Implements a World UID.

src/main/java/net/minecraft/server/Entity.java

@@ -910,7 +910,7 @@ public abstract class Entity {
         nbttagcompound.a("Air", (short) this.airTicks);
         nbttagcompound.a("OnGround", this.onGround);
         // CraftBukkit start
-        nbttagcompound.setString("World", this.world.worldData.name);
+        nbttagcompound.setLong("WorldUID", this.world.worldData.getWorldUID());
         nbttagcompound.setLong("UUIDLeast", this.uniqueId.getLeastSignificantBits());
         nbttagcompound.setLong("UUIDMost", this.uniqueId.getMostSignificantBits());
         // CraftBukkit end
@@ -982,18 +982,25 @@ public abstract class Entity {
         org.bukkit.Server server = Bukkit.getServer();
         org.bukkit.World bworld = null;
 
+        // TODO: Remove World related checks, replaced with WorldUID.
         if (this instanceof EntityPlayer) {
             EntityPlayer entityPlayer = (EntityPlayer) this;
             String worldName = nbttagcompound.getString("World");
 
-            if (worldName == "") {
+            if (nbttagcompound.hasKey("WorldUID")) {
+                bworld = server.getWorld(nbttagcompound.getLong("WorldUID"));
+            } else if ("".equals(worldName)) {
                 bworld = ((org.bukkit.craftbukkit.CraftServer) server).getServer().getWorldServer(entityPlayer.dimension).getWorld();
             } else {
                 bworld = server.getWorld(worldName);
             }
+        } else {
+            if (nbttagcompound.hasKey("WorldUID")) {
+                bworld = server.getWorld(nbttagcompound.getLong("WorldUID"));
             } else {
                 bworld = server.getWorld(nbttagcompound.getString("World"));
             }
+        }
 
         this.spawnIn(bworld == null ? null : ((org.bukkit.craftbukkit.CraftWorld) bworld).getHandle());
         // CraftBukkit end

src/main/java/net/minecraft/server/WorldData.java

@@ -19,6 +19,7 @@ public class WorldData {
     private int m;
     private boolean n;
     private int o;
+    private long worldUID; // CraftBukkit
 
     public WorldData(NBTTagCompound nbttagcompound) {
         this.a = nbttagcompound.getLong("RandomSeed");
@@ -38,11 +39,20 @@ public class WorldData {
             this.h = nbttagcompound.k("Player");
             this.i = this.h.e("Dimension");
         }
+        // CraftBukkit start
+        if (nbttagcompound.hasKey("WorldUID")) {
+            this.worldUID = nbttagcompound.getLong("WorldUID");
+        } else {
+            this.worldUID = (System.nanoTime() << 20) + this.a;
+            nbttagcompound.setLong("WorldUID", this.worldUID);
+        }
+        // CraftBukkit end
     }
 
     public WorldData(long i, String s) {
         this.a = i;
         this.name = s;
+        this.worldUID = (System.nanoTime() << 20) + this.a; // CraftBukkit
     }
 
     public WorldData(WorldData worlddata) {
@@ -105,6 +115,7 @@ public class WorldData {
         if (nbttagcompound1 != null) {
             nbttagcompound.a("Player", nbttagcompound1);
         }
+        nbttagcompound.setLong("WorldUID", this.worldUID); // CraftBukkit
     }
 
     public long getSeed() {
@@ -192,4 +203,10 @@ public class WorldData {
     public void setWeatherDuration(int i) {
         this.m = i;
     }
+
+    // CraftBukkit start
+    public long getWorldUID() {
+        return this.worldUID;
+    }
+    // CraftBukkit end
 }

src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -490,6 +490,16 @@ public final class CraftServer implements Server {
         return worlds.get(name.toLowerCase());
     }
 
+    public World getWorld(long uid) {
+        for (String worldName : worlds.keySet()) {
+            org.bukkit.World world = worlds.get(worldName);
+            if (world.getUID() == uid) {
+                return world;
+            }
+        }
+        return null;
+    }
+
     public void addWorld(World world) {
         worlds.put(world.getName().toLowerCase(), world);
     }

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -395,10 +395,15 @@ public class CraftWorld implements World {
         return world.worldData.name;
     }
 
+    @Deprecated
     public long getId() {
         return world.worldData.getSeed();
     }
 
+    public long getUID() {
+        return world.worldData.getWorldUID();
+    }
+
     @Override
     public String toString() {
         return "CraftWorld{name=" + getName() + '}';