Vert.x

Vert.x is an excellent framework to build reactive applications that do not block resources (threads) and hence are very scalable and elastic, and hence resilient. It is a relatively new kid on the block (with major upgrades in version 3.0).

However, its Reactive programming model is not very popular in the industry yet and hence it is just gaining adoption, especially for use cases that require very high resiliency and scalability.