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

[David Holmes]

Hi Volker,

You know I'm not going to be a big fan of this :), but as long as we 
don't pay for it if we don't want it, then that's okay. (I'm still 
trying to gauge that)

I have a little test for this that I ran through your patch:

public class NPE {
   static class B {
     C b() { return null; }
   }
   static class C {
     int c(Object o, String s) {  return 0; }
   }

   public static void main(String[] args) {
     int x  = a().b().c(d(), e().toString());
   }

   static B a() { return null; }
   static Object d() { return null; }
   static Object e() { return null; }

}

and the result was a bit confusing for me:

java.lang.NullPointerException: while trying to invoke the method 
NPE$B.b(()LNPE$C;) of a null object returned from NPE.a(()LNPE$B;)

The placement and format of the return type descriptors obfuscates 
things to me - especially the Lxxx; format. Can we make that more Java 
programmer friendly eg:

"while trying to invoke the method 'NPE$C NPE$B.b()' ..."

though I think trying to produce signatures within the message is going 
to be very awkward in the general case. The key part is the "method 
NPE.b() ... returned from NPE.a()"

Also "of a null object" would read better as "on a null reference".


A couple of initial comments

First you will need to file a CSR request for the new product flags.

Second, I don't understand why you need to call into the VM with 
JVM_SetDefaultMessage, to set a field in the Java object? Why isn't that 
done in Java?

Thanks,
David

On 8/02/2019 2:43 am, Lindenmaier, Goetz wrote:
> Hi,
> 
> since Java 5, our internal VM reports verbose null pointer exception
> messages. I would like to contribute this feature to OpenJDK.
> 
> With this change, messages as
>     "java.lang.NullPointerException: while trying to load from a null int array loaded from local variable 'ia1'"
> are printed.  For more examples see the JBS bug or the test included.
> https://bugs.openjdk.java.net/browse/JDK-8218628
> 
> The messages are generated by parsing the bytecodes. For not to have any overhead when the
> NPE is allocated, the message is only generated when it is accessed by getMessage() or
> serialization. For this I added a field to NPE to indicate that the message still needs to be
> computed lazily.
> 
> Please review:
> http://cr.openjdk.java.net/~goetz/wr19/8218628-exMsg-NPE/01/
> I'm happy to incorporate your comments.
> 
> Best regards,
>    Goetz
> 
>