It's not too late for access control
David M. Lloyd
david.lloyd at redhat.com
Mon Jul 11 14:21:46 UTC 2016
The crux of this access control discussion is that, up until JDK 9,
"public" meant "public". End of story. If you did not want something
to be visible, you made it not public. Very simple and very clear. The
word "public" literally means "accessible to all" after all; that's why
the term was selected in the first place, and it is nearly certain that
this was the intent of the thing up to this date.
What is being required in the JPMS spec, and, I dare to suggest, what
has been generally asked for by the public (and even required by the
JDK), is a way to provide an additional capability - the ability to
selectively share otherwise unshared code.
The existing Java language accessibility model is (of course)
well-understood by experts. For new developers, the concepts takes a
bit of time to explain but can generally be grasped. In the end the
accessibility of a member is generally easily determined by examining
the qualifiers of that member.
What has been proposed and implemented in Jigsaw is essentially a
completely new approach to access checking. Because it is new, and
because it is essentially untried, I predicted that issues would arise
not unlike the ones being currently discussed. I argue that this
approach is not optimal, for at least reasons discussed on this list and
in this email, but that even now, it's not too late to change the approach.
I propose, once again, that rather than changing the meaning of "public"
to something unintuitive (and indeed counter to the definition of the
actual word), we instead allow the selective extension of
package-private. Users would make public any type or member which is
*meant* to be public, i.e. accessible by all. Rather than (at best)
changing their expectations as to the behavior of "public" only to
immediately betray that expectation by forcing them through a backdoor
in order to meet practical needs, we ensure that their expectations
remain: public members are public, and things that are secret are not
Conceptually (and, hopefully, technically) this should not be too far
away from where we've arrived at now in Jigsaw, at least as far as the
package inventory is shared between modules for the purpose of access
control. Can anyone think of any good reason we should *not* do this,
or ways that this would be substantially weaker than restricting public?
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