The following AppCDS tests rely on the non-public ClassLoader.defineClass to load the class.
    test/hotspot/jtreg/runtime/cds/appcds/RewriteBytecodesTest.java
    test/hotspot/jtreg/runtime/cds/appcds/customLoader/LoaderSegregationTest.java

The tests will break with the following exception once java.base is fully encapsulated at run-time:

Exception in thread "main" java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @4a574795
        at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:349)
        at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:289)
        at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:196)
        at java.base/java.lang.reflect.Method.setAccessible(Method.java:190)
        at Util.defineClassFromJAR(Util.java:139)
        at Util.defineClassFromJAR(Util.java:111)
        at LoaderSegregation.main(LoaderSegregation.java:85)

The tests are working currently with the following warning because they have the --add-opens java.base/java.lang=ALL-UNNAMED set.

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by Util (file:/scratch/jtreg_test/JTwork/classes/0/runtime/cds/appcds/customLoader/LoaderSegregationTest.d/LoaderSegregation_app.jar) to method java.lang.ClassLoader.defineClass(byte[],int,int)
WARNING: Please consider reporting this to the maintainers of Util
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

The test should be fixed to use Lookup.defineClass.