Paper/nms-patches/EntityTypes.patch

--- a/net/minecraft/server/EntityTypes.java
+++ b/net/minecraft/server/EntityTypes.java
@@ -148,10 +148,16 @@
 
     @Nullable
     public T a(World world, @Nullable NBTTagCompound nbttagcompound, @Nullable IChatBaseComponent ichatbasecomponent, @Nullable EntityHuman entityhuman, BlockPosition blockposition, boolean flag, boolean flag1) {
+        // CraftBukkit start
+        return spawnCreature(world, nbttagcompound, ichatbasecomponent, entityhuman, blockposition, flag, flag1, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.SPAWNER_EGG);
+    }
+
+    @Nullable
+    public T spawnCreature(World world, @Nullable NBTTagCompound nbttagcompound, @Nullable IChatBaseComponent ichatbasecomponent, @Nullable EntityHuman entityhuman, BlockPosition blockposition, boolean flag, boolean flag1, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason) {
         Entity entity = this.b(world, nbttagcompound, ichatbasecomponent, entityhuman, blockposition, flag, flag1);
 
-        world.addEntity(entity);
-        return entity;
+        return world.addEntity(entity, spawnReason) ? (T) entity : null; // Don't return an entity when CreatureSpawnEvent is canceled
+        // CraftBukkit end
     }
 
     @Nullable
@@ -185,7 +191,7 @@
             }
 
             a(world, entityhuman, entity, nbttagcompound);
-            return entity;
+            return (T) entity; // CraftBukkit - decompile error
         }
     }
 
@@ -240,7 +246,7 @@
 
     @Nullable
     public T a(World world) {
-        return (Entity) this.aU.apply(world);
+        return this.aU.apply(world); // CraftBukkit - decompile error
     }
 
     @Nullable