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

[Lindenmaier, Goetz]

Hi Remi,

Hmm, Throwable.stackTrace is used for the stack trace at some
point.  
Another way to set a marker would be to handle it similar
to the hidden_top_frames in javaClasses.cpp.
Anyways, there are other places where the message is omitted, 
e.g. in case of a hidden top frame.

Remi, as we agree that the proposed solution is the 
most handy one, can I count you as reviewer?

Best regards,
  Goetz.




> -----Original Message-----
> From: Remi Forax <forax at univ-mlv.fr>
> Sent: Tuesday, June 30, 2020 9:54 AM
> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>
> Cc: Christoph Dreis <christoph.dreis at freenet.de>; hotspot-runtime-dev
> <hotspot-runtime-dev at openjdk.java.net>
> Subject: Re: [15] RFR: 8248476: No helpful NullPointerException message
> after calling fillInStackTrace
> 
> ----- 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.
> 
> > 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
> >>