CraftServer.java

class CraftServer {
    /** etc */
    
    public boolean registerPlayerMetadataProvider(String metadataKey, MetadataProvider<OfflinePlayer> provider) {
        return playerMetadata.registerProvider(metadataKey, provider);
    }

    public boolean registerWorldMetadataProvider(String metadataKey, MetadataProvider<World> provider) {
        return worldMetadata.registerProvider(metadataKey, provider);
    }

    public boolean registerEntityMetadataProvider(String metadataKey, MetadataProvider<Entity> provider) {
        return entityMetadata.registerProvider(metadataKey, provider);
    }
}

CraftWorld.java

class CraftWorld {
    /** etc */
    public boolean registerBlockMetadataProvider(String metadataKey, MetadataProvider<Block> provider) {
        return blockMetadata.registerProvider(metadataKey, provider);
    }
}

ProposedGenericRegistrationCraftServer.java

class CraftServer {
    /** etc */
    
    public <T> boolean registerMetadataProvider(String metadataKey, MetadataProvider<T> provider, Class<T> clazz) {
        if (OfflinePlayer.class.isAssignableFrom(clazz)) {
            return playerMetadata.registerProvider(metadataKey, (MetadataProvider<OfflinePlayer>) provider);
        } else if (Entity.class.isAssignableFrom(clazz)) {
            return entityMetadata.registerProvider(metadataKey, (MetadataProvider<Entity>) provider);
        } else if (World.class.isAssignableFrom(clazz)) {
            return worldMetadata.registerProvider(metadataKey, (MetadataProvider<World>) provider);
        } else if (Block.class.isAssignableFrom(clazz)) {
            // TODO: how do we handle the block Case, since there's a storage per-world?
            return false;
        } else {
            throw new UnsupportedOperationException("Cannot determine type of metadata provider.");
        }
    }
}