RFR: JDK-8283416: Update java.lang.invoke.MethodHandle to use sealed classes

[ExE Boss]

On Sun, 20 Mar 2022 21:31:15 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> Small refactoring to use sealed classes in the MethodHandle implementation hierarchy.
> 
> DelegatingMethodHandle is non-sealed rather than sealed since it has two subclasses, one defined as a nested class and only a separate type in the same package. The permits clause does not allow listed those two kinds of subclasses.
> 
> Please also review the corresponding CSR https://bugs.openjdk.java.net/browse/JDK-8283434

But the private inner classes are visible to the compiled `PermittedSubclasses` class attribute (otherwise it wouldn’t be possible for private inner classes to extend a sealed outer class with an implicit `permits` clause).

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Technically, `private` means accessible only within the current nest.

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PR: https://git.openjdk.java.net/jdk/pull/7881