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

Lindenmaier, Goetz goetz.lindenmaier at sap.com
Fri Feb 8 11:51:28 UTC 2019

Hi David, 

> Hi Volker,
... I assume Volker could have contributed this as well, but actually 
I must mention Ralf Schmelter as the original author of this :)
> 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()"
Actually, I have left out code that changes the signatures to the 
Java source code wording. I already left that out in my former 
exception message contributions.  For example see the messages in 
they have the same bad format:
"class test.Runner4 tried to access private method test.IllegalAccessErrorTest.iae4_m()V"

I would like to fix them altogether in a follow-up, is that acceptable to 
> Also "of a null object" would read better as "on a null reference".
Makes sense, fixed.

But I'm not that sure about changing these:
"while trying to read the field '%s' of a null object"
--> "while trying to read the field '%s' from a null reference"
"while trying to write the field %s of a null object"
--> "while trying to write the field %s  of a null reference"

> First you will need to file a CSR request for the new product flags.
I'm not sure whether I need the product flags altogether. I would 
prefer removing them.
> 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?
Obviously, the problem is that the field is private.
As Christoph points out, there are several ways to implement this. 
Please give advice:
  * reflection
  * shared secret
  * Add package visible "void setMessage (String msg)" to Throwable. 

Best regards,

> 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
> >
> >

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