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

[Maurizio Cimadamore]

Hi Goetz,
please find the attached ASM-based patch. It is just a PoC, as such it 
does not provide as fine-grained messages as the one discussed in the 
RFE/JEP, but can be enhanced to cover custom debugging attribute, I believe.

When running this:

Object o = null;
o.toString();

you get:

Exception in thread "main" java.lang.NullPointerException: attempt to 
dereference 'null' when calling method 'toString'
     at org.oracle.npe.NPEHandler.main(NPEHandler.java:103)

While when running this:

Foo foo = null;
int y = foo.x;

You get this:

Exception in thread "main" java.lang.NullPointerException: attempt to 
dereference 'null' when accessing field 'x'
     at org.oracle.npe.NPEHandler.main(NPEHandler.java:105)

One problem I had is that ASM provides no way to get the instruction 
given a program counter - which means we have to scan all the bytecodes 
and update the sizes as we go along, and, ASM unfortunately doesn’t 
expose opcode sizes either. A more robust solution would be to have a 
big switch which returned the opcode size of any given opcode. Also, 
accessing to StackWalker API on exception creation might not be 
desirable in terms of performances, so this might be one of these area 
where some VM help could be beneficial. Another problem is that we 
cannot distinguish between user-generated exceptions (e.g. `throw new 
NullPointerException`) and genuine NPE issued by the VM.

But I guess the upshot is that it works to leave all the gory detail of 
bytecode grovelling to a bytecode API - if the logic is applied lazily, 
then the impact on performances should be minimal, and the solution more 
maintainable longer term.

Cheers
Maurizio

On 15/03/2019 07:59, Lindenmaier, Goetz wrote:
> Yes, it would be nice if you shared that.
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