JDK-4834738 NullPointerException: Better info

[David Holmes]

On 15/04/2014 5:16 PM, Fedor Bobin wrote:
> Hello.
>
> What is the best way to submit patch?
> Currently you can see it at https://gist.github.com/Fuud/10709048

If you become a regular contributor to OpenJDK you can get access to 
cr.openjdk.java.net to upload patches. Otherwise inline attachments or a 
link to something like github is okay.

>>>Can you explain what information you add?
>
> This java code will produce NPE;
>
> ((String)null).length();
>
> Before patch exception stack trace looks like this:
>
> java.lang.NullPointerException
>          at Test.testNPE(Test.java:14)
>          at Test.main(Test.java:8)
>
> After patch if code is executing by interpreter NPE will have additional
> information
>
> java.lang.NullPointerException: can not invoke method
> java.lang.String.length()I on null object
>          at Test.testNPE(Test.java:14)
>          at Test.main(Test.java:8)
>
> This additional information can help to understand cause of NPE in case
> of call chains. For example, let we have java code
>
> a.b().c();
>
> if we get NPE without message we can not understand is a==null or
> a.b()==null. After patch we will get enough information:
>
> java.lang.NullPointerException: can not invoke method A.b() on null object
>
> So we can understand that a is null.

There have been a couple of RFEs on this in the past, one very recently, 
and they have been closed as "will not fix" - mainly because except in 
some limited circumstances the VM does not have much additional 
information to add. That said your patch is the "inverse" of the other 
requests - they wanted to know which "variable" was null, whereas you 
are showing which method invocation was attempted on a null reference.

Hopefully some folks from the runtime team will be able to take a look 
at this and evaluate it. But it would be nice to be able to get similar 
information from compiled code too - though that may not be feasible.

Thanks,
David

>
> Thanks.
> Fedor Bobin
>
>
>
> 2014-04-15 10:32 GMT+04:00 David Holmes <david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>>:
>
>     Hi Fedor,
>
>
>     On 14/04/2014 1:09 AM, Fedor Bobin wrote:
>
>         Can you please review patch for JDK-4834738.
>
>
>     Patches are stripped when sent as attachments.
>
>
>         Currently I added more information to NPE if it is handled by
>         interpreter
>         or template interpreter on x86-64. On other platforms and after
>         compilation
>         jdk will handle NPE as usual.
>
>
>     Can you explain what information you add?
>
>     Thanks,
>     David
>
>