JEP 181: Nest-Based Access Control

RoboVM stopped to be Java7+ runtime (Android 4.4 to be exact). However it doesn’t stop users to compile it against more recent Java like java 11.
It works with some amount of constraints but sometime changes are breaking. Like JEP181 that causes java8 scenarios doesn’t work anymore with RoboVM (issue #852).
Long story short: with JEP181 changes compiler doesn’t generate accessors to private fields for nested classes. And RoboVM truly produces IllegalAccessError: Attempt to access private method/variable.
Here is a sample code to reproduce:

public class Test {
    private int a;
    public void foo() {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                System.out.println(a);
            }
        };
        r.run();
    }
}

Instead, Java11+ compiler generates NestHost and NestMembers attributes for classes:

• NestHost specifies host class for nested one;
• NestMembers of host class specifies nested members.

And up to JEP181:

A field or method R is accessible to a class or interface D if 
and only if any of the following conditions are true:  
  * ...  
  *  R is private and is declared in a different class or
     interface C, and C and D, are nestmates.
For types C and D to be nestmates they must have the same nest host. 
A type C claims to be a member of the nest hosted by D, if it lists D 
in its NestHost attribute. The membership is validated if D also 
lists C in its NestMembers attribute. D is implicitly a member of 
the nest that it hosts.
A class with no NestHost or NestMembers attribute, implicitly forms 
a nest with itself as the nest host, and sole nest member.

Things sound like cpp friend classes now…

Compiler time fix

To support JEP181 during compilation it is enough to check NestHost and NestMembers attributes as described.

Limitation

Beside static case (compilation time) it shall work with runtime/regression as well. This to be done as part2, probably in scope of Libcore10 experiment where corresponding API is exposed.

Code

• Changes proposed to Soot #3 to expose NestHost and NestMembers attributes.
• RoboVM changes proposed as PR653.
• Both there already available in RoboVMx/Libcore10 branch and subject for build3.

Happy coding and please open an issue if any.