See the mail thread beginning here:
http://mail.openjdk.java.net/pipermail/compiler-dev/2018-March/011736.html

Right now, the format of the data in ct.sym, used to support --release, is not formally specified, and the data is only available indirectly via the unsupported internal PlatformProvider API, or within a compilation.

It would generally be useful to other tools to be able to configure a file manager to be able to return the data in ct.sym as the platform or system classes (i.e. StandardLocation.PLATFORM_CLASS_PATH, StandardLocation.SYSTEM)

This would be in line with the file manager supporting access to multi-release jars as well.