Truffle interop design

[Ghadi Shayban]

One of the many reasons JVM languages flourished is that they had access to
all Java libs.

It seems straightforward to call Truffle entry nodes from Java and return a
value.  Are there any established design patterns for the opposite case,
calling arbitrary Java from the Truffle node tree?  e.g. an instance
j.u.List is in a frame local, and you want to call add() on it.

MethodHandles?  Does Truffle/Graal optimize through those?