@EventHandler
public void serverStopped(FMLServerStoppedEvent evt)
{
    Set<IWorldGenerator> worldGens = ObfuscationReflectionHelper.getPrivateValue(GameRegistry.class, null, "worldGenerators");
    Map<IWorldGenerator,Integer> worldGenIdx = ObfuscationReflectionHelper.getPrivateValue(GameRegistry.class, null, "worldGeneratorIndex");

    for (TargetWorldWrapper tww : delegates.values())
    {
        worldGens.remove(tww);
        Integer idx = worldGenIdx.remove(tww);
        worldGens.add(tww.delegate);
        worldGenIdx.put(tww.delegate,idx);
    }

    delegates.clear();
}
 

@Mod.EventHandler
public void preinit(FMLPreInitializationEvent init)
{
    final Logger modLog = init.getModLog();
    IWorldGenerator gen = new IWorldGenerator() {
        @Override
        public void generate(Random random, int chunkX, int chunkZ, World world, IChunkGenerator chunkGenerator, IChunkProvider chunkProvider)
        {
            modLog.log(Level.INFO, "Calling!");
        }
    };
    GameRegistry.registerWorldGenerator(gen, 10);
}
 

@EventHandler
public void serverAboutToStart(FMLServerAboutToStartEvent evt)
{
    this.pendingWork = new MapMaker().weakKeys().makeMap();
    this.completedWork = new MapMaker().weakKeys().makeMap();
    this.completedWorkLocks = new MapMaker().weakKeys().makeMap();

    Set<IWorldGenerator> worldGens = ObfuscationReflectionHelper.getPrivateValue(GameRegistry.class, null, "worldGenerators");
    Map<IWorldGenerator,Integer> worldGenIdx = ObfuscationReflectionHelper.getPrivateValue(GameRegistry.class, null, "worldGeneratorIndex");

    for (String retro : ImmutableSet.copyOf(retros.keySet()))
    {
        if (!delegates.containsKey(retro))
        {
            FMLLog.info("Substituting worldgenerator %s with delegate", retro);
            for (Iterator<IWorldGenerator> iterator = worldGens.iterator(); iterator.hasNext();)
            {
                IWorldGenerator wg = iterator.next();
                if (wg.getClass().getName().equals(retro))
                {
                    iterator.remove();
                    TargetWorldWrapper tww = new TargetWorldWrapper();
                    tww.delegate = wg;
                    tww.tag = retro;
                    worldGens.add(tww);
                    Integer idx = worldGenIdx.remove(wg);
                    worldGenIdx.put(tww, idx);
                    FMLLog.info("Successfully substituted %s with delegate", retro);
                    delegates.put(retro, tww);
                    break;
                }
            }

            if (!delegates.containsKey(retro))
            {
                FMLLog.warning("WorldRetrogen was not able to locate world generator class %s, it will be skipped, found %s", retro, worldGens);
                retros.remove(retro);
            }
        }
    }
}