()).build(null)
);
|kotlin=val EXAMPLE_BLOCK_ENTITY: RegistryObject<BlockEntityType<ExampleBlockEntity>> = REGISTER.register("example_block_entity") { BlockEntityType.Builder.of(::ExampleBlockEntity, EXAMPLE_BLOCK.get()).build(null)) }
|scala=final val EXAMPLE_BLOCK_ENTITY = REGISTER.register("example_block_entity", () => BlockEntityType.Builder.of(() => new ExampleBlockEntity(), GeneralRegistrar.EXAMPLE_BLOCK.get).build(null))
|}}
}}

=== Non-Forge Registries ===

Not all vanilla registries are wrapped as a Forge registry. To register objects to any one of these registries, create a <code>DeferredRegister</code> via the <code>#create</code> overload which takes in a resource key of the registry and the mod id to register the entries for. Then simply call <code>#register</code> like any other <code>DeferredRegister</code>.

{{Template:Tabs/Code_Snippets
|java=private static final DeferredRegister<LootItemConditionType> REGISTER = DeferredRegister.create(Registry.LOOT_ITEM_REGISTRY, MODID);

public static final RegistryObject<LootItemConditionType> EXAMPLE_LOOT_ITEM_CONDITION_TYPE = REGISTER.register("example_loot_item_condition_type", () -> new LootItemConditionType(...));
|kotlin=private val REGISTER = DeferredRegister.create(Registry.LOOT_ITEM_REGISTRY, MODID)

val EXAMPLE_LOOT_ITEM_CONDITION_TYPE : RegistryObject<LootItemConditionType> = REGISTER.register("example_loot_item_condition_type") {
    LootItemConditionType(...)
}
|scala=private final val REGISTER = DeferredRegister.create(Registry.LOOT_ITEM_REGISTRY, MODID)

final val EXAMPLE_LOOT_ITEM_CONDITION_TYPE = REGISTER.register("example_loot_item_condition_type", () => new LootItemConditionType(...))
|groovy=private static final DeferredRegister<LootItemConditionType> REGISTER = DeferredRegister.create(Registry.LOOT_ITEM_REGISTRY, MODID);

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

If you attempt to make one of these instances require an instance of another registry object, you should use the lazy initialization method mentioned above to register the object in the correct order.

=== Data Driven Entries ===

Registries are considered to be data driven if they are located within <code>RegistryAccess</code> with the exception of <code>LevelStem</code> and <code>Level</code>.

These registry objects only need to be registered within code if they are to be used within a pre-existing registry object (e.g. a <code>PlacedFeature</code> for ore generation within an overworld <code>Biome</code>). Otherwise, their instance can be purely registered using a JSON file.

If a data driven registry object has to be registered within code, a dummy object should be supplied to hold a "registry name" and then constructed within a JSON file.

== Referencing Registered Objects ==

Each forge registered object should not be statically initialized nor reference another instance being registered. They must always be a new, singleton instance that is resolved during when <code>RegisterEvent</code> is called for their registry. This is to maintain a sane loading order for registries and their objects along with dynamic loading/unloading of mods.

Forge registered objects must always be referenced through a <code>RegistryObject</code> or a field with <code>@ObjectHolder</code>.

===Using RegistryObjects===
<code>RegistryObject</code>s can be used to retrieve references to registered objects once they become available. Their references are updated along with all <code>@ObjectHolder</code> annotations after the registry that <code>RegisterEvent</code> is called for is dispatched and frozen.

A <code>RegistryObject</code> can be retrieved as a result of using <code>DeferredRegister</code> or calling the static factory <code>RegistryObject#create</code>. Each static factory takes in the "registry name" of the object being referenced and one of the following: a <code>IForgeRegistry</code>, a registry name of the type <code>ResourceLocation</code>, or a registry key of the type <code>ResourceKey<? extends Registry<?>></code>. The <code>RegistryObject</code> can be stored within some field and retrieve the registered object using <code>#get</code>.

An example using <code>RegistryObject</code>:
{{Template:Tabs/Code_Snippets
|java=public static final RegistryObject<Item> EXAMPLE_ITEM = RegistryObject.create(new ResourceLocation("examplemod", "example_item"), ForgeRegistries.ITEMS);

// Assume that 'examplemod:example_registry' is a valid registry, and 'examplemod:example_object' is a valid object within that registry
public static final RegistryObject<ExampleRegistry> EXAMPLE_OBJECT = RegistryObject.create(new ResourceLocation("examplemod", "example_object"), new ResourceLocation("examplemod", "example_registry"), "examplemod"); 
|kotlin=val EXAMPLE_ITEM: RegistryObject<Item> = RegistryObject.create(ResourceLocation("examplemod", "example_item"), ForgeRegistries.ITEMS)

// Assume that 'examplemod:example_registry' is a valid registry, and 'examplemod:example_object' is a valid object within that registry
val EXAMPLE_OBJECT: RegistryObject<ExampleRegistry> = RegistryObject.create(ResourceLocation("examplemod", "example_object"), new ResourceLocation("examplemod", "example_registry"), "examplemod")
|scala=final val EXAMPLE_ITEM = RegistryObject.create(new ResourceLocation("examplemod", "example_item"), ForgeRegistries.ITEMS)

// Assume that 'examplemod:example_registry' is a valid registry, and 'examplemod:example_object' is a valid object within that registry
final val EXAMPLE_OBJECT = RegistryObject.create(new ResourceLocation("examplemod", "example_object"), new ResourceLocation("examplemod", "example_registry"), "examplemod"); 
|}}

{{Tip/Important|All vanilla objects are bootstrapped and registered before mods are loaded. As such, they can be referenced as is without any issues.}}

=== Using @ObjectHolder ===

Forge registry objects can also be injected into <code>public static</code> fields with either their class or that field annotated with <code>@ObjectHolder</code>. There must be enough information to construct a <code>ResourceLocation</code> to identify a single object within a specific registry.

The rules for <code>@ObjectHolder</code> are as follows:

* If the class is annotated with <code>@ObjectHolder</code>, its value will be the default namespace for all fields within if not explicitly defined
* If the class is annotated with <code>@Mod</code>, 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 <code>public static</code>; and
** one of the following conditions are true:
*** the '''enclosing class''' has an <code>@ObjectHolder</code> annotation, and the field is <code>final</code>, 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 <code>@ObjectHolder</code>, 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. <code>Item</code> or <code>ArrowItem</code> for the <code>Item</code> registry)
** ''An exception is thrown if a field does not have a corresponding registry.''
* ''An exception is thrown if the resulting <code>ResourceLocation</code> 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)

<code>@ObjectHolder</code> annotated fields are injected with their associated object values after <code>RegisterEvent</code> is fired for their registry, along with <code>RegistryObject</code>s.

{{Tip/Warning|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. If the object is found, but the field cannot be set, a warning message will be logged instead.}}

As these rules are rather complicated, here are some examples:
<div class="mw-collapsible mw-collapsed" style="border: solid 2px; padding: 2px 5px; margin-top: 3px">
<div style="font-weight:bold;line-height:1.6;">Example uses of <code>@ObjectHolder</code></div>
<div class="mw-collapsible-content" style="overflow: auto; white-space: nowrap;">
<syntaxhighlight lang="java">
@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.
}
</syntaxhighlight>
</div>
</div>

== Creating Custom Registries ==

Creating custom registries for your mod might be useful if you want other mods to add new things to your system. For example, you might have magic spells and want to allow other mods to add new spells. For this you will want to make a registry (eg. "mymagicmod:spells"). This way, other mods will be able to register things to that list, and you won't have to do anything else.

Just like with registering a new item or block you have two ways of making a new registry. Each method takes in a <code>RegistryBuilder</code> which is used to build an <code>IForgeRegistry</code>. Each builder should have its name set via <code>#setName</code> before being created.

=== With DeferredRegister ===

The first method involves the second static constructor: <code>DeferredRegister#create(ResourceLocation, String)</code>. From there, we can construct the registry using <code>#makeRegistry</code>. This will already populate <code>#setName</code> for us. This method also returns a supplier of the registry which we can use after the <code>NewRegistryEvent</code> is called.

Here is an example:

{{Template:Tabs/Code_Snippets
|java=public static final DeferredRegister<ExampleRegistry> EXAMPLE = DeferredRegister.create(new ResourceLocation(MODID, "example_registry"), MODID);

public static final Supplier<IForgeRegistry<ExampleRegistry>> REGISTRY = EXAMPLE.makeRegistry(RegistryBuilder::new);
|kotlin=val EXAMPLE: DeferredRegister<ExampleRegistry> = DeferredRegister.create(ResourceLocation(MODID, "example_registry"), MODID)

val REGISTRY: IForgeRegistry<ExampleRegistry> by lazy {
    EXAMPLE.makeRegistry(::RegistryBuilder).get()
}
|scala=final val EXAMPLE = DeferredRegister.create(new ResourceLocation(MODID, "example_registry"), MODID)

final lazy val REGISTRY = EXAMPLE.makeRegistry(() => new RegistryBuilder).get
|}}

=== Using `NewRegistryEvent` ===

The second method can be done during the <code>NewRegistryEvent</code> event. Using <code>NewRegistryEvent#create</code>, you can pass in a <code>RegistryBuilder</code> directly. This method will return a <code>Supplier<IForgeRegistry<V>></code> that can be stored and queried after the event is fired to gain access to your <code>IForgeRegistry</code> instance.

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

<syntaxhighlight lang="Java">
public static Supplier<IForgeRegistry<ExampleRegistry>> registrySupplier = null;

@SubscribeEvent
public void onNewRegistry(NewRegistryEvent event){
    RegistryBuilder<ExampleRegistry> registryBuilder = new RegistryBuilder<>();
    registryBuilder.setName(new ResourceLocation(MODID, "example_registry");
    registrySupplier = event.create(registryBuilder);
}
</syntaxhighlight>

== Handling Missing Entries ==

When loading a pre-existing world after removing mods or updating versions, there are cases where certain registry objects will cease to exist. In these cases, it is possible to specify actions to remove a mapping, prevent the world from loading, or remap the name as needed. This can be done through the third of the registry events: <code>MissingMappingsEvent</code>. Within the event, you can grab an immutable list of missing mappings associated with a mod id for a given registry via <code>#getMappings</code> or a list of all mappings via <code>#getAllMappings</code>.

For each <code>Mapping</code>, you can either execute one of the following methods:
* <code>#ignore</code> which abandons the entry when loading
* <code>#warn</code> which warns the user about the missing entry but continues loading
* <code>#fail</code> which prevents the world from loading
* <code>#remap</code> which remaps the entry to the specified non-null object in the same registry

If none of the above are specified, then the default action of notifying the user about the missing mappings occur.

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

{{Template:Tabs/Code_Snippets
|java=// This will ignore any missing test items from the specified world
@SubscribeEvent
public void onMissing(final MissingMappingsEvent event) {
    event.getMappings(ForgeRegistries.Keys.ITEMS, MODID).stream()
        .filter(mapping -> mapping.key.getPath().contains("test"))
            .forEach(Mapping::ignore);
}
|groovy=// This will ignore any missing test items from the specified world
@SubscribeEvent
void onMissing(final MissingMappingsEvent event) {
    event.getMappings(ForgeRegistries.Keys.ITEMS, MODID).stream()
        .filter(mapping -> mapping.key.path.contains("test"))
            .forEach(Mapping::ignore);
}
|}}


[[Category:Common Concepts]]