getPermittedSubclasses() on j.l.rClass returning an array of ClassDesc

Brian Goetz brian.goetz at
Sat May 9 14:53:11 UTC 2020

It's not just that it is _inconvenient_ to return the Class, "because of 
class loading."  It may not be _possible_, as the class may not be 
there, but we would still like to be able to reflect what the class says.

So the choices are:
  - Don't make this information accessible at all
  - Return descriptor strings
  - Return ClassDesc

Of the latter two, clearly ClassDesc is better than descriptor strings.  
(You could argue that we should go from array to List; this is an OK 
option but then the "for consistency" patrol will criticize it.)

I think a better question is: what do we expect people to DO with this 
information?  That class C permits D doesn't confer anything about what 
any given user can do with D; D might be nonexistent, or inaccessible to 
some clients.  Or it might not have any members that are accessible to 
some client.  So what would a client plan to do with the Class?

On 5/8/2020 6:53 PM, Remi Forax wrote:
> The current draft of the reflection API for the sealed keyword adds a method getPermittedSubclasses() [1] to java.lang.Class.
> I'm not fully sure that returning an array of ClassDesc is the right choice here, mainly for two reasons,
> 1/ it's weird to return an array of ClassDesc when all others similar methods return an array of Class,
>     I know why a ClassDesc might be "better" because it avoid the class loading,
>     but it also means that now to fully understand java.lang.Class, people has to understand how java.lang.constant works.
>     The java.lang.constant API was not designed for that, the first line of the overview of this package talks about descriptors, constant pool and indy, not something beginners should worry about.
> 2/ returning a symbolic view (ClassDesc) instead of a Class is *very* error prone from a user POV, to resolve a ClassDesc to a class, the user as to provide a Lookup
>     and there is a good chance that users will pick the wrong ones. The number of people that understand classloading and how Lookup works is < 10,
>     even experts struggle given the number of time the Lookup API as to be patched in recent years. Returning a ClassDesc in this context is like asking a child
>     to read the serial number of a loaded gun.
>     Perhaps a way to mitigate that is to provide the code a user should use to get the equivalent classes in the javadoc of getPermittedSubclasses().
> cheers,
> Rémi
> [1]

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