Alternative mechanism for reflective access control (#ReflectiveAccessToNonExportedTypes / #AwkwardStrongEncapsulation)

[Stephen Colebourne]

Having read this proposal a number of times, and considering how the
talks explained things at JavaOne, I have come to the conclusion that
this proposal is too complex. FWIW, I like the idea that a module
should be able to declare that it needs reflective access from its
users, however given that the proposal is what results from the idea,
it doesn't seem as appealing as it should.

The reason why I put forward the exports/exposes approach [1] is that
it keeps the questions that must be asked when creating a module
simple:
- what do I depend on publicly (requires)
- what do I publish publicly (exports)
- what do I publish privately (exposes)

>From a security point of view it also seems that it should be the
responsibility of a module to allow the publishing of its private
details, and simply depending on another module seems very minimal
(and easy to miss) as a mechanism to allow that extra permission.

Stephen

[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2016-September/009370.html


On 21 September 2016 at 17:39, David M. Lloyd <david.lloyd at redhat.com> wrote:
> In our internal discussion of the proposal for
> #ReflectiveAccessToNonExportedTypes, we discussed the ins and outs of
> various behaviors and have come up with a few ideas or starting points for
> solutions that we think would be more workable in conjunction with existing
> middleware (ours and others').
>
> For reasons previously explained, we do not think that weak modules are a
> good way forward; I won't go into that again here.  But the logical
> re-starting point is: If not weak modules, then what?
>
> I will boil it down to a few basic requirements that we have established.
> This list is probably non-exhaustive but hopefully complete enough to go on
> for now:
>
> • A module definition must be able to establish that a dependent has (or all
> modules have) access to one or more (or all) packages for public reflection
> only.
> • A module definition must be able to establish that a dependent has (or all
> modules have) access to one or more (or all) packages for public or private
> reflection only.
> • A module definition must be able to establish that a dependent has (or all
> modules have) access to one or more (or all) packages for public reflection
> and compilation/linkage (i.e. it's an export by today's terminology).
> • A module definition must be able to establish that a dependent has (or all
> modules have) access to one or more (or all) packages for public or private
> reflection and compilation/linkage (i.e. it's a "private" export by today's
> terminology).
> • As today, any packages not declared in one or more of the above categories
> is inaccessible outside of the module in any way (note that as I showed
> previously we have also concluded that it should continue to be impossible
> to export a package for compilation/linkage without public reflection, as we
> have not discovered any use for such a mode).
>
> More generally:
>
> • The syntax for all of the above has no particular constraint (in fact I
> will try to actively avoid touching what could be a very bikeshedding-rich
> discussion), except that it should not be construable as being pejorative
> against the usage of reflective frameworks; rather, it should be clear what
> level of trust is being established without raising undue warning.
> • Applications should not need gratuitous amounts of declarations in their
> module(s) in order to utilize frameworks.
> • As previously established, it should not be possible for one declaration
> to reduce the scope of access of another declaration in a module definition.
> • Access to a module (for reflective purposes only) must not cause conflicts
> if multiple such modules which contain identical packages are accessible to
> a single consumer; in other words, reflection-only access into
> non-dependency modules is not bound by duplicate package restrictions as
> long as each package is unique per class loader, as per the current (Java 8)
> class loader rules.
>
> The above cover the useful access modes that we have identified.  This is
> _nearly_ adequate to cover the use cases that we are currently concerned
> about; for example, I could export all packages for public reflection only
> to a specific framework, if only I know the module name of the
> implementation.
>
> Unfortunately, this does not work well in the case where a module may
> consume a framework whose specification is separate from the implementation.
> An application module may need to use (say) EJB and JPA; there is presently
> no clean way to do so without either (a) relying on a container environment
> to rewrite the descriptor or (b) opening up the module and defeating the
> security mechanism (e.g. "weak").  Without either of these workarounds, the
> application developer must have a good deal of knowledge about what modules
> provide what services within a framework-rich environment, possibly
> resulting in a very verbose (and error-prone) descriptor; none of these
> options is really satisfactory.
>
> Thus, apart from the option of redesigning (to an extent) the security
> mechanism (thereby eliminating the need to seal off access to public
> reflection, which is definitely still an attractive option for various
> reasons from our perspective, but which is also a very different
> discussion), we need some sort of mechanism which decouples the literal
> dependency system from access permission (much like uses/provides does).
>
> For example if I could declare that my module uses "javax.ejb", and, in so
> doing, automatically grants public and private reflective access to the
> module that provides that service, this would be a good outcome.  A module
> which answers to that service name could be responsible for reflective
> access to the application module, providing that information privately to
> any other framework modules which require it.
>
> The migration story looks much better in this light: module descriptors
> still can be quite terse and specific.  Applications which use reflective
> frameworks do not need gratuitous exports; in fact it's much more fluid for
> a user to say "I require these helper libraries; I use EJB; that's it" which
> means they don't have to worry about the details of whatever particular
> environment they run in.  This also has the advantage of allowing new Java
> 9-generation specifications to stipulate standard service names for each
> specification (e.g. "javax.ejb", "javax.cdi", that sort of thing).
>
> While this doesn't cover 100% of our remaining issues with Jigsaw (of
> course; we'll all continue moving through the issues list as we have been to
> get us there), meeting these requirements would go a long way towards at
> least having a reflection story that is more practical for present-day
> frameworks to move forward with.  So the last requirement would be:
>
> • A module definition must be able to establish that an "indirect"
> dependency exists on an otherwise unknown module providing a capability,
> wherein that module may require public or public+private reflection access
> to some or all packages without compile/link access.  This could possibly
> exist in conjunction with, or as an evolution of, the current services
> mechanism, however a complicating factor is that the current mechanism is
> based specifically on types, whereas a purely symbolic relationship might be
> better for this purpose (this is not a requirement though if it can be made
> to work as-is).  Note that any symbolic relationship system would need some
> in-code discovery mechanism such that consumers of the capability are made
> available to the provider and/or vice-versa, in order to make practical use
> of the relationship.
>
> The following example syntax is meant to be unambiguous and illustrative; no
> specific attempt is made to reuse existing keywords (for example), or even
> to imply an endorsement of the current descriptor mechanism at all, but to
> clarify how this might look in practice and provide a practical application
> of the ideas herein.
>
> Example 1: A contrived provider of the fictional framework
> "javax.fictional.orm" illustrating provides/uses-based access granting
>
> module org.foo.orm.provider {
>
>       // Require a module dependency, and give it private reflection access
> to everything
>       requires org.apache.commons.beanutils with private reflection on *;
>
>       // Require a module dependency with no reflection
>       requires org.apache.commons.logging;
>
>       // Provide the framework
>       provides javax.fictional.orm.ORM
>           using private reflection
>           with org.foo.orm.provider.ORMImpl1,
>                org.foo.orm.provider.ORMImpl2;
> }
>
> Example 2: A contrived consumer of #1
>
> module com.mycompany.application {
>       uses javax.fictional.orm.ORM; // automatically gives private
> reflection
> }
>
> Example 3: Grant reflection access to a couple of packages to a named
> non-dependency module
>
> module com.mycompany.application {
>       grant public reflection on
>           com.mycompay.application.package1,
>           com.mycompay.application.package2
>       to org.foo.framework;
> }
>
> Example 4: Behave like Java 8
>
> module com.mycompany.application {
>       grant private reflection on * to *;
> }
>
> Example 5: Behave like Java 8, but restrict private access without requiring
> a security manager
>
> module com.mycompany.application {
>       grant public reflection on * to *;
> }
>
> Example 6: An example of using CDI and EJB with symbolic capabilities
>
> module com.mycompany.application {
>       uses capability javax.ejb, javax.cdi
> }
>
> Example 7: An example of providing EJB with symbolic capabilities
>
> module org.foo.ejb.provider {
>       [...]
>       provides capability javax.ejb using private reflection;
> }
>
>
> --
> - DML