JDK17 compatibility

Problem

JEP 403 targeted for JDK 17 (release planned for Sept 2021) will remove --illegal-access flag. That will be equivalent to --illegal-access=deny.

Running with --illegal-access=warn apps using jackson-databind and other jackson modules reported here https://github.com/FasterXML/jackson-databind/issues/2856#issuecomment-700581797 can discover issues like WARNING: Illegal reflective access by com.fasterxml.jackson.databind.util.ClassUtil

Workaround

For illegal reflective access pointing to jackson use –add-open for the package containing the restricted field.

Update Not related to jackson library, since JDK 9 a number of standard replacements are provided for the most commonly used internal elements of the JDK.

[JJBRT]

Please take a look at my new article that explain how to export all modules to all modules at runtime in Java 16 and later without using any JVM parameter

[cowtowncoder]

Unfortunately most if not all remaining issues that surface with that rather non-descriptive warning are due to dynamic introspection for various JDK types and depend on actual usage. Jackson core itself does not force access to JDK, but to access fields, getters, setters, constructors, introspection is used. ClassUtil specifically gets called to try to force access when it is deemed necessary based on types user tries to serialize/deserialize; it has no “agenda of its known”.

So unfortunately this is not as easy as (for example) replacing use of sun.misc.Unsafe with alternatives.

So what tends to be necessary are user reports for specific usage that triggers these issues: this helps figure out how to prevent specific problems. I am not confident there is a general simple solution; although more and more general improvements can be developed when finding patterns common to reported problems.