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

forax at forax at
Thu Apr 2 20:18:55 UTC 2020

> De: "mandy chung" <mandy.chung at>
> À: "Remi Forax" <forax at>, "Brian Goetz" <brian.goetz at>
> Cc: "John Rose" <john.r.rose at>, "valhalla-dev"
> <valhalla-dev at>
> Envoyé: Jeudi 2 Avril 2020 21:58:29
> Objet: Re: java.lang.constant.ClassDesc and TypeDescriptor for hidden class??

> On 4/2/20 12:03 PM, Remi Forax wrote:

>> ----- Mail original -----

>>> De: "Brian Goetz" [ mailto:brian.goetz at | <brian.goetz at> ]
>>> À: "mandy chung" [ mailto:mandy.chung at | <mandy.chung at> ] ,
>>> "John Rose" [ mailto:john.r.rose at | <john.r.rose at> ] Cc:
>>> "valhalla-dev" [ mailto:valhalla-dev at |
>>> <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.
Hidden class is not something we want every developers to know, only the ones that write frameworks. So it's a secondary feature, that's why you ask if creating a new runtime exception worth the cost or not ? 
I have no strong feeling, i'm fine with IllegalStateException but having an exception in java.lang.constant is fine too. 

> Mandy


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