Summary

Lookup::in throws IllegalArgumentException if the target class is a primitive type, void or an array class.

Problem

The current spec of Lookup::in allows to produce a Lookup object on a primitive type, void or an array class which is against the original design that assumes the lookup class of a Lookup object is never non-array and non-primitive class (method handle lookup is as if a specified member is referenced via bytecode)

The static factory methods MethodHandles::lookup, MethodHandles::privateLookupIn never produce a Lookup object on primitive type, void or array class.

Solution

Change the spec of Lookup::in to throw IllegalArgumentException if requestedLookupClass is a primitive type, void or array type.

An alternative would be that Lookup::in returns a Lookup object successfully but drop all lookup modes if requestedLookupClass is a primitive type, void or array type. This delays the failure point to method handle lookup time. It makes it harder to diagnose if any existing code is depending on the existing behavior.

It is expected that it's rare for existing code to call Lookup::in on a primitive type, void or array type. It'd be more helpful to throw IAE when a bad requestLookupClass argument is passed to Lookup::in.

Specification

The spec of Lookup::in method has the following new @throws IAE:

+         * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class

@throw IAE in the spec of MethodHandles::privateLookupIn method is updated to explicitly call out void:

-     * @throws IllegalArgumentException if {@code targetClass} is a primitive type or array class
+     * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class