As the demand for UDTs increases beyond sparse/dense vectors in MLlib, it would be nice to make the UDT API public in 1.5.
Expose UserDefinedType make sure could extends it
Fixes and enhancements addressing UDTs' api and several usability concerns
Hide UserDefinedType in Spark 2.0
[Github] Pull Request #16478 (viirya)
GitHub Pull Request #16478