The 3.1-SNAPSHOT and 4.0-SNAPSHOT are the unreleased versions of GeoAPI currently in development. Developers working on projects who need long term stability should use the released 3.0.0 version of the API rather than the snapshots, formalized with the OGC 09-083 specification document. The 3.1-SNAPSHOT version contains the same interfaces than 4.0-SNAPSHOT without the incompatible changes.
Those milestones require a Java environment Java 7 or higher. Nightly builds can be obtained from the Jenkins automated build tool's output page.
A milestone is created on a regular basis. API changes compared to the previous versions are documented in the changes page.