This is log error:

Caused by: com.google.gson.JsonParseException: Error loading registry data: No key feature in MapLike[{"type":"epictrees:epic_trees_codec","replace":false,"entries":[{"biomes":["minecraft:plains","minecraft:meadow"],"feature":"epictrees:weep_tree"},{"biomes":"minecraft:beach","feature":"epictrees:beach_palm"}],"forge:registry_name":"forge:biome_modifier"}]; No key biomes in MapLike[{"type":"epictrees:epic_trees_codec","replace":false,"entries":[{"biomes":["minecraft:plains","minecraft:meadow"],"feature":"epictrees:weep_tree"},{"biomes":"minecraft:beach","feature":"epictrees:beach_palm"}],"forge:registry_name":"forge:biome_modifier"}]

 

This is my file located at data/epictrees/forge/biome_modifier/epic_trees_codec.json

{
  "type":"epictrees:epic_trees_codec",
  "replace":false,
  "entries": [
    {
      "biomes":["minecraft:plains","minecraft:meadow"],
      "feature":"epictrees:weep_tree"
    },
    {
      "biomes":"minecraft:beach",
      "feature":"epictrees:beach_palm"
    }
  ],"forge:registry_name":"epictrees:epic_trees_codec"
}

And I get the same error with this code:

{
  "type":"epictrees:epic_trees_codec",
  "replace":false,
  "entries": [
    {
      "biomes":["minecraft:plains","minecraft:meadow"],
      "feature":"epictrees:weep_tree"
    },
    {
      "biomes":"minecraft:beach",
      "feature":"epictrees:beach_palm"
    }
  ],"forge:registry_name":"forge:biome_modifier"
}

My main class:

  static DeferredRegister<Codec<? extends BiomeModifier>> BIOME_MODIFIER_SERIALIZERS = DeferredRegister.create(ForgeRegistries.Keys.BIOME_MODIFIER_SERIALIZERS, MODID);

    public static final RegistryObject<Codec<EpicTreesBiomeModifier>> EPIC_TREES_CODEC = BIOME_MODIFIER_SERIALIZERS.register("epic_trees_codec", () ->
            RecordCodecBuilder.create(builder -> builder.group(
                    // declare fields
                    Biome.LIST_CODEC.fieldOf("biomes").forGetter(EpicTreesBiomeModifier::biomes),
                    PlacedFeature.CODEC.fieldOf("feature").forGetter(EpicTreesBiomeModifier::feature)
                    // declare constructor
            ).apply(builder, EpicTreesBiomeModifier::new)));

    public EpicTrees()
    {
        final IEventBus modEventBus = FMLJavaModLoadingContext.get().getModEventBus();

        // Register the Deferred Register to the mod event bus so blocks get registered
        BLOCKS.register(modEventBus);
        // Register the Deferred Register to the mod event bus so items get registered
        ITEMS.register(modEventBus);
        BLOCK_ENTITIES.register(modEventBus);
        ModFeatures.register(modEventBus);
        BIOME_MODIFIER_SERIALIZERS.register(modEventBus);


        // Register ourselves for server and other game events we are interested in
        MinecraftForge.EVENT_BUS.register(this);

        //ModEntities.ENTITIES.register(modEventBus);
        //MinecraftForge.EVENT_BUS.register(WanderingArboristSpawner.class);


        //modEventBus.addListener(this::clientSetup);
        // Register the commonSetup method for modloading
        modEventBus.addListener(this::commonSetup);

    }

public record EpicTreesBiomeModifier(HolderSet<Biome> biomes, Holder<PlacedFeature> feature) implements BiomeModifier
{
    @Override
    public void modify(Holder<Biome> biome, Phase phase, ModifiableBiomeInfo.BiomeInfo.Builder builder) {
        if (phase == Phase.ADD && biomes.contains(biome))
        {
            // add things to biomes
            builder.getGenerationSettings().addFeature(GenerationStep.Decoration.VEGETAL_DECORATION, feature);
        }
    }

    @NotNull
    public Codec<? extends BiomeModifier> codec()
    {
        // This must return a registered Codec, see Biome Modifier Serializers below.
        return EpicTrees.EPIC_TREES_CODEC.get();
    }

}

And I have printed out the names of the Deferred Register objects to confirm the feature names are specified correctly and each feature, configured feature and the placed feature versions are registered with the same name.

You don't need to write your write your own biome modifier for this.

https://github.com/MinecraftForge/MinecraftForge/blob/b0caea8e204a7f026646396558d6d58a02bff9d6/src/main/java/net/minecraftforge/common/world/ForgeBiomeModifiers.java#L31

You put your "entries" in different files. Example from the testsuite:

https://github.com/MinecraftForge/MinecraftForge/blob/1.19.x/src/generated_test/resources/data/biome_modifiers_test/forge/biome_modifier/add_basalt.json

 

If you want to get yours to work with that json format, then you will need to change your codec and BiomeModifier implementation so it knows what you mean by "entries"s.

Edited September 16, 20223 yr by warjort