Add hooks to allow modification of structures spawn lists (#7344)

patches/minecraft/net/minecraft/server/MinecraftServer.java.patch

@@ -25,7 +25,15 @@
           BiomeProvider biomeprovider = chunkgenerator.func_202090_b();
           Random random = new Random(p_240786_0_.func_72905_C());
           BlockPos blockpos = biomeprovider.func_225531_a_(0, p_240786_0_.func_181545_F(), 0, 256, (p_244265_0_) -> {
-@@ -563,6 +565,7 @@
+@@ -449,6 +451,7 @@
+    }
+ 
+    private void func_213186_a(IChunkStatusListener p_213186_1_) {
++      net.minecraftforge.common.world.StructureSpawnManager.gatherEntitySpawns();
+       ServerWorld serverworld = this.func_241755_D_();
+       field_147145_h.info("Preparing start region for dimension {}", (Object)serverworld.func_234923_W_().func_240901_a_());
+       BlockPos blockpos = serverworld.func_241135_u_();
+@@ -563,6 +566,7 @@
        for(ServerWorld serverworld1 : this.func_212370_w()) {
           if (serverworld1 != null) {
              try {
@@ -33,7 +41,7 @@
                 serverworld1.close();
              } catch (IOException ioexception1) {
                 field_147145_h.error("Exception closing the level", (Throwable)ioexception1);
-@@ -611,6 +614,7 @@
+@@ -611,6 +615,7 @@
     protected void func_240802_v_() {
        try {
           if (this.func_71197_b()) {
@@ -41,7 +49,7 @@
              this.field_211151_aa = Util.func_211177_b();
              this.field_147147_p.func_151315_a(new StringTextComponent(this.field_71286_C));
              this.field_147147_p.func_151321_a(new ServerStatusResponse.Version(SharedConstants.func_215069_a().getName(), SharedConstants.func_215069_a().getProtocolVersion()));
-@@ -640,7 +644,10 @@
+@@ -640,7 +645,10 @@
                 this.func_240795_b_(longtickdetector);
                 this.field_71296_Q = true;
              }
@@ -52,7 +60,7 @@
              this.func_71228_a((CrashReport)null);
           }
        } catch (Throwable throwable1) {
-@@ -659,6 +666,7 @@
+@@ -659,6 +667,7 @@
              field_147145_h.error("We were unable to save this crash report to disk.");
           }
  
@@ -60,7 +68,7 @@
           this.func_71228_a(crashreport);
        } finally {
           try {
-@@ -667,6 +675,7 @@
+@@ -667,6 +676,7 @@
           } catch (Throwable throwable) {
              field_147145_h.error("Exception stopping the server", throwable);
           } finally {
@@ -68,7 +76,7 @@
              this.func_71240_o();
           }
  
-@@ -768,6 +777,7 @@
+@@ -768,6 +778,7 @@
  
     protected void func_71217_p(BooleanSupplier p_71217_1_) {
        long i = Util.func_211178_c();
@@ -76,7 +84,7 @@
        ++this.field_71315_w;
        this.func_71190_q(p_71217_1_);
        if (i - this.field_147142_T >= 5000000000L) {
-@@ -782,6 +792,7 @@
+@@ -782,6 +793,7 @@
  
           Collections.shuffle(Arrays.asList(agameprofile));
           this.field_147147_p.func_151318_b().func_151330_a(agameprofile);
@@ -84,7 +92,7 @@
        }
  
        if (this.field_71315_w % 6000 == 0) {
-@@ -809,6 +820,7 @@
+@@ -809,6 +821,7 @@
        long i1 = Util.func_211178_c();
        this.field_213215_ap.func_181747_a(i1 - i);
        this.field_71304_b.func_76319_b();
@@ -92,7 +100,7 @@
     }
  
     protected void func_71190_q(BooleanSupplier p_71190_1_) {
-@@ -816,7 +828,8 @@
+@@ -816,7 +829,8 @@
        this.func_193030_aL().func_73660_a();
        this.field_71304_b.func_219895_b("levels");
  
@@ -102,7 +110,7 @@
           this.field_71304_b.func_194340_a(() -> {
              return serverworld + " " + serverworld.func_234923_W_().func_240901_a_();
           });
-@@ -827,6 +840,7 @@
+@@ -827,6 +841,7 @@
           }
  
           this.field_71304_b.func_76320_a("tick");
@@ -110,7 +118,7 @@
  
           try {
              serverworld.func_72835_b(p_71190_1_);
-@@ -835,9 +849,11 @@
+@@ -835,9 +850,11 @@
              serverworld.func_72914_a(crashreport);
              throw new ReportedException(crashreport);
           }
@@ -122,7 +130,7 @@
        }
  
        this.field_71304_b.func_219895_b("connection");
-@@ -912,7 +928,7 @@
+@@ -912,7 +929,7 @@
     }
  
     public String getServerModName() {
@@ -131,7 +139,7 @@
     }
  
     public CrashReport func_71230_b(CrashReport p_71230_1_) {
-@@ -925,6 +941,7 @@
+@@ -925,6 +942,7 @@
        p_71230_1_.func_85056_g().func_189529_a("Data Packs", () -> {
           StringBuilder stringbuilder = new StringBuilder();
  
@@ -139,7 +147,7 @@
           for(ResourcePackInfo resourcepackinfo : this.field_195577_ad.func_198980_d()) {
              if (stringbuilder.length() > 0) {
                 stringbuilder.append(", ");
-@@ -1271,6 +1288,7 @@
+@@ -1271,6 +1289,7 @@
           this.func_184103_al().func_193244_w();
           this.field_200258_al.func_240946_a_(this.field_195576_ac.func_240960_a_());
           this.field_240765_ak_.func_195410_a(this.field_195576_ac.func_240970_h_());
@@ -147,7 +155,7 @@
        }, this);
        if (this.func_213162_bc()) {
           this.func_213161_c(completablefuture::isDone);
-@@ -1280,10 +1298,13 @@
+@@ -1280,10 +1299,13 @@
     }
  
     public static DatapackCodec func_240772_a_(ResourcePackList p_240772_0_, DatapackCodec p_240772_1_, boolean p_240772_2_) {
@@ -163,7 +171,7 @@
        } else {
           Set<String> set = Sets.newLinkedHashSet();
  
-@@ -1433,6 +1454,31 @@
+@@ -1433,6 +1455,31 @@
  
     public abstract boolean func_213199_b(GameProfile p_213199_1_);
  
@@ -195,7 +203,7 @@
     public void func_223711_a(Path p_223711_1_) throws IOException {
        Path path = p_223711_1_.resolve("levels");
  
-@@ -1561,6 +1607,10 @@
+@@ -1561,6 +1608,10 @@
        return this.field_240768_i_;
     }
  

patches/minecraft/net/minecraft/world/gen/NoiseChunkGenerator.java.patch

@@ -11,3 +11,21 @@
                                chunkprimer.func_201637_h(blockpos$mutable);
                             }
  
+@@ -582,6 +582,9 @@
+    }
+ 
+    public List<MobSpawnInfo.Spawners> func_230353_a_(Biome p_230353_1_, StructureManager p_230353_2_, EntityClassification p_230353_3_, BlockPos p_230353_4_) {
++      List<MobSpawnInfo.Spawners> spawns = net.minecraftforge.common.world.StructureSpawnManager.getStructureSpawns(p_230353_2_, p_230353_3_, p_230353_4_);
++      if (spawns != null) return spawns;
++      if (false) {//Forge: We handle these hardcoded cases above in StructureSpawnManager#getStructureSpawns, but allow for insideOnly to be changed and allow for creatures to be spawned in ones other than just the witch hut
+       if (p_230353_2_.func_235010_a_(p_230353_4_, true, Structure.field_236374_j_).func_75069_d()) {
+          if (p_230353_3_ == EntityClassification.MONSTER) {
+             return Structure.field_236374_j_.func_202279_e();
+@@ -605,6 +608,7 @@
+             return Structure.field_236378_n_.func_202279_e();
+          }
+       }
++      }
+ 
+       return super.func_230353_a_(p_230353_1_, p_230353_2_, p_230353_3_, p_230353_4_);
+    }

patches/minecraft/net/minecraft/world/gen/feature/structure/FortressStructure.java.patch

@@ -0,0 +1,12 @@
+--- a/net/minecraft/world/gen/feature/structure/FortressStructure.java
++++ b/net/minecraft/world/gen/feature/structure/FortressStructure.java
+@@ -30,7 +30,8 @@
+       return FortressStructure.Start::new;
+    }
+ 
+-   public List<MobSpawnInfo.Spawners> func_202279_e() {
++   @Override
++   public List<MobSpawnInfo.Spawners> getDefaultSpawnList() {
+       return field_202381_d;
+    }
+ 

patches/minecraft/net/minecraft/world/gen/feature/structure/OceanMonumentStructure.java.patch

@@ -0,0 +1,12 @@
+--- a/net/minecraft/world/gen/feature/structure/OceanMonumentStructure.java
++++ b/net/minecraft/world/gen/feature/structure/OceanMonumentStructure.java
+@@ -49,7 +49,8 @@
+       return OceanMonumentStructure.Start::new;
+    }
+ 
+-   public List<MobSpawnInfo.Spawners> func_202279_e() {
++   @Override
++   public List<MobSpawnInfo.Spawners> getDefaultSpawnList() {
+       return field_175803_h;
+    }
+ 

patches/minecraft/net/minecraft/world/gen/feature/structure/PillagerOutpostStructure.java.patch

@@ -0,0 +1,12 @@
+--- a/net/minecraft/world/gen/feature/structure/PillagerOutpostStructure.java
++++ b/net/minecraft/world/gen/feature/structure/PillagerOutpostStructure.java
+@@ -19,7 +19,8 @@
+       super(p_i231977_1_, 0, true, true);
+    }
+ 
+-   public List<MobSpawnInfo.Spawners> func_202279_e() {
++   @Override
++   public List<MobSpawnInfo.Spawners> getDefaultSpawnList() {
+       return field_214558_a;
+    }
+ 

patches/minecraft/net/minecraft/world/gen/feature/structure/Structure.java.patch

@@ -5,7 +5,28 @@
  import org.apache.logging.log4j.Logger;
  
 -public abstract class Structure<C extends IFeatureConfig> {
-+public abstract class Structure<C extends IFeatureConfig> extends net.minecraftforge.registries.ForgeRegistryEntry<Structure<?>> {
++public abstract class Structure<C extends IFeatureConfig> extends net.minecraftforge.registries.ForgeRegistryEntry<Structure<?>> implements net.minecraftforge.common.extensions.IForgeStructure {
     public static final BiMap<String, Structure<?>> field_236365_a_ = HashBiMap.create();
     private static final Map<Structure<?>, GenerationStage.Decoration> field_236385_u_ = Maps.newHashMap();
     private static final Logger field_208204_b = LogManager.getLogger();
+@@ -237,13 +237,18 @@
+    }
+ 
+    public List<MobSpawnInfo.Spawners> func_202279_e() {
+-      return ImmutableList.of();
++      return getSpawnList(net.minecraft.entity.EntityClassification.MONSTER);
+    }
+ 
+    public List<MobSpawnInfo.Spawners> func_214469_f() {
+-      return ImmutableList.of();
++      return getSpawnList(net.minecraft.entity.EntityClassification.CREATURE);
+    }
+ 
++   @Override
++   public final List<MobSpawnInfo.Spawners> getSpawnList(net.minecraft.entity.EntityClassification classification) {
++      return net.minecraftforge.common.world.StructureSpawnManager.getSpawnList(getStructure(), classification);
++   }
++
+    public interface IStartFactory<C extends IFeatureConfig> {
+       StructureStart<C> create(Structure<C> p_create_1_, int p_create_2_, int p_create_3_, MutableBoundingBox p_create_4_, int p_create_5_, long p_create_6_);
+    }

patches/minecraft/net/minecraft/world/gen/feature/structure/SwampHutStructure.java.patch

@@ -0,0 +1,18 @@
+--- a/net/minecraft/world/gen/feature/structure/SwampHutStructure.java
++++ b/net/minecraft/world/gen/feature/structure/SwampHutStructure.java
+@@ -24,11 +24,13 @@
+       return SwampHutStructure.Start::new;
+    }
+ 
+-   public List<MobSpawnInfo.Spawners> func_202279_e() {
++   @Override
++   public List<MobSpawnInfo.Spawners> getDefaultSpawnList() {
+       return field_202384_d;
+    }
+ 
+-   public List<MobSpawnInfo.Spawners> func_214469_f() {
++   @Override
++   public List<MobSpawnInfo.Spawners> getDefaultCreatureSpawnList() {
+       return field_214559_aS;
+    }
+ 

src/main/java/net/minecraftforge/common/extensions/IForgeStructure.java

@@ -0,0 +1,73 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.extensions;
+
+import java.util.Collections;
+import java.util.List;
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.world.biome.MobSpawnInfo;
+import net.minecraft.world.gen.feature.structure.Structure;
+
+public interface IForgeStructure
+{
+    default Structure<?> getStructure()
+    {
+        return (Structure<?>) this;
+    }
+
+    /**
+     * Gets the default list of {@link EntityClassification#MONSTER} spawns for this structure.
+     *
+     * @apiNote Implement this over {@link Structure#getSpawnList()}
+     */
+    default List<MobSpawnInfo.Spawners> getDefaultSpawnList()
+    {
+        return Collections.emptyList();
+    }
+
+    /**
+     * Gets the default list of {@link EntityClassification#CREATURE} spawns for this structure.
+     *
+     * @apiNote Implement this over {@link Structure#getCreatureSpawnList()}
+     */
+    default List<MobSpawnInfo.Spawners> getDefaultCreatureSpawnList()
+    {
+        return Collections.emptyList();
+    }
+
+    /**
+     * Gets the default for if entity spawns are restricted to being inside of the pieces making up the structure or if being in the bounds of the overall structure
+     * is good enough.
+     * @return {@code true} if the location to check spawns for has to be inside of the pieces making up the structure, {@code false} otherwise.
+     */
+    default boolean getDefaultRestrictsSpawnsToInside()
+    {
+        //The Pillager Outpost and Ocean Monument check the full structure by default instead of limiting themselves to being within the structure's bounds
+        return getStructure() != Structure.field_236366_b_ && getStructure() != Structure.field_236376_l_;
+    }
+
+    /**
+     * Helper method to get the list of entity spawns for this structure for the given classification.
+     * @param classification The classification of entities.
+     * @apiNote This method is marked as final in {@link Structure} so as to not be overridden by modders and breaking support for
+     * {@link net.minecraftforge.event.world.StructureSpawnListGatherEvent}.
+     */
+    List<MobSpawnInfo.Spawners> getSpawnList(EntityClassification classification);
+}

src/main/java/net/minecraftforge/common/world/StructureSpawnManager.java

@@ -0,0 +1,134 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.world;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import javax.annotation.Nullable;
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.biome.MobSpawnInfo;
+import net.minecraft.world.gen.feature.structure.Structure;
+import net.minecraft.world.gen.feature.structure.StructureManager;
+import net.minecraftforge.common.MinecraftForge;
+import net.minecraftforge.event.world.StructureSpawnListGatherEvent;
+import net.minecraftforge.registries.ForgeRegistries;
+
+/**
+ * Class to help manage entity spawns inside of structures
+ */
+public class StructureSpawnManager
+{
+    private static Map<Structure<?>, StructureSpawnInfo> structuresWithSpawns = Collections.emptyMap();
+
+    /**
+     * Gathers potential entity spawns for all the different registered structures.
+     * @apiNote Internal
+     */
+    public static void gatherEntitySpawns()
+    {
+        //We use a linked hash map to ensure that we check the structures in an order that if there are multiple structures a position satisfies
+        // then we have the same behavior as vanilla as vanilla checks, swamp huts, pillager outposts, ocean monuments, and nether fortresses in
+        // that order.
+        Map<Structure<?>, StructureSpawnInfo> structuresWithSpawns = new LinkedHashMap<>();
+        gatherEntitySpawns(structuresWithSpawns, Structure.field_236374_j_);
+        gatherEntitySpawns(structuresWithSpawns, Structure.field_236366_b_);
+        gatherEntitySpawns(structuresWithSpawns, Structure.field_236376_l_);
+        gatherEntitySpawns(structuresWithSpawns, Structure.field_236378_n_);
+        for (Structure<?> structure : ForgeRegistries.STRUCTURE_FEATURES.getValues())
+        {
+            if (structure != Structure.field_236374_j_ && structure != Structure.field_236366_b_ && structure != Structure.field_236376_l_ &&
+                structure != Structure.field_236378_n_)
+            {
+                //If we didn't already gather the spawns already to ensure we do vanilla ones already
+                // gather the spawns for this structure
+                gatherEntitySpawns(structuresWithSpawns, structure);
+            }
+        }
+        StructureSpawnManager.structuresWithSpawns = structuresWithSpawns;
+    }
+
+    private static void gatherEntitySpawns(Map<Structure<?>, StructureSpawnInfo> structuresWithSpawns, Structure<?> structure)
+    {
+        StructureSpawnListGatherEvent event = new StructureSpawnListGatherEvent(structure);
+        MinecraftForge.EVENT_BUS.post(event);
+        ImmutableMap.Builder<net.minecraft.entity.EntityClassification, List<MobSpawnInfo.Spawners>> builder = ImmutableMap.builder();
+        event.getEntitySpawns().forEach((classification, spawns) -> {
+            if (!spawns.isEmpty())
+                builder.put(classification, ImmutableList.copyOf(spawns));
+        });
+        Map<EntityClassification, List<MobSpawnInfo.Spawners>> entitySpawns = builder.build();
+        if (!entitySpawns.isEmpty())
+            structuresWithSpawns.put(structure, new StructureSpawnInfo(entitySpawns, event.isInsideOnly()));
+    }
+
+    /**
+     * Looks up if a given position is within a structure and returns any entity spawns that structure has for the given classification, or null if
+     * none are found.
+     * @param structureManager Structure Manager, used to check if a position is within a structure.
+     * @param classification   Entity classification
+     * @param pos              Position to get entity spawns of
+     */
+    @Nullable
+    public static List<MobSpawnInfo.Spawners> getStructureSpawns(StructureManager structureManager, EntityClassification classification, BlockPos pos)
+    {
+        for (Entry<Structure<?>, StructureSpawnInfo> entry : structuresWithSpawns.entrySet())
+        {
+            Structure<?> structure = entry.getKey();
+            StructureSpawnInfo spawnInfo = entry.getValue();
+            //Note: We check if the structure has spawns for a type first before looking at the world as it should be a cheaper check
+            if (spawnInfo.spawns.containsKey(classification) && structureManager.func_235010_a_(pos, spawnInfo.insideOnly, structure).isValid())
+                return spawnInfo.spawns.get(classification);
+        }
+        return null;
+    }
+
+    /**
+     * Gets the entity spawn lists for entities of a given classification for a given structure.
+     * @param structure      The Structure
+     * @param classification The classification to lookup
+     */
+    public static List<MobSpawnInfo.Spawners> getSpawnList(Structure<?> structure, EntityClassification classification)
+    {
+        if (structuresWithSpawns.containsKey(structure))
+            return structuresWithSpawns.get(structure).spawns.getOrDefault(classification, Collections.emptyList());
+        return Collections.emptyList();
+    }
+
+    /**
+     * Helper class to keep track of spawns and if the spawns should be restricted to inside the structure pieces.
+     */
+    private static class StructureSpawnInfo
+    {
+        private final Map<EntityClassification, List<MobSpawnInfo.Spawners>> spawns;
+        private final boolean insideOnly;
+
+        private StructureSpawnInfo(Map<EntityClassification, List<MobSpawnInfo.Spawners>> spawns, boolean insideOnly)
+        {
+            this.spawns = spawns;
+            this.insideOnly = insideOnly;
+        }
+    }
+}

src/main/java/net/minecraftforge/event/world/StructureSpawnListGatherEvent.java

@@ -0,0 +1,150 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.event.world;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.world.biome.MobSpawnInfo;
+import net.minecraft.world.gen.feature.structure.Structure;
+import net.minecraftforge.eventbus.api.Event;
+import net.minecraftforge.eventbus.api.EventPriority;
+
+/**
+ * This event fires when a Structure is gathering what mobs/creatures can spawn in it.
+ *
+ * In order to maintain the most compatibility possible with other mods' modifications to a structure,
+ * the event should be assigned a {@link net.minecraftforge.eventbus.api.EventPriority} as follows:
+ *
+ * - Additions : {@link EventPriority#HIGH}
+ * - Removals : {@link EventPriority#NORMAL}
+ * - Any other modification : {@link EventPriority#LOW}
+ *
+ * Be aware that another mod could have done an operation beforehand, so an expected value out of a vanilla structure might not
+ * always be the same, depending on other mods.
+ */
+public class StructureSpawnListGatherEvent extends Event
+{
+
+    private final Structure<?> structure;
+    private final Map<EntityClassification, List<MobSpawnInfo.Spawners>> entitySpawns = new HashMap<>();
+    private final Map<EntityClassification, List<MobSpawnInfo.Spawners>> entitySpawnsUnmodifiableLists = new HashMap<>();
+    private final Map<EntityClassification, List<MobSpawnInfo.Spawners>> entitySpawnsUnmodifiable = Collections.unmodifiableMap(entitySpawnsUnmodifiableLists);
+    private boolean insideOnly;
+
+    public StructureSpawnListGatherEvent(Structure<?> structure)
+    {
+        this.structure = structure;
+        this.insideOnly = this.structure.getDefaultRestrictsSpawnsToInside();
+        addEntitySpawns(EntityClassification.MONSTER, this.structure.getDefaultSpawnList());
+        addEntitySpawns(EntityClassification.CREATURE, this.structure.getDefaultCreatureSpawnList());
+    }
+
+    /**
+     * @return Structure to add or remove spawns to/from.
+     */
+    public Structure<?> getStructure()
+    {
+        return structure;
+    }
+
+    /**
+     * Change if entity spawn location checks are done against the entire bounds of the structure or only the inside the pieces of the structure.
+     * @param insideOnly {@code true} to restrict the spawn checks to inside the pieces of the structure, {@code false} to allow spawns outside
+     */
+    public void setInsideOnly(boolean insideOnly)
+    {
+        this.insideOnly = insideOnly;
+    }
+
+    /**
+     * Checks if spawns for the structure are restricted to being inside the individual pieces of the structure.
+     */
+    public boolean isInsideOnly()
+    {
+        return insideOnly;
+    }
+
+    /**
+     * Gets an unmodifiable view of the the list representing the entity spawns for the given classification.
+     * @param classification Entity Classification
+     * @return The list of spawns for the given classification.
+     */
+    public List<MobSpawnInfo.Spawners> getEntitySpawns(EntityClassification classification)
+    {
+        return this.entitySpawnsUnmodifiableLists.getOrDefault(classification, Collections.emptyList());
+    }
+
+    /**
+     * Gets the internal spawn list for a given entity classification, or adds one if needed. (This includes adding it to the unmodifiable view)
+     */
+    private List<MobSpawnInfo.Spawners> getOrCreateEntitySpawns(EntityClassification classification)
+    {
+        return this.entitySpawns.computeIfAbsent(classification, c -> {
+            List<MobSpawnInfo.Spawners> spawners = new ArrayList<>();
+            //If the classification isn't in entitySpawns yet, also add an unmodifiable view of the list to
+            // the unmodifiable list spawn map
+            this.entitySpawnsUnmodifiableLists.put(c, Collections.unmodifiableList(spawners));
+            return spawners;
+        });
+    }
+
+    /**
+     * Adds a spawn to the list of spawns for the given classification.
+     * @param classification Entity Classification
+     * @param spawner        Spawner
+     */
+    public void addEntitySpawn(EntityClassification classification, MobSpawnInfo.Spawners spawner)
+    {
+        getOrCreateEntitySpawns(classification).add(spawner);
+    }
+
+    /**
+     * Adds spawns to the list of spawns for the given classification.
+     * @param classification Entity Classification
+     * @param spawners       Spawners to add
+     */
+    public void addEntitySpawns(EntityClassification classification, List<MobSpawnInfo.Spawners> spawners)
+    {
+        getOrCreateEntitySpawns(classification).addAll(spawners);
+    }
+
+    /**
+     * Removes a spawn from the list of spawns for the given classification.
+     * @param classification Entity Classification
+     * @param spawner        Spawner
+     */
+    public void removeEntitySpawn(EntityClassification classification, MobSpawnInfo.Spawners spawner)
+    {
+        if (this.entitySpawns.containsKey(classification))
+            this.entitySpawns.get(classification).remove(spawner);
+    }
+
+    /**
+     * Gets an unmodifiable view of the map of spawns based on entity classification that is used to fill in the various spawn lists for the structure.
+     */
+    public Map<EntityClassification, List<MobSpawnInfo.Spawners>> getEntitySpawns()
+    {
+        return entitySpawnsUnmodifiable;
+    }
+}

src/test/java/net/minecraftforge/debug/world/StructureSpawnListGatherEventTest.java

@@ -0,0 +1,59 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.debug.world;
+
+import net.minecraft.entity.EntityClassification;
+import net.minecraft.entity.EntityType;
+import net.minecraft.world.biome.MobSpawnInfo;
+import net.minecraft.world.gen.feature.structure.Structure;
+import net.minecraftforge.common.MinecraftForge;
+import net.minecraftforge.event.world.StructureSpawnListGatherEvent;
+import net.minecraftforge.eventbus.api.EventPriority;
+import net.minecraftforge.fml.common.Mod;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+@Mod(StructureSpawnListGatherEventTest.MODID)
+public class StructureSpawnListGatherEventTest
+{
+
+    public static final String MODID = "structure_spawn_list_event_test";
+    private static final Logger LOGGER = LogManager.getLogger(MODID);
+
+    public StructureSpawnListGatherEventTest()
+    {
+            MinecraftForge.EVENT_BUS.addListener(EventPriority.HIGH, this::onStructureSpawnListGather);
+    }
+
+    private void onStructureSpawnListGather(StructureSpawnListGatherEvent event)
+    {
+        if (event.getStructure() == Structure.field_236375_k_)
+        {
+            event.addEntitySpawn(EntityClassification.MONSTER, new MobSpawnInfo.Spawners(EntityType.WITHER_SKELETON, 100, 5, 15));
+            LOGGER.info("Adding wither skeleton spawns to strong holds");
+        }
+        else if (event.getStructure() == Structure.field_236373_i_)
+        {
+            event.setInsideOnly(false);
+            event.addEntitySpawn(EntityClassification.MONSTER, new MobSpawnInfo.Spawners(EntityType.GUARDIAN, 100, 5, 15));
+            LOGGER.info("Adding guardians spawns to shipwrecks");
+        }
+    }
+}

src/test/resources/META-INF/mods.toml

@@ -87,4 +87,6 @@ license="LGPL v2.1"
 [[mods]]
     modId="custom_tag_types_test"
 [[mods]]
-    modId="biome_loading_event_test"
+    modId="biome_loading_event_test"
+[[mods]]
+    modId="structure_spawn_list_event_test"