MethodHandle accessing private method of outer class

[David Holmes]

On 1/04/2018 2:47 AM, Jochen Theodorou wrote:
> Hi all,
> 
> I just noticed something I would like to know if that is correct and as 
> specified (before I start using that in my application logic) using java 
> 9.0.4
> 
>> public class Application {
>>   public Lookup foo() {
>>     Supplier<Lookup> s = new Supplier<Lookup>() {
>>       public Lookup get() { return MethodHandles.lookup); }
>>     }
>>     return s.get();
>>   }
>> }
> 
> what I noticed is that the lookup object returned by the call to foo is 
> one with full access rights for the inner class of Application, but it 
> seems I am also having full access rights on Application itself. I can 
> for example call a private method in Application using a handle created 
> using this lookup object.
> 
> Is this really as intended? I am asking because Java would realize this 
> afaik with a helper method (at least in the past).
> Was this always possible with MethodHandles, or did this change?

The Java language has always permitted this. But at the source level it 
is achieved through helper methods as the VM does not allow it (yet). 
Core reflection does not allow it (but by spec should). But 
MethodHandles went to a lot of trouble to allow the same accesses as the 
language permits (though sometimes needing to use special mechanisms 
like Lookup.in). I can't say for sure MH has "always" allowed this 
(sometimes the implementation has lagged when it comes to private method 
access, particularly when private interface methods were introduced) - 
though it would be easy to check.

Going forward, for JDK 11 we expect JEP 181 "Nestmates" to be in place, 
which implements the language access semantics consistently across the 
VM, MH and core reflection. So yes a nested type has full access to its 
enclosing types, and full access to all other nested types in the same 
nest (ie all types directly or indirectly nested in a given top-level 
class).

Cheers,
David

> bye Jochen
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