There are a few issues here:
1. LoadersConfigurationBuilder stores individual loader builders in a List<LoaderConfigurationBuilder>, but that list is never instantiated, thus NPEs
2. LoaderConfigurationBuilder stores properties in a Properties object, but that too is never instantiated. Thus one must call withProperties(Properties) before calling addProperty(..., ...).
3. The LoaderConfiguration constructor, via the AbstractTypedPropertiesConfiguration constructor, calls Immutables.immutableTypedPropreties(...) on the properties object. However, this method always fails for a non-empty properties object, since the ImmutableTypedProperties constructor tries to call ImmutableTypedProperties.putAll(...) which throws an UnsupportedOperationException. I'm guess this issue affects more than just the loaders, but probably any configuration object that uses generic properties.