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

[Christoph Dreis]

Hi Goetz,

> One design principle of the new message was not 
> to introduce overhead if the message is not printed. 
> Therefore I don't want to record any additional 
> information about what happened, which I would 
> need to distinguish the cases.
> Also, reporting a wrong message as the one you detected
> is a no-go.
> Thus, I prefer to skip the message in this case.

I prefer no message in this edge case, too.
Rather have none than a wrong one.

I wanted to state the other edge-case we're knowingly "break" for completeness reasons, though.

> 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.
> But this would also compute the message in cases
> where it is not printed.

That would again violate the design principle if the message is not consumed, I guess.

So overall I'd be fine with the patch.
As said: This was my local approach before reporting, but I didn't want to break the mentioned case.

Cheers,
Christoph

> -----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
> 
>