RFR(L): 8218628: Add detailed message to NullPointerException describing what is null.

[Lindenmaier, Goetz]

Hi Mandy, 

Thanks for supporting my intend of adding the message as such!
I'll start implementing this in Java and come back with a webrev
in a while. 

In parallel, I would like to continue discussing the other 
topics, e.g., the wording of the message. I will probably come up 
with a separate webrev for that.

Best regards,
  Goetz.



> -----Original Message-----
> From: core-libs-dev <core-libs-dev-bounces at openjdk.java.net> On Behalf
> Of Mandy Chung
> Sent: Tuesday, February 12, 2019 7:32 PM
> To: Roger Riggs <Roger.Riggs at oracle.com>
> Cc: Java Core Libs <core-libs-dev at openjdk.java.net>; hotspot-runtime-
> dev at openjdk.java.net
> Subject: Re: RFR(L): 8218628: Add detailed message to NullPointerException
> describing what is null.
> 
> On 2/8/19 11:46 AM, Roger Riggs wrote:
> > Hi,
> >
> > A few higher level issues should be considered, though the details
> > of the webrev captured my immediate attention.
> >
> > Is this the right feature and is this the right level of implementation
> > (C++/native)?
> > :
> > How much of this can be done in Java code with StackWalker and other
> > java APIs?
> > It would be a shame to add this much native code if there was a more
> robust
> > way to implement it using APIs with more leverage.
> 
> Improving the NPE message for better diagnosability is helpful while
> I share the same concern Roger raised.
> 
> Implementing this feature in Java and the library would be a better
> choice as this isn't absolutely required to be done in VM in native.
> 
> NPE keeps a backtrace capturing the method id and bci of each stack
> frame.  One option to explore is to have StackWalker to accept a
> Throwable object that returns a stream of StackFrame which allows
> you to get the method and BCI and also code source (I started a
> prototype for JDK-8189752 some time ago).  It can use the bytecode
> library e.g. ASM to read the bytecode.  For NPE message, you can
> implement a specialized StackFrameTraverser just for building
> an exception message purpose.
> 
> Mandy