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

[Lindenmaier, Goetz]

Hi, 

I had promised to work on a better wording of the messages.

This I deliver with this webrev:
http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/05-otherMessages/

The test in the webrev is modified to just print messages along with the
code that raised the messages.

Please have a look at these files with test output contained in the webrev:
Messages with debug information: 
http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/05-otherMessages/output_with_debug_info.txt
Messages without debug information:
http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/05-otherMessages/output_no_debug_info.txt

Especially look at the first few messages, they point out the usefulness
of this change.  They precisely say what was null in a chain of dereferences.

Best regards,
  Goetz.


> -----Original Message-----
> From: Lindenmaier, Goetz
> Sent: Wednesday, February 13, 2019 10:09 AM
> To: 'Mandy Chung' <mandy.chung at oracle.com>; 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.
> 
> 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