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

[Lindenmaier, Goetz]

Hi,

while waiting for progress on corresponding the JEP, I improved 
the implementation of generating the NPE message.  It now uses
a single outputStream. This removes several allocations of temporary
data. I also removed TrackingStackSource. The analysis code originally 
addressed several use cases, for NullPointerExceptions this is 
not needed.  I cleaned up bytecodeUtils from some code not (really) needed.

I split get_null_pointer_slot() into two methods: get_NPE_null_slot() 
and print_NPE_failed_action(). This simplifies the
implementation, and streamlines it more with the text in the JEP.

I print methods using the code added in 
"8221470: Print methods in exception messages in java-like Syntax.",
so it now prints 'void m(int)' instead of 'm(I)V'.

I implemented a row of new test cases, and rearranged the test 
to test the message part of print_NPE_failed_action() and 
print_NPE_cause() separated. I made sure all bytecodes 
handled in these methods are covered.  
Further I arranged the tests in methods according to the
functional properties as discussed in the JEP.

http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/07

Best regards,
  Goetz.





> -----Original Message-----
> From: Lindenmaier, Goetz
> Sent: Donnerstag, 14. März 2019 21:56
> 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' <hotspot-runtime-dev at openjdk.java.net>
> Subject: RE: RFR(L): 8218628: Add detailed message to NullPointerException
> describing what is null.
> 
> 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