For JEP 12 (https://openjdk.org/jeps/12) the concept of a preview API may require:

""a new and cohesive set of modules, packages, classes, interfaces, methods, constructors, and fields in the java.* or javax.* namespace""

Until now it was not possible to deliver alternate versions of existing classes packaged in the jimage modules files. However for Valhalla (and in future, other preview features) the is a need to supply such classes (e.g. alternate value class bytecode for Valhalla).

Valhalla was previously using module patching to achieve this, but now uses a new "preview mode" mechanism which can now be integrated back into mainline JDK, without any Valhalla specific details.

The changes for implementing this can be split into 4 parts:
1. Implement jimage support (Java & C++)
2. Implement exploded image support (Java & C++)
3. JRT file-system support (Java)
4. Plumbing of flags to control "preview mode" in the above (Java & C++)

Valhalla specific changes, such as the compilation and inclusion of value classes will be left until the eventual Valhalla merge.