Across Framework helps Java developers build modular web applications using Spring.
Across Platform is collection of modules for bootstrapping your web application.
For development of Across itself, start with the instructions in the Maintenance section.
5.5.1 upgrades Spring Framework and Spring Security to the very last (public) 5.y.z releases. The Foreach Common Java Libraries are upgraded to 2.0.0.
The Foreach Common Java Libraries
(FCJL)
are a set of small libraries, with limited dependencies and limited
scope. Some of these libraries are used by Across, and had to be
updated for Jakarta EE (a requirement for Spring Boot 3 and thus for
Across 6). We made 2.0 and 3.0 releases, and a new module
common-file-manager was added.
Despite being a minor release, 5.5 is definitely not backward compatible. You will almost certainly have to make adaptations w.r.t. Spring Security, and thoroughly test that.
On the bright side, the upgrade to Spring Boot 2.7 means you can upgrade to Java 21 (at least in theory, it has not been tested yet).