Ensure that API implementations can never be null.

fabric-blockrenderlayer-v1/src/main/java/net/fabricmc/fabric/api/blockrenderlayer/v1/BlockRenderLayerMap.java

@@ -36,7 +36,7 @@ import net.fabricmc.fabric.impl.blockrenderlayer.BlockRenderLayerMapImpl;
  * <p>Client-side only.
  */
 public interface BlockRenderLayerMap {
-	BlockRenderLayerMap INSTANCE = BlockRenderLayerMapImpl.INSTANCE;
+	BlockRenderLayerMap INSTANCE = new BlockRenderLayerMapImpl();
 
 	/**
 	 * Map (or re-map) a block state with a render layer.  Re-mapping is not recommended but if done, last one in wins.

fabric-blockrenderlayer-v1/src/main/java/net/fabricmc/fabric/impl/blockrenderlayer/BlockRenderLayerMapImpl.java

@@ -28,7 +28,7 @@ import net.minecraft.item.Item;
 import net.fabricmc.fabric.api.blockrenderlayer.v1.BlockRenderLayerMap;
 
 public class BlockRenderLayerMapImpl implements BlockRenderLayerMap {
-	private BlockRenderLayerMapImpl() { }
+	public BlockRenderLayerMapImpl() { }
 
 	@Override
 	public void putBlock(Block block, RenderLayer renderLayer) {
@@ -75,8 +75,6 @@ public class BlockRenderLayerMapImpl implements BlockRenderLayerMap {
 		}
 	}
 
-	public static final BlockRenderLayerMap INSTANCE = new BlockRenderLayerMapImpl();
-
 	private static Map<Block, RenderLayer> blockRenderLayerMap = new HashMap<>();
 	private static Map<Item, RenderLayer> itemRenderLayerMap = new HashMap<>();
 	private static Map<Fluid, RenderLayer> fluidRenderLayerMap = new HashMap<>();

fabric-containers-v0/src/main/java/net/fabricmc/fabric/api/client/screen/ScreenProviderRegistry.java

@@ -29,7 +29,7 @@ import net.fabricmc.fabric.impl.client.container.ScreenProviderRegistryImpl;
  */
 @Deprecated
 public interface ScreenProviderRegistry {
-	ScreenProviderRegistry INSTANCE = ScreenProviderRegistryImpl.INSTANCE;
+	ScreenProviderRegistry INSTANCE = new ScreenProviderRegistryImpl();
 
 	/**
 	 * Register a "Container -> ContainerScreen" factory. This is used only on the client side.

fabric-containers-v0/src/main/java/net/fabricmc/fabric/api/container/ContainerProviderRegistry.java

@@ -32,7 +32,7 @@ import net.fabricmc.fabric.impl.container.ContainerProviderImpl;
  */
 @Deprecated
 public interface ContainerProviderRegistry {
-	ContainerProviderRegistry INSTANCE = ContainerProviderImpl.INSTANCE;
+	ContainerProviderRegistry INSTANCE = new ContainerProviderImpl();
 
 	/**
 	 * Register a "packet buffer -> container" factory. This is used both on the client and server side.

fabric-containers-v0/src/main/java/net/fabricmc/fabric/impl/client/container/ScreenProviderRegistryImpl.java

@@ -31,14 +31,10 @@ import net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking;
 import net.fabricmc.fabric.api.client.screen.ContainerScreenFactory;
 import net.fabricmc.fabric.api.client.screen.ScreenProviderRegistry;
 import net.fabricmc.fabric.api.container.ContainerFactory;
+import net.fabricmc.fabric.api.container.ContainerProviderRegistry;
 import net.fabricmc.fabric.impl.container.ContainerProviderImpl;
 
 public class ScreenProviderRegistryImpl implements ScreenProviderRegistry {
-	/**
-	 * Use the instance provided by ScreenProviderRegistry.
-	 */
-	public static final ScreenProviderRegistry INSTANCE = new ScreenProviderRegistryImpl();
-
 	private static final Logger LOGGER = LogManager.getLogger();
 
 	private static final Map<Identifier, ContainerFactory<HandledScreen>> FACTORIES = new HashMap<>();
@@ -55,7 +51,7 @@ public class ScreenProviderRegistryImpl implements ScreenProviderRegistry {
 	@Override
 	public <C extends ScreenHandler> void registerFactory(Identifier identifier, ContainerScreenFactory<C> containerScreenFactory) {
 		registerFactory(identifier, (syncId, identifier1, player, buf) -> {
-			C container = ContainerProviderImpl.INSTANCE.createContainer(syncId, identifier1, player, buf);
+			C container = ((ContainerProviderImpl) ContainerProviderRegistry.INSTANCE).createContainer(syncId, identifier1, player, buf);
 
 			if (container == null) {
 				LOGGER.error("Could not open container for {} - a null object was created!", identifier1.toString());

fabric-containers-v0/src/main/java/net/fabricmc/fabric/impl/container/ContainerProviderImpl.java

@@ -37,10 +37,6 @@ import net.fabricmc.fabric.mixin.container.ServerPlayerEntityAccessor;
 
 public class ContainerProviderImpl implements ContainerProviderRegistry {
 	public static final Identifier OPEN_CONTAINER = new Identifier("fabric", "container/open");
-	/**
-	 * Use the instance provided by ContainerProviderRegistry.
-	 */
-	public static final ContainerProviderImpl INSTANCE = new ContainerProviderImpl();
 
 	private static final Logger LOGGER = LogManager.getLogger();
 

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/api/registry/FuelRegistry.java

@@ -23,5 +23,5 @@ import net.fabricmc.fabric.impl.content.registry.FuelRegistryImpl;
  * Registry of items to 0-32767 fuel burn time values, in in-game ticks.
  */
 public interface FuelRegistry extends Item2ObjectMap<Integer> {
-	FuelRegistry INSTANCE = FuelRegistryImpl.INSTANCE;
+	FuelRegistry INSTANCE = new FuelRegistryImpl();
 }

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/api/registry/LootEntryTypeRegistry.java

@@ -26,7 +26,7 @@ import net.fabricmc.fabric.impl.content.registry.LootEntryTypeRegistryImpl;
 @Deprecated
 public interface LootEntryTypeRegistry {
 	@Deprecated
-	LootEntryTypeRegistry INSTANCE = LootEntryTypeRegistryImpl.INSTANCE;
+	LootEntryTypeRegistry INSTANCE = new LootEntryTypeRegistryImpl();
 
 	@Deprecated
 	void register(LootPoolEntry.Serializer<?> serializer);

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/impl/content/registry/FuelRegistryImpl.java

@@ -34,7 +34,6 @@ import net.fabricmc.fabric.api.registry.FuelRegistry;
 
 // TODO: Clamp values to 32767 (+ add hook for mods which extend the limit to disable the check?)
 public final class FuelRegistryImpl implements FuelRegistry {
-	public static final FuelRegistryImpl INSTANCE = new FuelRegistryImpl();
 	private static final Logger LOGGER = LogManager.getLogger();
 	private final Object2IntMap<ItemConvertible> itemCookTimes = new Object2IntLinkedOpenHashMap<>();
 	private final Object2IntMap<Tag<Item>> tagCookTimes = new Object2IntLinkedOpenHashMap<>();

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/impl/content/registry/LootEntryTypeRegistryImpl.java

@@ -25,7 +25,6 @@ import net.fabricmc.fabric.api.registry.LootEntryTypeRegistry;
 
 @Deprecated
 public final class LootEntryTypeRegistryImpl implements LootEntryTypeRegistry {
-	public static final LootEntryTypeRegistryImpl INSTANCE = new LootEntryTypeRegistryImpl();
 	private static final Method REGISTER_METHOD;
 
 	static {
@@ -49,7 +48,7 @@ public final class LootEntryTypeRegistryImpl implements LootEntryTypeRegistry {
 		}
 	}
 
-	private LootEntryTypeRegistryImpl() { }
+	public LootEntryTypeRegistryImpl() { }
 
 	@Override
 	public void register(LootPoolEntry.Serializer<?> serializer) {

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/mixin/content/registry/MixinAbstractFurnaceBlockEntity.java

@@ -27,22 +27,23 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
 import net.minecraft.block.entity.AbstractFurnaceBlockEntity;
 import net.minecraft.item.Item;
 
+import net.fabricmc.fabric.api.registry.FuelRegistry;
 import net.fabricmc.fabric.impl.content.registry.FuelRegistryImpl;
 
 @Mixin(AbstractFurnaceBlockEntity.class)
 public class MixinAbstractFurnaceBlockEntity {
 	@Inject(at = @At("RETURN"), method = "createFuelTimeMap")
 	private static void fuelTimeMapHook(CallbackInfoReturnable<Map<Item, Integer>> info) {
-		FuelRegistryImpl.INSTANCE.apply(info.getReturnValue());
+		((FuelRegistryImpl) FuelRegistry.INSTANCE).apply(info.getReturnValue());
 	}
 
 	@Redirect(method = "canUseAsFuel", at = @At(value = "INVOKE", target = "Lnet/minecraft/block/entity/AbstractFurnaceBlockEntity;createFuelTimeMap()Ljava/util/Map;"))
 	private static Map<Item, Integer> canUseAsFuelRedirect() {
-		return FuelRegistryImpl.INSTANCE.getFuelTimes();
+		return ((FuelRegistryImpl) FuelRegistry.INSTANCE).getFuelTimes();
 	}
 
 	@Redirect(method = "getFuelTime", at = @At(value = "INVOKE", target = "Lnet/minecraft/block/entity/AbstractFurnaceBlockEntity;createFuelTimeMap()Ljava/util/Map;"))
 	private Map<Item, Integer> getFuelTimeRedirect() {
-		return FuelRegistryImpl.INSTANCE.getFuelTimes();
+		return ((FuelRegistryImpl) FuelRegistry.INSTANCE).getFuelTimes();
 	}
 }

fabric-content-registries-v0/src/main/java/net/fabricmc/fabric/mixin/content/registry/client/MixinClientPlayNetworkHandler.java

@@ -24,12 +24,13 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 import net.minecraft.client.network.ClientPlayNetworkHandler;
 import net.minecraft.network.packet.s2c.play.SynchronizeTagsS2CPacket;
 
+import net.fabricmc.fabric.api.registry.FuelRegistry;
 import net.fabricmc.fabric.impl.content.registry.FuelRegistryImpl;
 
 @Mixin(ClientPlayNetworkHandler.class)
 public abstract class MixinClientPlayNetworkHandler {
 	@Inject(at = @At("TAIL"), method = "onSynchronizeTags")
 	private void onSynchronizeTagsHook(SynchronizeTagsS2CPacket packet, CallbackInfo info) {
-		FuelRegistryImpl.INSTANCE.resetCache();
+		((FuelRegistryImpl) FuelRegistry.INSTANCE).resetCache();
 	}
 }

fabric-loot-tables-v1/src/main/java/net/fabricmc/fabric/api/loot/v1/LootEntryTypeRegistry.java

@@ -29,7 +29,7 @@ import net.fabricmc.fabric.impl.loot.table.LootEntryTypeRegistryImpl;
  * @see #register
  */
 public interface LootEntryTypeRegistry {
-	LootEntryTypeRegistry INSTANCE = LootEntryTypeRegistryImpl.INSTANCE;
+	LootEntryTypeRegistry INSTANCE = new LootEntryTypeRegistryImpl();
 
 	/**
 	 * Registers a loot entry type serializer by its ID.

fabric-loot-tables-v1/src/main/java/net/fabricmc/fabric/impl/loot/table/LootEntryTypeRegistryImpl.java

@@ -23,9 +23,7 @@ import net.minecraft.util.JsonSerializer;
 import net.fabricmc.fabric.mixin.loot.table.LootPoolEntryTypesAccessor;
 
 public final class LootEntryTypeRegistryImpl implements net.fabricmc.fabric.api.loot.v1.LootEntryTypeRegistry {
-	public static final LootEntryTypeRegistryImpl INSTANCE = new LootEntryTypeRegistryImpl();
-
-	private LootEntryTypeRegistryImpl() { }
+	public LootEntryTypeRegistryImpl() { }
 
 	@Override
 	public void register(Identifier id, JsonSerializer<? extends LootPoolEntry> serializer) {

fabric-models-v0/src/main/java/net/fabricmc/fabric/api/client/model/ModelLoadingRegistry.java

@@ -24,7 +24,7 @@ import net.minecraft.util.Identifier;
 import net.fabricmc.fabric.impl.client.model.ModelLoadingRegistryImpl;
 
 public interface ModelLoadingRegistry {
-	ModelLoadingRegistry INSTANCE = ModelLoadingRegistryImpl.INSTANCE;
+	ModelLoadingRegistry INSTANCE = new ModelLoadingRegistryImpl();
 
 	/**
 	 * Register a model provider, which can request loading additional models.

fabric-models-v0/src/main/java/net/fabricmc/fabric/impl/client/model/ModelLoadingRegistryImpl.java

@@ -178,7 +178,6 @@ public class ModelLoadingRegistryImpl implements ModelLoadingRegistry {
 	}
 
 	private static final Logger LOGGER = LogManager.getLogger();
-	public static final ModelLoadingRegistryImpl INSTANCE = new ModelLoadingRegistryImpl();
 
 	private final List<Function<ResourceManager, ModelVariantProvider>> variantProviderSuppliers = new ArrayList<>();
 	private final List<Function<ResourceManager, ModelResourceProvider>> resourceProviderSuppliers = new ArrayList<>();
@@ -205,6 +204,6 @@ public class ModelLoadingRegistryImpl implements ModelLoadingRegistry {
 	}
 
 	public static LoaderInstance begin(ModelLoader loader, ResourceManager manager) {
-		return new LoaderInstance(INSTANCE, loader, manager);
+		return new LoaderInstance((ModelLoadingRegistryImpl) INSTANCE, loader, manager);
 	}
 }

fabric-rendering-fluids-v1/src/main/java/net/fabricmc/fabric/api/client/render/fluid/v1/FluidRenderHandler.java

@@ -82,7 +82,7 @@ public interface FluidRenderHandler {
 	 * @return Whether anything is tessellated.
 	 */
 	default boolean renderFluid(BlockPos pos, BlockRenderView world, VertexConsumer vertexConsumer, FluidState state) {
-		return FluidRenderHandlerRegistryImpl.INSTANCE.renderFluid(pos, world, vertexConsumer, state);
+		return ((FluidRenderHandlerRegistryImpl) FluidRenderHandlerRegistry.INSTANCE).renderFluid(pos, world, vertexConsumer, state);
 	}
 
 	/**

fabric-rendering-fluids-v1/src/main/java/net/fabricmc/fabric/api/client/render/fluid/v1/FluidRenderHandlerRegistry.java

@@ -30,7 +30,7 @@ import net.fabricmc.fabric.impl.client.rendering.fluid.FluidRenderHandlerRegistr
  * rendering.
  */
 public interface FluidRenderHandlerRegistry {
-	FluidRenderHandlerRegistry INSTANCE = FluidRenderHandlerRegistryImpl.INSTANCE;
+	FluidRenderHandlerRegistry INSTANCE = new FluidRenderHandlerRegistryImpl();
 
 	/**
 	 * Get a {@link FluidRenderHandler} for a given Fluid. Supports vanilla and

fabric-rendering-fluids-v1/src/main/java/net/fabricmc/fabric/impl/client/rendering/fluid/FluidRenderHandlerRegistryImpl.java

@@ -41,7 +41,6 @@ import net.fabricmc.fabric.api.client.render.fluid.v1.FluidRenderHandler;
 import net.fabricmc.fabric.api.client.render.fluid.v1.FluidRenderHandlerRegistry;
 
 public class FluidRenderHandlerRegistryImpl implements FluidRenderHandlerRegistry {
-	public static final FluidRenderHandlerRegistryImpl INSTANCE = new FluidRenderHandlerRegistryImpl();
 	private static final int DEFAULT_WATER_COLOR = BuiltinRegistries.BIOME.get(BiomeKeys.OCEAN).getWaterColor();
 	private final Map<Fluid, FluidRenderHandler> handlers = new IdentityHashMap<>();
 	private final Map<Fluid, FluidRenderHandler> modHandlers = new IdentityHashMap<>();
@@ -49,7 +48,7 @@ public class FluidRenderHandlerRegistryImpl implements FluidRenderHandlerRegistr
 
 	private FluidRenderer fluidRenderer;
 
-	private FluidRenderHandlerRegistryImpl() {
+	public FluidRenderHandlerRegistryImpl() {
 	}
 
 	@Override

fabric-rendering-fluids-v1/src/main/java/net/fabricmc/fabric/mixin/client/rendering/fluid/MixinFluidRenderer.java

@@ -37,6 +37,7 @@ import net.minecraft.util.math.BlockPos;
 import net.minecraft.world.BlockRenderView;
 
 import net.fabricmc.fabric.api.client.render.fluid.v1.FluidRenderHandler;
+import net.fabricmc.fabric.api.client.render.fluid.v1.FluidRenderHandlerRegistry;
 import net.fabricmc.fabric.impl.client.rendering.fluid.FluidRenderHandlerRegistryImpl;
 import net.fabricmc.fabric.impl.client.rendering.fluid.FluidRendererHookContainer;
 
@@ -56,7 +57,7 @@ public class MixinFluidRenderer {
 	@Inject(at = @At("RETURN"), method = "onResourceReload")
 	public void onResourceReloadReturn(CallbackInfo info) {
 		FluidRenderer self = (FluidRenderer) (Object) this;
-		FluidRenderHandlerRegistryImpl.INSTANCE.onFluidRendererReload(self, waterSprites, lavaSprites, waterOverlaySprite);
+		((FluidRenderHandlerRegistryImpl) FluidRenderHandlerRegistry.INSTANCE).onFluidRendererReload(self, waterSprites, lavaSprites, waterOverlaySprite);
 	}
 
 	@Inject(at = @At("HEAD"), method = "render", cancellable = true)
@@ -82,7 +83,7 @@ public class MixinFluidRenderer {
 	@Unique
 	private void tessellateViaHandler(BlockRenderView view, BlockPos pos, VertexConsumer vertexConsumer, FluidState state, CallbackInfoReturnable<Boolean> info) {
 		FluidRendererHookContainer ctr = fabric_renderHandler.get();
-		FluidRenderHandler handler = FluidRenderHandlerRegistryImpl.INSTANCE.getOverride(state.getFluid());
+		FluidRenderHandler handler = ((FluidRenderHandlerRegistryImpl) FluidRenderHandlerRegistry.INSTANCE).getOverride(state.getFluid());
 
 		ctr.view = view;
 		ctr.pos = pos;
@@ -146,7 +147,7 @@ public class MixinFluidRenderer {
 	public Sprite modSideSpriteForOverlay(Sprite chk) {
 		Block block = fabric_neighborBlock.get();
 
-		if (FluidRenderHandlerRegistryImpl.INSTANCE.isBlockTransparent(block)) {
+		if (FluidRenderHandlerRegistry.INSTANCE.isBlockTransparent(block)) {
 			FluidRendererHookContainer ctr = fabric_renderHandler.get();
 			return ctr.handler != null && ctr.hasOverlay ? ctr.overlay : waterOverlaySprite;
 		}

fabric-rendering-v1/src/main/java/net/fabricmc/fabric/api/client/rendering/v1/BuiltinItemRendererRegistry.java

@@ -38,7 +38,7 @@ public interface BuiltinItemRendererRegistry {
 	 * The singleton instance of the renderer registry.
 	 * Use this instance to call the methods in this interface.
 	 */
-	BuiltinItemRendererRegistry INSTANCE = BuiltinItemRendererRegistryImpl.INSTANCE;
+	BuiltinItemRendererRegistry INSTANCE = new BuiltinItemRendererRegistryImpl();
 
 	/**
 	 * Registers the renderer for the item.

fabric-rendering-v1/src/main/java/net/fabricmc/fabric/impl/client/rendering/BuiltinItemRendererRegistryImpl.java

@@ -33,11 +33,9 @@ import net.fabricmc.fabric.api.client.rendering.v1.BuiltinItemRendererRegistry;
 
 @Environment(EnvType.CLIENT)
 public final class BuiltinItemRendererRegistryImpl implements BuiltinItemRendererRegistry {
-	public static final BuiltinItemRendererRegistryImpl INSTANCE = new BuiltinItemRendererRegistryImpl();
-
 	private static final Map<Item, DynamicItemRenderer> RENDERERS = new HashMap<>();
 
-	private BuiltinItemRendererRegistryImpl() {
+	public BuiltinItemRendererRegistryImpl() {
 	}
 
 	@Override