We do not yet offer an official asynchronous Java SDK. However, there is an in-progress Scala plugin and Play sample application that demonstrate how you can use our Java SDK (which is blocking) for non-blocking operations by wrapping the interaction in a future.
Scala Plugin: https://github.com/stormpath/stormpath-scala/tree/dev
Sample Play app: https://github.com/stormpath/stormpath-play-sample/tree/dev
The plugin provides an asynchronous authentication mechanism, returning a future to collect the authentication result in a non-blocking way. The sample application shows how to use the plugin and how to update an Accounts customData resource, also in a non-blocking fashion.
We hope they provide a solid starting point for developers working in Scala! However, it is important to note that neither project is complete, or ready for production.