The reference implementation of the Spine runtime, the code that plays back animations exported from Spine, is written in Java. However, it has a dependency on libGDX, a popular Java game development framework. You'd have to remove those dependencies from the code (should be easy), then add a custom renderer for JavaFX. We currently have no plans for a dedicated JavaFX runtime due to the non-popularity of JavaFX for game development (despite it's potentially nice features).
