[jsr294-modularity-eg] Problems for JSR 294 to address

[Alex Buckley]

Of course IDEs can do that. What about javac? Again:

javac Foo.java Bar.java

Is class Foo in the same module as class Bar? Let's answer that by 
assuming all files passed on the command line are in the same module. 
Then, what about:

javac Quux.java

where Quux uses a module-private member of Foo.

Even if the compiler could figure out Foo's module now, what is Quux's? 
You'd need to pass it on the command line, which will get old real fast.

Alex

Since javac is not comparing versions,

Hal Hildebrand wrote:
> It seems to me that the issue is a lack of context.  The actual 
> identities of the modules don't matter at all, rather the only issue 
> that matters is whether the caller and the callee are in the same 
> module.  Alex believes that this identity must be obtained by encoding 
> the module identities into the source and class files so that the 
> compiler can determine their equivalence.  I would only point out that 
> modern IDEs provide a very good approximation of the module concept 
> already.  Whether it be Eclipse, IntelliJ or whatever, the IDE sets up 
> the context such that the compiler can deal with the visibility 
> constraints of what amounts to a module system.  The IDE knows what the 
> current context of the code being compiled is - e.g. it has membership 
> to a Project, in the Eclipse case - and it knows about the context of 
> the code that the compiled class is dependent on.  While it's not there 
> already, it seems like it would be fairly trivial for the compiler to 
> simply compare the two contexts (the calling code's module context and 
> the callee code's module context) and if they are not equals() then the 
> compilation fails for module private members.
> 
> On Feb 2, 2009, at 4:47 PM, 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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