IllegalAccessError when method reference resolves to method returning inaccessible class

[Alex Buckley]

I believe this program is accepted by JLS 15.13, and that accepting the 
program is correct. It has always been possible in Java to invoke a 
method whose return type is inaccessible to the caller, provided that 
the caller doesn't use the method's result at the inaccessible type. 
Fundamentally the same rules apply to the indirect invocation denoted by 
a method reference expression. Foo::bar is typed w.r.t. [the function 
type of] Supplier<Object>, for which an access check _is_ performed; 
however, Foo::bar is not typed w.r.t. anything involving the 
inaccessible nested type Foo.Bar, so Foo::bar is as legal as its 
"equivalent" lambda expression ()->Foo.bar().

Alex

On 5/4/2022 7:40 AM, Tagir Valeev wrote:
> Consider the following code:
> 
> // foo/Foo.java
> package foo;
> 
> public class Foo {
>      public static Bar bar() {
>          return new Bar();
>      }
> 
>      static class Bar {}
> }
> 
> // bar/Baz.java
> package bar;
> 
> import foo.Foo;
> import java.util.function.Supplier;
> 
> public class Baz {
>      public static  void foo(Supplier<Object> supplier) {
>          System.out.println(supplier.get());
>      }
> 
>      public static void main(String[] args) {
>          Baz.foo(Foo::bar);
>      }
> }
> 
> It's perfectly compilable (tried javac 17 and 19-ea) but immediately
> fails at runtime with
> 
> Exception in thread "main" java.lang.IllegalAccessError: failed to
> access class foo.Foo$Bar from class bar.Baz (foo.Foo$Bar and bar.Baz
> are in unnamed module of loader 'app')
> at bar.Baz.main(Baz.java:14)
> 
> It looks like VM doesn't like if bootstrap method refers to a method
> that mentions inaccessible type in a signature. However, we can
> normally call the same method in lambda:
> 
> public static void main(String[] args) {
>      Baz.foo(() -> Foo.bar());
> }
> 
> I'm not sure whether this is a compiler or VM problem but to me, it
> looks wrong when the program after successful complete recompilation
> fails with IllegalAccessError. Please correct me if this compiler
> behavior is specified.
> 
> With best regards,
> Tagir Valeev