Re: [Clirr-devel] Java doesn't appear to do runtime checks for class/method access rights

[Simon K. <si...@ec...>]

On Tue, 2004-07-06 at 06:06, Lars K=FChne wrote:
> Simon Kitching wrote:
> >Attached is an archive which contains some java code that appears to
> >demonstrate that the Java JVM/runtime does not check for class or meth=
od
> >access rights.
> >
> >The java lang spec document seems clear that this *should* throw an
> >IllegalAccessError, but it doesn't. I would like someone to confirm th=
at
> >I am not having hallucinations here before I add a note about this to
> >the message description document.
> >
> I can confirm your results:
>=20
> access> java Main
> public method f1 of package scope class Foo called!
> private method f2 of package scope class Foo called!
>=20
> Wow!!
>=20
> This is scary, but on the other hand it's quite logical. Try to=20
> disassemble the code of Main:
[...]
> It seems the bytecode for both method calls is the same. The Main class=
=20
> simply seems to have no record whether the called methods are private o=
r=20
> public, so the VM can't check anything... ?

The information must be in the class file somewhere, because in BCEL we
can ask:
   somemethod.isPrivate() etc.
Also, when compiling source with javac against a library the compiler
can report attempted calls to inaccessable methods.

>=20
> I couldn't find the IllegalAccessError requirement you mentioned, but i=
n=20
> $13.4.6 the JLS says:
>=20
> "Changing the declared access of a member or constructor to permit less=
=20
> access may break compatibility with pre-existing binaries, causing a=20
> linkage error to be thrown when these binaries are resolved.
>=20
> Less access is permitted if the access modifier is changed from default=
=20
> access to |private| access; from |protected| access to default or=20
> |private| access; or from |public| access to |protected|, default, or=20
> |private| access"
>=20
> Not quite sure what to make of this, to me the spec is quite confusing =
-=20
> aren't those two sentences contradicting each other?

The sentence beginning "Less access" *is* poorly worded, but correct.=20

To rephrase:=20
  "<em>Less</em> access is permitted if ..."
or:
  "The access permissions are defined to be 'less' if the access =20
   modifier is changed from...."=20

The sentence starting "Changing..." says what happens if a change
resulting in "less access" is made: a linkage error is thrown.

Hmm..I can't find the bit that specifies "IllegalAccessError" either,
though I'm sure this *is* the exception that should be thrown.

>=20
> Still surprised,

Me too. And I think that clirr still should report an ERROR here,
because this change is a violation of the spec, even if it isn't
currently enforced by either Sun or IBM JVMs. What do you think?

Cheers,

Simon