JDK-4834738 NullPointerException: Better info

[Staffan Larsen]

On 15 apr 2014, at 09:35, David Holmes <david.holmes at oracle.com> wrote:

> 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.

I don’t feel comfortable reviewing the code, by I support the effort even if it is only for the interpreter. If at a later stage we can add the same information from the compilers, that would of course be good.

/Staffan

> 
> 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
>> 
>>