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

[Mandy Chung]

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