Using java.awt.Toolkit.getDefaultToolkit().getScreenSize() reflectively causes InaccessibleObjectException

[Alex Buckley]

On 1/5/2017 1:16 PM, Rony G. Flatscher wrote:
> The rule for using the Java bridge for the Rexx programmers has always been very simple: you are
> only allowed to use public Java classes, public Java fields and public Java methods as these are
> guaranteed to be available to everyone at all times.
>
> This is what happens in this case too: the public Java class java.awt.Toolkit gets loaded, its
> public class method getDefaultToolkit() gets invoked and a java.awt.Toolkit object gets returned by
> that method.
>
> The returned Toolkit object is known from the documentation to allow access to all documented public
> methods, so its class (or one of its superclasses) must have the public abstract method
> getScreenDimension() implemented (being a subclass of java.awt.Toolkit it must implement all
> abstract members otherwise no instance could be created from it), therefore getting that Method
> object and invoking it.
>
> There has been no reason since the beginning of Java to know from a programmer's point of view how
> the implementation of the class took place for the returned object (e.g. which package serves as its
> home) as long as it was a subclass of that public Java class and as long as we have adhered to its
> its public members only.

The j.l.r.Method object on which you call invoke() should not be 
obtained by inspecting the methods of the implementation class given by 
getDefaultToolkit().getClass(). Implementation classes (i.e. classes in 
non-exported packages) cannot be instantiated, nor their members 
manipulated, by code outside their module.

The j.l.r.Method object on which you call invoke() should be obtained by 
inspecting the methods of the "public Java class" java.awt.Toolkit. The 
first argument that you pass to invoke(), indicating the receiver, can 
still be instanceof the implementation class.

> (Also, I would expect to be able to access any additional public members of
> such a subclass, irrespectible of its implementation, just from knowing that in Java one can always
> access public classes and public members.)

Nope, because access to members is gated by access to the enclosing 
class. If the enclosing class is inaccessible then its members are 
inaccessible.

Alex