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.
Thanks!
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 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
--
- DML
