[15] RFR: 8248476: No helpful NullPointerException message after calling fillInStackTrace

[David Holmes]

On 30/06/2020 5:53 pm, Remi Forax wrote:
> ----- Mail original -----
>> De: "Goetz Lindenmaier" <goetz.lindenmaier at sap.com>
>> À: "Christoph Dreis" <christoph.dreis at freenet.de>, "hotspot-runtime-dev" <hotspot-runtime-dev at openjdk.java.net>
>> Envoyé: Lundi 29 Juin 2020 14:07:08
>> Objet: RE: [15] RFR: 8248476: No helpful NullPointerException message after calling fillInStackTrace
> 
> [...]
> 
>> An alternative fix I could imagine would be to
>> override fillInStackTrace in NPE.java. It could
>> call getMessage() and then super.fillInStackTrace,
>> and return a new exception with the message.
> 
> overriding fillInStackTrace() and marking it when it is called twice is the other solution.

That was my thought now I understand how/when this process kicks in. 
Just add a "filled" count that the overriding fillInStackTrace 
increments, and have getMessage check the count before calling 
getExtendedNPEMessage.

Cheers,
David
-----

>> But this would also compute the message in cases where it is not printed.
> 
> you don't need that, you can change the field Throwable.stackTrace with a marker object not unlike UNASSIGNED_STACK to detect if NPE.fillInStackTrace is called twice,
> but as is said earlier, it's not a small change because Throwable should stay thread safe and serializable.
> 
> So i agree with you and Christoph that the "best" solution is to document that null.fillInStackTrace() doesn't get a detailed error message, apart if someone has a better fix.
> 
>>
>> Best regards,
>>   Goetz.
> 
> Rémi
> 
>>
>>> -----Original Message-----
>>> From: Christoph Dreis <christoph.dreis at freenet.de>
>>> Sent: Monday, June 29, 2020 1:28 PM
>>> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-runtime-
>>> dev at openjdk.java.net
>>> Subject: Re: [15] RFR: 8248476: No helpful NullPointerException message
>>> after calling fillInStackTrace
>>>
>>> Hi Goetz,
>>>
>>>> If changing the stack trace by calling fillInStackTrace in user code, the
>>>> NPE algorithm lacks the proper information to compute the message.
>>>> Thus, we must omit it after that call.
>>>
>>>> I implement this by checking for a call to fillInStackTrace at the bci
>>>> recorded in the exception.
>>>> http://cr.openjdk.java.net/~goetz/wr20/8248476-NPE_fillInStackTrace-
>>> jdk15/01/
>>>
>>> I tried this when reporting the issue already. The problem with this is, that it
>>> suppresses any valid exception.
>>>
>>> E.g. the following example would not throw any helpful NPE anymore.
>>>
>>> public class Main {
>>> 	public static void main(String[] args) {
>>> 		NullPointerException ex = null;
>>> 		ex.fillInStackTrace();
>>> 	}
>>> }
>>>
>>> Cheers,
>>> Christoph
>>>