CrossWorlds is a feature extension to IBM Websphere Liberty that gives direct Java access to a Domino server or Client. It requires:
IBM Websphere Liberty
IBM Websphere Liberty is a lightweight Java EE server, analagous to Apache Tomcat. It is free for development or test use, as well as for limites production use. It can be installed and configured in Eclipse via a plugin available in the Eclipse Marketplace. It can be set up within a few minutes. Websphere Liberty supports Java 8, unlike full Websphere or Domino1.
Domino is a very powerful server. Amongst many other features, it provides:
These are all available out of the box! Domino is regularly under-valued and compared pejoratively to other products that offer better options for part of what Domino provides. But never do they compare all features.
On top of this, the OpenNTF Domino API (ODA) provides:
CrossWorlds allows you to use a modern Java EE web server instead of Domino's HTTP server. This means servlets coded to Servlet 3.0 specification and standard web applications, without the need to wrap the application in a plugin. It also means being able to leverage Java 1.8, which is currently not available in Domino 9.0.1. But you still have access to everything else that's useful. Native access.
Think of it like preferring to store your data in a SQL database rather than an NSF, and using JDBC access to the data from XPages. Or like the recent work on adding a node.js or vert.x engine on top of Domino. You're swapping out one element of the stack, but still able to leverage the rest of the stack. With CrossWorlds, you're replacing the HTTP server with a newer flavour, but keeping everything else - including years worth of critical data. And why pay to migrate that data when you don't have to (and the business don't have time to review it!)