jtreg @modules specifies $MODULE[/$PACKAGE] that a test depends on:

1. for test selection

jtreg can determine if a test should be selected when running on an image with a limited set of modules. This replaces the adhoc mechanism "needs_jre", "needs_charsets" etc TEST.group mechanism for testing compact profiles.

2. break encapsulation for tests to access JDK internal APIs for testing purpose.

Right now, one has to run the tests with --limit-modules in order to verify @modules is correct but it won't find if any dependence is stale.

It's useful to extend jdeps to list the module dependences and the internal APIs to aid test development. jtreg can also make use of it to validate if @modules is properly specified and catch issues.