RFR(M): 8221077: No NullPointerException message if top frame is hidden.

[Lindenmaier, Goetz]

Hi David, 

I'm not really happy with the design of hiding the frames, 
either.
For 'inner' frames this seems ok, but for the top frames
up to the first not-hidden frame it is questionable to 
hide them.  As you say, there might be exceptions where 
this is desirable, for NPE it seems wrong.

I’ll have a look how it could be implemented to 
suppress the hiding in case of NPE.

Best regards,
  Goetz.


> -----Original Message-----
> From: David Holmes <david.holmes at oracle.com>
> Sent: Montag, 6. Mai 2019 13:50
> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-runtime-
> dev at openjdk.java.net; Coleen Phillimore (coleen.phillimore at oracle.com)
> <coleen.phillimore at oracle.com>
> Subject: Re: RFR(M): 8221077: No NullPointerException message if top frame is
> hidden.
> 
> Hi Goetz,
> 
> Just commenting on the situation ...
> 
> On 6/05/2019 8:22 pm, Lindenmaier, Goetz wrote:
> > Hi,
> >
> > Exceptions can print the stack trace of when the exception occurred.
> > There can be methods on the stack that have been generated by
> > the runtime. To not confuse the reader of the stackTrace,
> > these are omitted (if -XX:-ShowHiddenFrames, which is default.)
> >
> > If the top frame is omitted/hidden,
> > it is inconsistent to print a message telling where
> > exactly in the hidden method the error occurred.
> >
> > Further, if the top frame is hidden, the information
> > needed to compute the message is missing. A wrong
> > message will be printed.
> >
> > This change suppresses the NPE message if the NPE
> > occurred in a hidden top frame.
> 
> This sounds to me like "hidden frames" are being applied too broadly.
> Frames should only be hidden with respect to throwing specific
> exceptions - those for which the frame is  part of the "throwing
> mechanics". If that code hits a real exception then the frame should not
> be treated as hidden IMO. But I don't know whether we can clearly
> identify which kinds of exceptions should really lead to a frame being
> hidden.
> 
> David
> -----
> 
> > It adds a java.lang.Boolean(true) to the backtrace
> > in case the real top frame is a hidden one and is dropped.
> >
> > When the NullPointerException message text is generated, this is
> > checked and the message is skipped if the proper frame is not
> > available.
> >
> > I handle this in a bug itself to keep the discussion of
> > the basic feature separated from this technical issue.
> >
> > Please review:
> > https://bugs.openjdk.java.net/browse/JDK-8221077
> > http://cr.openjdk.java.net/~goetz/wr19/8221077-hidden_top_frame/02/
> > This change must be applied on top of
> > http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/08/
> >
> > Best regards,
> >    Goetz.
> >