Proposal: #ReflectiveAccessToNonExportedTypes (revised) & #AwkwardStrongEncapsulation: Weak modules & private exports

Stephen Colebourne scolebourne at
Mon Sep 12 22:58:41 UTC 2016

Quite a radical proposal. Overall, I think it is better, although I do
have semantic and syntax concerns.

I'm happy that there is a way to achieve real strong encapsulation.
This seems like a good thing and will be a benefit in the long run.
For the weak modules, the proposal does not provide a way to have
packages exposed for runtime use, but not hidden at compile time.
Given the benefits of hiding internal classes from IDEs, this seems
rather unfortunate. Here is my counter proposal, hopefully a
relatively small conceptual change:

The proposal module:

  weak module {
      // No exports
      requires hibernate.core;
      requires hibernate.entitymanager;

could be written with an alternative syntax of (same semantics):

  module {
      exports private *;
      requires hibernate.core;
      requires hibernate.entitymanager;

Taking this one step further, we could separate what is exported
(public things) from what is exposed (at runtime):

  module {
      exports *;
      exposes *;
      requires hibernate.core;
      requires hibernate.entitymanager;

(again, this is the same semantics as the previous two, just different syntax)

This would then provide a straightforward way to control exports
tightly but still expose everything for reflection:

  module {
      exposes *;
      requires hibernate.core;
      requires hibernate.entitymanager;

- "exports" defines whether the public elements can be accessed
- "exposes" defines whether the package can be accessed for reflection
- there may be a one * exports clause or zero to many package name
ones, but not both types
- there may be a one * exposes clause or zero to many package name
ones, but not both types
- a package may be listed in both exports and exposes, they are orthogonal
- the "to" clause would apply to exports and exposes, but only with
package names

The above does not deviate radically from today's proposal, yet offers
a clearer syntax more power and simpler migration from weak to strong

FWIW, I'm doing a presentation on modules on Wednesday, and I think
the above would be easier to explain than the "weak" keyword.


On 12 September 2016 at 16:08, Mark Reinhold <mark.reinhold at> wrote:
> Issue summary
> -------------
>   #ReflectiveAccessToNonExportedTypes --- Some kinds of framework
>   libraries require reflective access to members of the non-exported
>   types of other modules; examples include dependency injection (Guice),
>   persistence (JPA), debugging tools, code-automation tools, and
>   serialization (XStream).  In some cases the particular library to be
>   used is not known until run time (e.g., Hibernate and EclipseLink both
>   implement JPA).  This capability is also sometimes used to work around
>   bugs in unchangeable code.  Access to non-exported packages can, at
>   present, only be done via command-line flags, which is extremely
>   awkward.  Provide an easier way for reflective code to access such
>   non-exported types. [1]
>   #AwkwardStrongEncapsulation --- A non-public element of an exported
>   package can still be accessed via the `AccessibleObject::setAccessible`
>   method of the core reflection API.  The only way to strongly
>   encapsulate such an element is to move it to a non-exported package.
>   This makes it awkward, at best, to encapsulate the internals of a
>   package that defines a public API. [2]
> Proposal
> --------
> (Warning: This is somewhat long, and in the end it affects both `exports`
>  and `requires` directives.)
> Extend the language of module declarations with the concept of _weak
> modules_.  Weak modules make it easy to modularize components whose
> internals will be accessed by reflection-based frameworks.  Every
> package in a weak module has the following properties:
>   (A) It is exported at both compile time and run time, as if by an
>       `exports` directive, and
>   (B) Its non-public elements are available for _deep_ reflection, i.e.,
>       at run time they can be made accessible to code outside the module
>       via the `AccessibleObject::setAccessible` method of the core
>       reflection API.
> In other words, every type defined in a weak module, whether public or
> not, is subject to exactly the same access checks as in Java SE 8 and
> earlier releases.
> A weak module is declared by placing the modifier `weak` before the
> `module` keyword.  The declaration of a weak module cannot contain any
> explicit `exports` directives.  If the `weak` modifier does not appear
> before the `module` keyword then the declared module is _strong_, and
> it can contain explicit `exports` directives.
> Suppose we have a module `` which has an internal package
> `` that contains entity classes to be manipulated by
> Hibernate, via core reflection.  Then the module declaration
>     weak module {
>         // No exports
>         requires hibernate.core;
>         requires hibernate.entitymanager;
>     }
> exports the public types in ``, and those of any other
> packages, in all phases.  It additionally makes all non-public elements
> of all packages available for deep reflection, enabling Hibernate to
> access such elements in the `` package via the
> `setAccessible` method.
> Weak modules simplify the process of migrating to modules.  The steps
> to convert an existing component into a module were, previously:
>   (1) Make any changes necessary to get it working as an automatic
>       module (e.g., eliminate duplicate packages), and then
>   (2) Write an explicit module declaration, which entails identifying
>       both the component's dependences (`requires`) and the packages
>       whose public types are to be made available to other modules
>       (`exports`).
> With weak modules we can now divide the second step into two steps:
>   (2a) Write an explicit module declaration for a weak module, which
>        entails identifying just the component's dependences (`requires`).
>   (2b) Convert the weak module into a strong module, which entails
>        identifying the packages of the component whose public types
>        are to be made available to other modules (`exports`).
> In other words, weak modules make it possible to focus first upon the
> reliable configuration of a module (`requires`), and then later think
> about its strong encapsulation (`exports`).
> Weak modules are "weak" in what they export, but they remain subject
> to all of the constraints required to achieve reliable configuration.
> They do not read the unnamed module (i.e., the class path), they do not
> allow cycles in the module graph, and they do not allow split packages.
> Weak modules read named modules only as indicated by their `requires`
> directives, and they consume and provide services only as indicated by
> their `uses` and `provides` directives.
>                                   * * *
> In a strong module, an ordinary `exports` directive exports a package at
> both compile time and run time (property (A) above) but does not make its
> non-public types available for deep reflection (B).  In order to enable a
> package in a strong module to be exported in the same way as in a weak
> module we introduce the per-export modifier `private` to denote this
> second property.
> If the above weak `` module, e.g., contains some other packages
> besides ``, and we wish to encapsulate those packages,
> we can convert it into a strong module with the declaration
>     module {
>         exports private;
>         requires hibernate.core;
>         requires hibernate.entitymanager;
>     }
> Now Hibernate can still access any public or non-public entity classes in
> the `` package, but all the other packages are strongly
> encapsulated.
> The `private` modifier should generally not be used to export a package
> containing an API, since normally an API's internal implementation
> details should be strongly encapsulated.  It may, however, be useful for
> legacy APIs whose internals are known to be accessed by existing code.
> Every package in a weak module, an automatic module, or an unnamed module
> is exported as if by an `exports private` directive.
> To ensure the integrity of the platform we expect few, if any, packages
> in the JDK itself to be exported with the `private` modifier.
>                                   * * *
> The new `private` modifier can also be used with qualified exports,
> though they interact with unqualified exports in a non-obvious way.
>   - If you write `exports p` then you can also write `exports private
>     p to m`, so that code in module `m` can access the non-public types
>     of `p` via deep reflection but code outside of `m` can only access
>     the public types of `p`.
>   - If you write `exports p to m1` then you can also write `exports
>     private p to m2`, so that code in `m2` can access the non-public
>     types of `p` via deep reflection, code in `m1` can access the
>     public types of `p`, but no code in any other module can access
>     any of the types of `p`.
>   - If you write `exports private p` then you cannot also have a
>     qualified export of `p`, since code in all other modules already
>     has access to the non-public types of `p` via deep reflection.
> Put informally, you can give your friends additional access, but you
> can't discriminate against them by giving them less access than everyone
> else.
> As before, duplicate `exports` directives are not permitted, in order to
> ensure easy readability.  At most one `exports` directive is relevant to
> any given package/module pair, and it's easy to determine which one.
>                                   * * *
> The introduction of `private` as a modifier of `exports` directives calls
> the existing syntax of `requires public` even more strongly into question
> than before.  A module declaration of the form
>     module {
>         exports private;
>         requires public java.sql;
>     }
> is likely to be very confusing to an uninformed reader.  The `private`
> modifier in the `exports` directive means that the private elements of
> the `` package are exported for deep reflection at run
> time.  The `public` modifier in the `requires` directive, however, does
> not mean that the public elements of the `java.sql` module are needed by
> this module; that is true of any plain `requires` directive.  It means
> that, additionally, any client of this module is granted implied
> readability to the `java.sql` module, thereby gaining access to all of
> its exported types.
> To reduce this confusion we rename the `public` modifier in `requires`
> directives to `transitive`.  Thus the above example becomes
>     module {
>         exports private;
>         requires transitive java.sql;
>     }
> This is potentially confusing in a different way, since in mathematics
> the term "transitive" is usually applied to an entire relation rather
> than to three specific elements of a set.  Its use here does not, in
> particular, mean that the resolver does not interpret plain `requires`
> directives when computing the transitive closure of a set of root
> modules.  "Transitive" as used here is in the more abstract sense,
> expressing the notion of conveying a property -- in this case, the
> readability of the required module -- from one thing to another.
> Notes
> -----
>   - This is significantly different from the first proposal [3].  It adds
>     the notion of weak modules, to ease migration, and also the notion of
>     exporting a package without enabling deep reflection, to strengthen
>     encapsulation.
>   - This proposal removes the notion of dynamic exports, which in the
>     presence of private exports would introduce considerable complexity
>     into the interactions between qualified and unqualified exports.
>     This means that it is no longer possible to export a package only at
>     run time, so it is no longer possible for the author of a module to
>     express the intent that the types of a non-API package are meant to
>     be available to frameworks for deep reflection at run time but
>     inaccessible at compile time.  The dynamic-export feature could, if
>     needed, be added in a future release.
>   - A strong module with no exports makes no types accessible to code in
>     other modules while a weak module makes all of its types accessible,
>     both directly and via deep reflection.  The declarations of such
>     modules are, however, visually similar since most of their text lies
>     between the curly braces:
>         module m1 {
>             requires ...;
>             uses ...;
>             provides ...;
>         }
>         weak module m2 {
>             requires ...;
>             uses ...;
>             provides ...;
>         }
>     We suspect that this visual similarity will not cause much confusion
>     in practice since strong modules that export no packages will be very
>     rare.
>   - If a container is to ensure that a package in an application module
>     is available for deep reflection only by a trusted framework then it
>     can arrange for that by rewriting that module's descriptor, as
>     suggested previously [4], to insert the appropriate qualified private
>     export.  If there is a possibility that two modules will need to
>     export a package of the same name to the same framework module, as
>     suggested by Jason Greene [5], then the container should instead
>     inject a small class into each module whose static initializer
>     invokes the `Module::addExports` method in order to export the
>     package to the framework module.  There is no need any longer for
>     the resolution algorithm to take this scenario into account [6].
>   - This proposal primarily addresses "friendly" uses of reflection, such
>     as dependency injection and persistence, in which the author of a
>     module knows in advance that one or more packages must be exported at
>     run time for deep reflective access by frameworks.  Intrusive access
>     to arbitrary packages of arbitrary modules by, e.g., serialization
>     frameworks or debugging tools, will still require the use of sharp
>     knives such as the `--add-exports` command-line option, the legacy
>     unsupported `sun.misc.Unsafe` API and related APIs, or JVM TI.
>   - Using the `--add-exports` option or its equivalent remains awkward,
>     and sometimes it's the only way out.  To ease migration it's worth
>     considering some way for an application packaged as a JAR file to
>     include such options in its `MANIFEST.MF` file, as suggested by Simon
>     Nash [7].  This is tracked as #AddExportsInManifest [8].
> [1]
> [2]
> [3]
> [4]
> [5]
> [6]
> [7]
> [8]

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