We try and log a message if we detect a world leak: it's probably not infallible, but it should

help mod developers- if you see this when testing your mod with, say, mystcraft, you're probably
keeping a hold of an invalid handle to the World (either directly, or indirectly via Entity or TileEntity)
and you should look to refactor to wrap those handles in WeakReferences

common/net/minecraftforge/common/DimensionManager.java

@@ -3,15 +3,23 @@ package net.minecraftforge.common;
 import java.io.File;
 import java.util.ArrayList;
 import java.util.BitSet;
+import java.util.HashSet;
 import java.util.Hashtable;
+import java.util.List;
+import java.util.ListIterator;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Set;
+import java.util.concurrent.ConcurrentMap;
 import java.util.logging.Level;
 
 import com.google.common.collect.ArrayListMultimap;
 import com.google.common.collect.ImmutableListMultimap;
 import com.google.common.collect.ListMultimap;
+import com.google.common.collect.Lists;
+import com.google.common.collect.MapMaker;
 import com.google.common.collect.Maps;
+import com.google.common.collect.Sets;
 
 import cpw.mods.fml.common.FMLCommonHandler;
 import cpw.mods.fml.common.FMLLog;
@@ -40,9 +48,10 @@ public class DimensionManager
     private static Hashtable<Integer, WorldServer> worlds = new Hashtable<Integer, WorldServer>();
     private static boolean hasInit = false;
     private static Hashtable<Integer, Integer> dimensions = new Hashtable<Integer, Integer>();
-    private static Map<World, ListMultimap<ChunkCoordIntPair, String>> persistentChunkStore = Maps.newHashMap(); //FIXME: Unused?
     private static ArrayList<Integer> unloadQueue = new ArrayList<Integer>();
     private static BitSet dimensionMap = new BitSet(Long.SIZE << 4);
+    private static ConcurrentMap<World, World> weakWorldMap = new MapMaker().weakKeys().weakValues().<World,World>makeMap();
+    private static Set<Integer> leakedWorlds = Sets.newHashSet();
 
     public static boolean registerProviderType(int id, Class<? extends WorldProvider> provider, boolean keepLoaded)
     {
@@ -115,6 +124,34 @@ public class DimensionManager
         return getWorld(dim).provider;
     }
 
+    public static Integer[] getIDs(boolean check)
+    {
+        if (check)
+        {
+            List<World> allWorlds = Lists.newArrayList(weakWorldMap.keySet());
+            allWorlds.removeAll(worlds.values());
+            Set<Integer> newLeaks = Sets.newHashSet();
+            for (ListIterator<World> li = allWorlds.listIterator(); li.hasNext(); )
+            {
+                World w = li.next();
+                if (leakedWorlds.contains(System.identityHashCode(w)))
+                {
+                    li.remove();
+                }
+                newLeaks.add(System.identityHashCode(w));
+            }
+            leakedWorlds = newLeaks;
+            if (allWorlds.size() > 0)
+            {
+                FMLLog.severe("Detected leaking worlds in memory. There are %d worlds that appear to be persisting. A mod is likely caching the world incorrectly\n", allWorlds.size() + leakedWorlds.size());
+                for (World w : allWorlds)
+                {
+                    FMLLog.severe("The world %x (%s) has leaked.\n", System.identityHashCode(w), w.getWorldInfo().getWorldName());
+                }
+            }
+        }
+        return getIDs();
+    }
     public static Integer[] getIDs()
     {
         return worlds.keySet().toArray(new Integer[worlds.size()]); //Only loaded dims, since usually used to cycle through loaded worlds
@@ -124,6 +161,7 @@ public class DimensionManager
     {
         if (world != null) {
             worlds.put(id, world);
+            weakWorldMap.put(world, world);
             MinecraftServer.getServer().worldTickTimes.put(id, new long[100]);
             FMLLog.info("Loading dimension %d (%s) (%s)", id, world.getWorldInfo().getWorldName(), world.getMinecraftServer());
         } else {

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

@@ -117,7 +117,7 @@
  
 -        for (var1 = 0; var1 < this.worldServers.length; ++var1)
 -        {
-+        Integer[] ids = DimensionManager.getIDs();
++        Integer[] ids = DimensionManager.getIDs(this.tickCounter % 200 == 0);
 +        for (int x = 0; x < ids.length; x++)
 +        {
 +            int id = ids[x];