How to add new buildings to villages and other jigsaw structures

addNewBuildingsVillages.java

// Note: Lithostitch mod allows you to add to template pools easily! No code needed. 
// https://github.com/Apollounknowndev/lithostitched/wiki/Worldgen-Modifiers#add_template_pool_elements    

// This gist code is public domain. Take it! Steal it! AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!!!!!!!!!!!!
    // 1.18.2 code at top. 1.16.5 version farther down

    private static final ResourceKey<StructureProcessorList> EMPTY_PROCESSOR_LIST_KEY = ResourceKey.create(
            Registry.PROCESSOR_LIST_REGISTRY, new ResourceLocation("minecraft", "empty"));
    
    public ExampleMod() {
        MinecraftForge.EVENT_BUS.addListener(this::addNewVillageBuilding);
    }

    /**
     * Adds the building to the targeted pool.
     * We will call this in addNewVillageBuilding method further down to add to every village.
     *
     * Note: This is an additive operation which means multiple mods can do this and they stack with each other safely.
     */
    private static void addBuildingToPool(Registry<StructureTemplatePool> templatePoolRegistry, 
                                          Registry<StructureProcessorList> processorListRegistry,
                                          ResourceLocation poolRL, 
                                          String nbtPieceRL,
                                          int weight) {
        
        // Grabs the processor list we want to use along with our piece.
        // This is a requirement as using the ProcessorLists.EMPTY field will cause the game to throw errors.
        // The reason why is the empty processor list in the world's registry is not the same instance as in that field once the world is started up.
        Holder<StructureProcessorList> emptyProcessorList = processorListRegistry.getHolderOrThrow(EMPTY_PROCESSOR_LIST_KEY);
        
        // Grab the pool we want to add to
        StructureTemplatePool pool = templatePoolRegistry.get(poolRL);
        if (pool == null) return;

        // Grabs the nbt piece and creates a SinglePoolElement of it that we can add to a structure's pool.
        // Use .legacy( for villages/outposts and .single( for everything else
        SinglePoolElement piece = SinglePoolElement.legacy(nbtPieceRL, emptyProcessorList).apply(StructureTemplatePool.Projection.RIGID);

        // Use AccessTransformer or Accessor Mixin to make StructureTemplatePool's templates field public for us to see.
        // Weight is handled by how many times the entry appears in this list.
        // We do not need to worry about immutability as this field is created using Lists.newArrayList(); which makes a mutable list.
        for (int i = 0; i < weight; i++) {
            pool.templates.add(piece);
        }

        // Use AccessTransformer or Accessor Mixin to make StructureTemplatePool's rawTemplates field public for us to see.
        // This list of pairs of pieces and weights is not used by vanilla by default but another mod may need it for efficiency.
        // So lets add to this list for completeness. We need to make a copy of the array as it can be an immutable list.
        //   NOTE: This is a com.mojang.datafixers.util.Pair. It is NOT a fastUtil pair class. Use the mojang class.
        List<Pair<StructurePoolElement, Integer>> listOfPieceEntries = new ArrayList<>(pool.rawTemplates);
        listOfPieceEntries.add(new Pair<>(piece, weight));
        pool.rawTemplates = listOfPieceEntries;
    }


    /**
     * We use FMLServerAboutToStartEvent as the dynamic registry exists now and all JSON worldgen files were parsed.
     * Mod compat is best done here.
     */
    public void addNewVillageBuilding(final ServerAboutToStartEvent event) {
        Registry<StructureTemplatePool> templatePoolRegistry = event.getServer().registryAccess().registry(Registry.TEMPLATE_POOL_REGISTRY).orElseThrow();
        Registry<StructureProcessorList> processorListRegistry = event.getServer().registryAccess().registry(Registry.PROCESSOR_LIST_REGISTRY).orElseThrow();

        // Adds our piece to all village houses pool
        // Note, the resourcelocation is getting the pool files from the data folder. Not assets folder.
        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new ResourceLocation("minecraft:village/plains/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new ResourceLocation("minecraft:village/snowy/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry, 
                new ResourceLocation("minecraft:village/savanna/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry, 
                new ResourceLocation("minecraft:village/taiga/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new ResourceLocation("minecraft:village/desert/houses"),
                "modid:structure_nbt_resourcelocation", 5);
    }

////////////////////////////////////////////////////////////////////////////////////////////////////////////////
    // 1.16.5 code

    public ExampleMod() {
            MinecraftForge.EVENT_BUS.addListener(this::addNewVillageBuilding);
    }
      

   /**
     * Adds the building to the targeted pool.
     * We will call this in addNewVillageBuilding method further down to add to every village.
     *
     * Note: This is an additive operation which means multiple mods can do this and they stack with each other safely.
     */
    private static void addBuildingToPool(MutableRegistry<JigsawPattern> templatePoolRegistry, ResourceLocation poolRL, String nbtPieceRL, int weight) {

        // Grab the pool we want to add to
        JigsawPattern pool = templatePoolRegistry.get(poolRL);
        if (pool == null) return;

        // Grabs the nbt piece and creates a SingleJigsawPiece of it that we can add to a structure's pool.
        SingleJigsawPiece piece = SingleJigsawPiece.single(nbtPieceRL).apply(JigsawPattern.PlacementBehaviour.RIGID);

        // AccessTransformer to make JigsawPattern's templates field public for us to see.
        // public net.minecraft.world.gen.feature.jigsaw.JigsawPattern field_214953_e #templates
        // Weight is handled by how many times the entry appears in this list.
        // We do not need to worry about immutability as this field is created using Lists.newArrayList(); which makes a mutable list.
        for (int i = 0; i < weight; i++) {
            pool.templates.add(piece);
        }

        // AccessTransformer to make JigsawPattern's rawTemplates field public for us to see.
        // net.minecraft.world.gen.feature.jigsaw.JigsawPattern field_214952_d #rawTemplates
        // This list of pairs of pieces and weights is not used by vanilla by default but another mod may need it for efficiency.
        // So lets add to this list for completeness. We need to make a copy of the array as it can be an immutable list.
        List<Pair<JigsawPiece, Integer>> listOfPieceEntries = new ArrayList<>(pool.rawTemplates);
        listOfPieceEntries.add(new Pair<>(piece, weight));
        pool.rawTemplates = listOfPieceEntries;
    }


    /**
     * We use FMLServerAboutToStartEvent as the dynamic registry exists now and all JSON worldgen files were parsed.
     * Mod compat is best done here.
     */
    public void addNewVillageBuilding(final FMLServerAboutToStartEvent event) {
        MutableRegistry<JigsawPattern> templatePoolRegistry = event.getServer().registryAccess().registry(Registry.TEMPLATE_POOL_REGISTRY).get();

        // Adds our piece to all village houses pool
        // Note, the resourcelocation is getting the pool files from the data folder. Not assets folder.
        addBuildingToPool(templatePoolRegistry, new ResourceLocation("minecraft:village/plains/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, new ResourceLocation("minecraft:village/snowy/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, new ResourceLocation("minecraft:village/savanna/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, new ResourceLocation("minecraft:village/taiga/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, new ResourceLocation("minecraft:village/desert/houses"),
                "modid:structure_nbt_resourcelocation", 5);
    }

addNewBuildingsVillagesFabric.java

// Note: Lithostitch mod allows you to add to template pools easily! No code needed. 
// https://github.com/Apollounknowndev/lithostitched/wiki/Worldgen-Modifiers#add_template_pool_elements   

    @Override
    public void onInitialize() {
        ServerLifecycleEvents.SERVER_STARTING.register(this::addNewVillageBuilding);
    }

    private static final RegistryKey<StructureProcessorList> EMPTY_PROCESSOR_LIST_KEY = RegistryKey.of(
            RegistryKeys.PROCESSOR_LIST, new Identifier("minecraft", "empty"));

    /**
     * Adds the building to the targeted pool.
     * We will call this in addNewVillageBuilding method further down to add to every village.
     *
     * Note: This is an additive operation which means multiple mods can do this and they stack with each other safely.
     */
    private static void addBuildingToPool(Registry<StructurePool> templatePoolRegistry,
                                          Registry<StructureProcessorList> processorListRegistry,
                                          Identifier poolRL,
                                          String nbtPieceRL,
                                          int weight) {

        // Grabs the processor list we want to use along with our piece.
        // This is a requirement as using the ProcessorLists.EMPTY field will cause the game to throw errors.
        // The reason why is the empty processor list in the world's registry is not the same instance as in that field once the world is started up.
        RegistryEntry<StructureProcessorList> emptyProcessorList = processorListRegistry.getEntry(EMPTY_PROCESSOR_LIST_KEY).get();

        // Grab the pool we want to add to
        StructurePool pool = templatePoolRegistry.get(poolRL);
        if (pool == null) return;

        // Grabs the nbt piece and creates a SinglePoolElement of it that we can add to a structure's pool.
        // Use .ofProcessedLegacySingle( for villages/outposts and .ofProcessedSingle( for everything else
        SinglePoolElement piece = SinglePoolElement.ofProcessedLegacySingle(nbtPieceRL, emptyProcessorList).apply(StructurePool.Projection.RIGID);

        // Use AccessWideners or Accessor Mixin to make StructurePool's templates field public for us to see.
        // Weight is handled by how many times the entry appears in this list.
        // We do not need to worry about immutability as this field is created using Lists.newArrayList(); which makes a mutable list.
        for (int i = 0; i < weight; i++) {
            ((StructurePoolAccessor)pool).getElements().add(piece);
        }

        // Use AccessWideners or Accessor Mixin to make StructurePool's elementCounts field public for us to see.
        // This list of pairs of pieces and weights is not used by vanilla by default but another mod may need it for efficiency.
        // So lets add to this list for completeness. We need to make a copy of the array as it can be an immutable list.
        //   NOTE: This is a com.mojang.datafixers.util.Pair. It is NOT a fastUtil pair class. Use the mojang class.
        List<Pair<StructurePoolElement, Integer>> listOfPieceEntries = new ArrayList<>(((StructurePoolAccessor)pool).getElementCounts());
        listOfPieceEntries.add(new Pair<>(piece, weight));
        ((StructurePoolAccessor)pool).setElementCounts(listOfPieceEntries);
    }

    /**
     * We use ServerLifecycleEvents.SERVER_STARTING as the dynamic registry exists now and all JSON worldgen files were parsed.
     * Mod compat is best done here.
     */
    public void addNewVillageBuilding(final MinecraftServer event) {
        Registry<StructurePool> templatePoolRegistry = event.getRegistryManager().get(RegistryKeys.TEMPLATE_POOL);
        Registry<StructureProcessorList> processorListRegistry = event.getRegistryManager().get(RegistryKeys.PROCESSOR_LIST);

        // Adds our piece to all village houses pool
        // Note, the resourcelocation is getting the pool files from the data folder. Not assets folder.
        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new Identifier("minecraft:village/plains/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new Identifier("minecraft:village/snowy/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new Identifier("minecraft:village/savanna/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new Identifier("minecraft:village/taiga/houses"),
                "modid:structure_nbt_resourcelocation", 5);

        addBuildingToPool(templatePoolRegistry, processorListRegistry,
                new Identifier("minecraft:village/desert/houses"),
                "modid:structure_nbt_resourcelocation", 5);
    }

For any confused chaps like me in 1.19 having problems related to private variables:

1. Join this discord https://discord.gg/forge // it's the official forge discord
2. in bot commands type !moj varName // for example: !moj rawTemplates
3. Copy the value that says AT: ... // like public net.minecraft.world.level.levelgen.structure.pools.StructureTemplatePool f_210559_ # rawTemplates
4. if the variable is final, add -f after the world public
5. in build.gradle uncomment this line accessTransformer = file('src/main/resources/META-INF/accesstransformer.cfg')
6. Follow that path and add and open the accesstransformer.cfg file
7. Paste any copied values
8. Rebuild gradle // It's the little refresh button on the right side in the gradle tab

Happy modding!