[jsr294-modularity-eg] Module boundaries

Christopher Brind brindy at brindy.org.uk
Fri Feb 13 19:37:22 EST 2009

Hi Richard,

So you are saying that masquerading is acceptable and JSR 294 is not going
to even try to prevent it?

If that is the case, why is what is being proposed/discussed for JSR 294 so

For instance, a simpler solution would be to create a new type modifier
which bridges the gap between public and package private, allowing types
with the modifier to be accessed by classes in the parent package only.

e.g. for want of a better name lets call it 'internal' (I think I read
internal suggested before, but can't think of a better name, sorry)

   Geo: internal class Geo { ... new GeoSupport() ...  }
   GeoSupport: class GeoSuport { ... }

   Main: public class Main { ... new Geo() ... }

That results in minimal changes to Java, is easy to understand and seems to
'fix' the problem described in the original JSR 294.


2009/2/13 Richard S. Hall <heavy at ungoverned.org>

> Isn't such masquerading possible in approaches that allow separate
> compilation? I am not sure this is an issue. For example, nothing stops me
> from creating a class in someone else's package and adding it to a JAR file
> to get package-private access.
> -> richard
> Christopher Brind wrote:
>> Hi,
>> I have to admit that I read the straw-man with a mixture of both
>> excitement and trepidation. :)
>> The first question that springs to mind is, what is to stop me from
>> contributing to the 'Felix' module example in a way that then exposes some
>> of the module private functionality, e.g. via a delegate?
>> So, for example, let's say that I have downloaded a JAR which contains the
>> Felix classes from the example in the straw-man.  I could compile the
>> following in order to contribute to the Felix module (source tree example
>> like in the straw-man):
>> module-info: module Felix;
>> org/apache/felix/framework
>>    PublicFelix: package org.apache.felix.framework;
>>              public class PublicFelix {  ... }
>> I then repackage the downloaded JAR adding the PublicFelix class.  The
>> result is that because I told the compiler that the PublicFelix class is in
>> the Felix module, it can access the Felix class.  The code in the
>> PublicFelix class is basically a delegate for the Felix class, but since the
>> PublicFelix class is accessible from anywhere the ultimate result is that
>> the module system has been circumvented.
>> The implication of this seems to be that if you want to prevent this from
>> happening then you will have to provide the binaries in a format which
>> prevents tampering.  For instance, if the original Felix module was compiled
>> and then bundled as a signed JAR I could not contribute to the module as
>> described above as that will break the JAR's signature.  However, the
>> proposal says that types in a module can be loaded from multiple containers,
>> e.g. URL Class Loader.  How do I protect my module's internal implementation
>> in this case?
>> Regards,
>> Chris
>> 2009/2/13 Peter Kriens <peter.kriens at aqute.biz <mailto:
>> peter.kriens at aqute.biz>>
>>    Enclosed find the first strawman proposal for nested modules. This
>>    was not easy :-( but I tried hard to make it as clear as possible.
>>    I guess sending docs will probably not work or create too much
>>    CO2, so here is the link: http://www.aqute.biz/JSR294/HomePage. Enjoy.
>>    Kind regards,
>>           Peter Kriens
>>    On 4 feb 2009, at 10:32, Alex Buckley wrote:
>>        Yes, setting module membership at runtime effectively
>>        simulates 'internal', where the .NET assembly-build operation
>>        is done lazily.
>>        Module nesting sounds interesting. Could you please send some
>>        source code which exhibits how it would impact module-private
>>        accessibility?
>>        We can certainly have a phone call. I am travelling
>>        internationally for the next week and, in practice, will not
>>        be back in the office until Tuesday 17th.
>>        EG members, please indicate when on 17-20th February you could
>>        be available. Good times would be 9am West Coast, 12pm East
>>        Coast, 6pm France.
>>        Alex
>>        Peter Kriens wrote:
>>            Ok, so you allow a module boundary to be set in runtime,
>>            this will make the module keyword in unbound modules
>>            reflect the semantics of the .NET 'internal' concept? I
>>            agree we should then have a discussion later about how the
>>            compiler gets the unbound module boundaries.(Do we have an
>>            action list for these things?)
>>            If I look at software projects than there will be a
>>            tendency to put a set of related packages into a bound
>>            module, where a number of these bound modules are packaged
>>            in a JAR (the unbound module). This seems to scream for
>>            nesting of bound modules?
>>             VM -> unbound module -> bound module +-> package -> type -+
>>                                  ^              |           ^        |
>>                                  +--------------+           +--------+
>>            And, can we please, pretty please, have a kick-off phone
>>            conference now we're a group starting to work on this?
>>            Kind regards,
>>              Peter Kriens
>>            On 3 feb 2009, at 01:47, Alex Buckley wrote:
>>                .NET is not Java.
>>                Compiled types in .NET live in non-executable modules
>>                (.netmodule files). To be executed, modules must be
>>                packaged into assemblies (.dll/.exe files).
>>                This packaging makes "internal" a bit tricky.
>>                "internal" is specified as "accessible from the same
>>                program", where a program is merely a bunch of source
>>                files. A C# compiler must take it on trust that the
>>                .netmodule which accesses an "internal" artifact will
>>                be packaged in the same assembly as the .netmodule
>>                which declares the artifact.
>>                This is not unreasonable, since .netmodules MUST be
>>                packaged. But if packaged "wrongly", code accessing an
>>                "internal" artifact which the compiler promised was
>>                accessible will fail at runtime. You do not want
>>                language features to behave like this. There is
>>                effectively a hole in the C# spec caused by the
>>                greater CLI spec stratifying loadable code into
>>                non-executable and executable.
>>                Java is different. A JVM does not require classfiles
>>                to be packaged in any way before executing them. Nor
>>                does the reference implementation of a Java compiler
>>                require classfiles to be packaged before compiling
>>                against them. These things are not going to change.
>>                Since module-private accessibility is a language and
>>                VM construct, it cannot depend on the future packaging
>>                of code still being compiled or compiled against.
>>                (What it can depend on is an API consulted by a Java
>>                compiler to discover whether module M version X and
>>                module M version Y have "equal" versions. The API
>>                implementation would be honor-bound to give consistent
>>                answers, similar to classloaders. We'll get to that
>>                API soon, I hope.)
>>                This boils down to saying the module-private
>>                accessibility and module membership go hand-in-hand
>>                for me. In any case, the first thing a programmer will
>>                ask when seeing a module-private member is: "What
>>                module does this member belong to?". The answer should
>>                NOT be "it depends on a compiler flag or invocation of
>>                jar".
>>                I will shortly send a Java language grammar and
>>                classfile format which reflect these views.
>>                Now, I know this issue is not going to go away. I
>>                therefore propose that IN ADDITION to encoding module
>>                membership in source, it should be possible for the
>>                host system to determine module membership. If you
>>                REALLY want the answer above to be "it depends on a
>>                compiler flag or invocation of jar", you can do that.
>>                But a Java compiler/VM must defer to the membership in
>>                a source/class file if it exists.
>>                Is that enough of a weakening of my requirements #1
>>                and #2?
>>                Alex
>>                Peter Kriens wrote:
>>                    Hmm. .NET shows that you do not have to know all
>>                    the module boundaries during compilation.
>>                    'internal' indicates inside the assembly and the
>>                    source assembly is clearly not known during
>>                    compilation. This concept looks attractively
>>                    simple and is in line with common practice.
>>                    For 294, during compilation, the JAR files and
>>                    project source can easily be used inside the
>>                    compiler for the scope. During runtime, the module
>>                    system can set this boundary.
>>                    However, I also like a module concept that is
>>                    defined in the language. This will be more complex
>>                    (and probably should be hierarchical) but it
>>                    should be not attempt to define the deployment
>>                    artifact nor dependencies. That is, it should be
>>                    more like namespaces.
>>                    Kind regards,
>>                     Peter Kriens
>>                     On 28 jan 2009, at 03:56, Alex Buckley wrote:
>>                        I am happy to discuss module-private
>>                        accessibility first, in that it addresses the
>>                        limitations of packages on which we all agree.
>>                        The issue we cannot ignore is that to check
>>                        module-private accessibility at compile-time
>>                        means knowing the modules to which the
>>                        requesting and requested types belong. If I
>>                        invoke:
>>                        javac Foo.java Bar.java
>>                        where Foo.java refers to Bar, then a compiler
>>                        must be able to compute at least the module
>>                        name to which Foo belongs as Foo is compiled,
>>                        if Bar is found to be module-private as it is
>>                        compiled. There are no deployment artifacts
>>                        around yet. It shouldn't be a surprise that
>>                        Foo.class and Bar.class are subsequently
>>                        emitted to encode their module membership.
>>                        So with module accessibility comes module
>>                        membership. Module dependencies are a quite
>>                        different topic which we'll get to soon
>>                        enough, and they are the place where
>>                        minimizing new concepts is essential.
>>                        Alex
>>                        Peter Kriens wrote:
>>                            I think it would make sense to work from
>>                            first principles
>>                            1. What problem are we trying to solve?
>>                            2. How do we scope this problem to an
>>                            acceptable area for all?
>>                            3. What requirements should the solution
>>                            fulfill
>>                            I would like to have a short discussion
>>                            about the problem so we are sure we are
>>                            all on the same page. I can start off with
>>                            the problem I think we are trying to solve.
>>                            Modularity
>>                            Modularity is the art of encapsulation and
>>                            hiding. A module limits the amount of
>>                            information outside its boundaries. This
>>                            reduces the overall complexity of a system
>>                            because it becomes possible to reason (and
>>                            change) locally about a module instead of
>>                            understanding the whole system, and it
>>                            provides a local namespace that is easier
>>                            to work with for humans than a global
>>                            namespace.
>>                            Java provides modularity in many places. A
>>                            type encapsulates its fields and methods
>>                            and selectively exposes them to other
>>                            types with the access modifies: private,
>>                            protected and public. A package
>>                            encapsulates a number of types and
>>                            resources. And last, but not least, the
>>                            class loader provides a class space that
>>                            can be distinct from other class spaces.
>>                            The modularity that is enabled with class
>>                            loaders has been exploited by many.
>>                            However, the Java Language has little to
>>                            say about class loaders; it is outside the
>>                            scope of the language.
>>                            So what problem do we need to solve,
>>                            seeing that Java provides already so many
>>                            forms of modularity? There is a sense in
>>                            the community that packages are too small
>>                            for modules. Though the package names hint
>>                            at a hierarchy (and this is how they are
>>                            found in a file system), this hierarchy
>>                            does not provide a preferential treatment
>>                            for children. Many programs consist of
>>                            hundreds of packages. However, except for
>>                            the limited package concept, the
>>                            programmer cannot indicate that the
>>                            visibility of an artifact should be
>>                            limited to the program, or part of the
>>                            program.
>>                            A large number of delivery formats have
>>                            been created over the years, most of the
>>                            based on the JAR format (which is also not
>>                            a part of the language): WAR, EAR, OSGi
>>                            bundles, midlets, xlets, and likely many
>>                            more proprietary formats. Usually, the
>>                            modularity of these modules is based on a
>>                            rather simplistic hierarchical class
>>                            loader model. That is, all classes in
>>                            ancestors are visible, but not any
>>                            siblings or their children. The language
>>                            is moot on the point of this type of
>>                            encapsulation.
>>                            Delivery modules contain types that depend
>>                            on types in other delivery modules. There
>>                            is no uniformly agreed Java standard to
>>                            model these dependencies. During build
>>                            time, the compiler is provided with a
>>                            linear list of JARs and the compiler picks
>>                            the first type that matches a name. Only
>>                            the name is encoded in the class file, not
>>                            the originating delivery module. During
>>                            runtime, the same process is used to find
>>                            classes, albeit in a hierarchical class
>>                            loader model. This has all so far been
>>                            outside the Java Language.
>>                            It is crucial that we distinguish the
>>                            language/logic modularity and the
>>                            modularity based on the deployment
>>                            artifact. Interestingly, in my
>>                            understanding, .NET makes such a
>>                            distinction between the modularity of a
>>                            delivery unit (called an assembly) and the
>>                            finer grained modularity inside a delivery
>>                            unit, represented by namespaces. The
>>                            "internal" keyword indicates that the
>>                            artifact is visible only inside an
>>                            assembly and the "namespace" keyword
>>                            provides a hierarchical namespace.
>>                            Therefore, one can identify two problems
>>                            in the Java platform concerning modularity
>>                            1. Packages are too limited for proper
>>                            language/logic modularity
>>                            2. Lack of a runtime module concept that
>>                            is related to deployment
>>                              artifacts (physical modularity)
>>                            Looking at the proposed time frame (EDR by
>>                            mid-April), attacking both problems
>>                            simultaneously seems rather unrealistic
>>                            based on my limited experience. I
>>                            therefore suggest to start with problem
>>                            #1: Packages are too limited for proper
>>                            language/logic modularity. What do you think?
>>                            Kind regards,
>>                            Peter Kriens
>>                            On 27 jan 2009, at 05:31, Alex Buckley wrote:
>>                                Comments welcome.
>>                                1) Packages are typically arranged in
>>                                hierarchies, but types can only be
>>                                used across different branches of the
>>                                hierarchy by being made public,
>>                                which exposes implementation details
>>                                too widely. Information hiding is
>>                                further reduced by interface members
>>                                always being public.
>>                                Non-solution: hierarchical package
>>                                membership. Redefining existing
>>                                well-known semantics is always a bad
>>                                idea, and this one is especially
>>                                complicated. There would need to be a
>>                                way to stop package-private
>>                                artifacts from being accessible to
>>                                subpackages, or else we would be
>>                                strengthening information hiding in
>>                                one place only to weaken it
>>                                elsewhere. Also, there would need to
>>                                be a way to configure the depth of
>>                                exposure of package-private artifacts,
>>                                since artifacts in package A
>>                                should sometimes be accessible to A.B
>>                                and not to A.B.C, and so on.
>>                                2) Dependencies of one type on another
>>                                are expressed in source and
>>                                classfiles, but there is no standard
>>                                way for a Java compiler or JVM to
>>                                interact with its environment to read
>>                                and resolve dependencies. This
>>                                causes compile-time and runtime
>>                                environments to differ, and complicates
>>                                any effort to version types available
>>                                in the (compile-time or runtime)
>>                                environment.
>>                                Non-solution: standardize the
>>                                CLASSPATH. One list for all programs and
>>                                libraries in the JVM makes versioning
>>                                almost meaningless, and packages
>>                                whose types occur in multiple
>>                                CLASSPATH entries can behave poorly. (See
>>                                Peter Kriens' presentation at Devoxx
>>                                2008.)
>>                                3) Many packages have grown large over
>>                                the years. They are effectively
>>                                impossible to refactor now, since it
>>                                is binary-incompatible to rename
>>                                types and dangerous to "split"
>>                                packages. The next-best option is to let
>>                                subsets be taken of existing packages,
>>                                and deliver subsets
>>                                independently, but there is no
>>                                mechanism in the Java language or JVM for
>>                                that.
>>                                Non-solution: a mechanism for renaming
>>                                packages and types outside the
>>                                Java language. This would require
>>                                updating Java's model of binary
>>                                compatibility, and would make source
>>                                code incomprehensible.
>>                                Alex
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