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

David M. Lloyd david.lloyd at
Wed Sep 28 12:13:03 UTC 2016

Hi all, I've been requested to ask if the OpenJDK development team have 
had a chance to review this email, and when we might expect a response.


On 09/21/2016 11:39 AM, David M. Lloyd 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 {
>       // 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,
>      ;
> }
> 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;
> }
> 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 {
>       [...]
>       provides capability javax.ejb using private reflection;
> }
> --
> - DML


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