Prevent worlds from unloading while players are in out-of-world states by tracking all worlds references by every player in the server's player list.

src/main/java/net/minecraftforge/common/DimensionManager.java

@@ -23,8 +23,10 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.ConcurrentMap;
 import java.util.stream.Collectors;
 
@@ -45,6 +47,7 @@ import net.minecraft.nbt.CompoundNBT;
 import net.minecraft.nbt.ListNBT;
 import net.minecraft.network.PacketBuffer;
 import net.minecraft.server.MinecraftServer;
+import net.minecraft.server.management.PlayerList;
 import net.minecraft.util.ResourceLocation;
 import net.minecraft.util.math.ChunkPos;
 import net.minecraft.util.registry.MutableRegistry;
@@ -80,6 +83,7 @@ public class DimensionManager
     private static final ConcurrentMap<World, World> weakWorldMap = new MapMaker().weakKeys().weakValues().makeMap();
     private static final Multiset<Integer> leakedWorlds = HashMultiset.create();
     private static final Map<ResourceLocation, SavedEntry> savedEntries = new HashMap<>();
+    private static volatile Set<World> playerWorlds = new HashSet<>();
 
     /**
      * Registers a real unique dimension, Should be called on server init, or when the dimension is created.
@@ -144,7 +148,7 @@ public class DimensionManager
     /**
      * Retrieves the world from the server allowing for null return, and optionally resetting it's unload timer.
      *
-     * @param server The server that controlls this world.
+     * @param server The server that controls this world.
      * @param dim Dimension to load.
      * @param resetUnloadDelay True to reset the unload timer, which is a delay that is used to prevent constant world loading/unloading cycle.
      * @param forceLoad True to attempt to load the dimension if the server has it unloaded.
@@ -222,7 +226,8 @@ public class DimensionManager
         return world.getDimension().getType() != DimensionType.OVERWORLD
                 && world.getPlayers().isEmpty()
                 && world.getForcedChunks().isEmpty()
-                && !getData(world.getDimension().getType()).keepLoaded;
+                && !getData(world.getDimension().getType()).keepLoaded
+                && !playerWorlds.contains(world);
     }
 
     /**
@@ -484,4 +489,10 @@ public class DimensionManager
         @Override public void statusChanged(ChunkPos p_219508_1_, ChunkStatus p_219508_2_) { }
         @Override public void stop() { }
     }
+
+    public static boolean rebuildPlayerMap(PlayerList players, boolean changed)
+    {
+        playerWorlds = players.getPlayers().stream().map(e -> e.world).collect(Collectors.toSet());
+        return changed;
+    }
 }

src/main/java/net/minecraftforge/common/ForgeInternalHandler.java

@@ -21,12 +21,14 @@ package net.minecraftforge.common;
 
 import net.minecraft.entity.Entity;
 import net.minecraft.entity.item.ItemEntity;
+import net.minecraft.entity.player.ServerPlayerEntity;
 import net.minecraft.item.Item;
 import net.minecraft.item.ItemStack;
 import net.minecraft.world.server.ServerWorld;
 import net.minecraftforge.client.CloudRenderer;
 import net.minecraftforge.common.util.FakePlayerFactory;
 import net.minecraftforge.event.entity.EntityJoinWorldEvent;
+import net.minecraftforge.event.entity.player.PlayerEvent;
 import net.minecraftforge.event.world.ChunkEvent;
 import net.minecraftforge.event.world.WorldEvent;
 import net.minecraftforge.eventbus.api.EventPriority;
@@ -46,18 +48,16 @@ public class ForgeInternalHandler
         {
             ItemStack stack = ((ItemEntity)entity).getItem();
             Item item = stack.getItem();
-/*
             if (item.hasCustomEntity(stack))
             {
                 Entity newEntity = item.createEntity(event.getWorld(), entity, stack);
                 if (newEntity != null)
                 {
-                    entity.setDead();
+                    entity.remove();
                     event.setCanceled(true);
-                    event.getWorld().spawnEntity(newEntity);
+                    event.getWorld().addEntity(newEntity);
                 }
             }
-*/
         }
     }
 
@@ -88,5 +88,18 @@ public class ForgeInternalHandler
         if (!event.getWorld().isRemote())
             FarmlandWaterManager.removeTickets(event.getChunk());
     }
+
+    @SubscribeEvent
+    public void playerChangeDimension(PlayerEvent.PlayerChangedDimensionEvent event)
+    {
+        if (event.getPlayer() instanceof ServerPlayerEntity)
+            DimensionManager.rebuildPlayerMap(((ServerPlayerEntity)event.getPlayer()).server.getPlayerList(), true);
+    }
+
+    @SubscribeEvent
+    public void playerLogin(PlayerEvent.PlayerLoggedInEvent event)
+    {
+        UsernameCache.setUsername(event.getPlayer().getUniqueID(), event.getPlayer().getGameProfile().getName());
+    }
 }
 

src/main/java/net/minecraftforge/common/ForgeMod.java

@@ -49,7 +49,6 @@ import net.minecraftforge.common.data.ForgeItemTagsProvider;
 import net.minecraftforge.common.data.ForgeRecipeProvider;
 import net.minecraftforge.common.model.animation.CapabilityAnimation;
 import net.minecraftforge.energy.CapabilityEnergy;
-import net.minecraftforge.event.entity.player.PlayerEvent;
 import net.minecraftforge.fml.event.lifecycle.GatherDataEvent;
 import net.minecraftforge.fluids.capability.CapabilityFluidHandler;
 import net.minecraftforge.items.CapabilityItemHandler;
@@ -93,7 +92,6 @@ public class ForgeMod implements WorldPersistenceHooks.WorldPersistenceHook
         modEventBus.addListener(this::preInit);
         modEventBus.addListener(this::gatherData);
         MinecraftForge.EVENT_BUS.addListener(this::serverStarting);
-        MinecraftForge.EVENT_BUS.addListener(this::playerLogin);
         MinecraftForge.EVENT_BUS.addListener(this::serverStopping);
         ModLoadingContext.get().registerConfig(ModConfig.Type.CLIENT, ForgeConfig.clientSpec);
         ModLoadingContext.get().registerConfig(ModConfig.Type.SERVER, ForgeConfig.serverSpec);
@@ -103,12 +101,6 @@ public class ForgeMod implements WorldPersistenceHooks.WorldPersistenceHook
         StartupMessageManager.addModMessage("Forge version "+ForgeVersion.getVersion());
     }
 
-    public void playerLogin(PlayerEvent.PlayerLoggedInEvent event)
-    {
-        UsernameCache.setUsername(event.getPlayer().getUniqueID(), event.getPlayer().getGameProfile().getName());
-    }
-
-
     public void preInit(FMLCommonSetupEvent evt)
     {
         CapabilityItemHandler.register();