java.lang.constant.ClassDesc and TypeDescriptor for hidden class??

Mandy Chung mandy.chung at
Thu Apr 2 19:58:29 UTC 2020

On 4/2/20 12:03 PM, Remi Forax wrote:
> ----- Mail original -----
>> De: "Brian Goetz" <brian.goetz at>
>> À: "mandy chung" <mandy.chung at>, "John Rose" <john.r.rose at>
>> Cc: "valhalla-dev" <valhalla-dev at>
>> Envoyé: Jeudi 2 Avril 2020 20:34:31
>> Objet: Re: java.lang.constant.ClassDesc and TypeDescriptor for hidden class??
>> Mandy reminded me that descriptorString() is specified to return a JLS
>> 4.2.3 field descriptor, and so (c) would violate that spec.  I think
>> this tilts the table away from (c), even though this string is arguably
>> useful (though not clear to whom.)
>> While the spec for TypeDescriptor doesn't explicitly say "for use in a
>> classfile" or "for reflective instantiation", its hard to imagine use
>> cases that don't terminate in one of these situations, at which point,
>> we're going to fail.  So it is not clear whether we do anyone favors by
>> having Class::descriptorString() return something "helpful" that is not
>> really helpful.
>> Given the choice between (a) and (b), well, both are going to result in
>> user surprises (though, the spec for TD::descriptorString doesn't
>> prohibit returning null.)  Taken together, I think (b) is less awful
>> than (a), so I revise my answer to (b).
> (a) and (b) are incompatible changes.
> returning null is equivalent of throwing a NPE because at least the existing callers will not expect null, so i think (a) is better because you can have a proper error message.
> And the javadoc can explain that testing myClass.isHidden() upfront is the right way to avoid the exception.

What exception would you propose to throw for this "not a valid type 
descriptor runtime entity"?    If it throws an exception, it may be 
better to define a new exception type in `java.lang.constant` - would it 
seem a little overkill?  The clients would need to be updated to handle 
hidden classes if they expect Class::descriptorType invoked on any 
class.  I consider a helpful error message is nice while returning null 
indicates this class does not have a valid field descriptor.


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