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

[Lindenmaier, Goetz]

Hi Remi,

But how does volatile help?
I see the test for numStackTracesFilledIn == 1 then gets always the
right value. 
But the backtrace may not be changed until I read it in 
getExtendedNPEMessage.  The other thread could change it after
checking numStackTracesFilledIn and before I read the backtrace.

I want to vote again for the much more simple version 
proposed in webrev 02: 
http://cr.openjdk.java.net/~goetz/wr20/8248476-NPE_fillInStackTrace-jdk15/02/

It's drawback is only that for this code:
  ex = null;
  ex.fillInStackTrace()
no message is created.

I think this really is acceptable.


Remi, I didn't comment on this statement from a previous mail:
> > Hmm, Throwable.stackTrace is used for the stack trace at some point.
> yes, it contains the Java stack trace, but if the Java stack trace is filled you don't 
> compute any helpful message anyway.
The internal structure is no more deleted when the stack trace 
is filled. So the message can be computed later, too.  

Best regards,
  Goetz.

> -----Original Message-----
> From: forax at univ-mlv.fr <forax at univ-mlv.fr>
> Sent: Thursday, July 2, 2020 8:52 PM
> To: Alan Bateman <Alan.Bateman at oracle.com>
> Cc: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; Christoph Dreis
> <christoph.dreis at freenet.de>; hotspot-runtime-dev <hotspot-runtime-
> dev at openjdk.java.net>; David Holmes <david.holmes at oracle.com>
> Subject: Re: [15] RFR: 8248476: No helpful NullPointerException message
> after calling fillInStackTrace
> 
> yes,
> it's what i was saying,
> given that a NPE can be thrown very early, before VarHandle is initialized, i
> believe that declaring numStackTracesFilledIn volatile is the best way to
> tackle that.
> 
> Rémi
> 
> ----- Mail original -----
> > De: "Alan Bateman" <Alan.Bateman at oracle.com>
> > À: "Goetz Lindenmaier" <goetz.lindenmaier at sap.com>, "Christoph Dreis"
> <christoph.dreis at freenet.de>
> > Cc: "hotspot-runtime-dev" <hotspot-runtime-dev at openjdk.java.net>,
> "David Holmes" <david.holmes at oracle.com>, "Remi Forax"
> > <forax at univ-mlv.fr>
> > Envoyé: Jeudi 2 Juillet 2020 20:47:31
> > Objet: Re: [15] RFR: 8248476: No helpful NullPointerException message
> after calling fillInStackTrace
> 
> > On 02/07/2020 17:45, Lindenmaier, Goetz wrote:
> >> Hi Christoph,
> >>
> >> I fixed the comment, thanks for pointing that out.
> >>
> > One other thing is that NPE::getMessage reads numStackTracesFilledIn
> > without synchronization.
> >
> > -Alan