Reusing module name token `*` in -d

Wed Jan 25 22:50:24 UTC 2017

Stephan,

There will be many cases where one project contains one module
and for those cases, the single module mode compilation will be
sufficient and bears the most similarity to the way people organize
their project today.

At the other end of the scale, there will be cases where a set of
closely-coupled modules have to be compiled together, and
multi-module mode was designed to address those cases. In this
situation, having the class files end up in a single directory hierarchy
organized by module should not be surprising.

In between, there may be projects that contain a set of loosely
related modules, such that they can be compiled one module
at a time in a topologically sorted order. For these projects, it
becomes a design choice for the IDE vendor and/or a use choice
for the developer whether to use a series of compilations, one
per module, into distinct class output directories, or whether to
compile them all together, and have the classes written to a
single module-oriented output directory. Either way, the effective
compilation will be the same, and the same checks for strong
encapsulation should be in effect between the modules.

(For your example of a set of separate projects, each with its own
sourcefolder and binfolder, I would expect each project to have its
own independent build "script", which each come down to one of the
three preceding cases.)

Comparing the two ways of compiling a set of modules ...

Compiling modules one at a time (when that is possible) is the
smallest overall change for both the IDE and developer.
Each module can be compiled and built separately, and the resulting
modules can be put individually on the module path.

Compiling modules together may yield a faster overall compilation
times, and the use of a single output directory makes it much easier
to set up the module path at runtime.

-- Jon

On 01/25/2017 12:05 PM, Stephan Herrmann wrote:
> I found the general direction of this proposal interesting,
> but from the reluctance to accept this, I start to think,
> that maybe the role of multi-module compilation was overrated?
>
> I assume that the majority of source code out there is organized
> in some structure like
>   Project1
>     + sourcefolder
>     + binfolder
>   Project2
>     + sourcefolder
>     + binfolder
>   ...
> Note there is nothing outside of projects.
> I don't believe this is going to change any time soon.
> Tools know how to find .class files in this structure.
>
> When projects become modules, at first look it sounds compelling to
> perform a multi-module compilation on several projects in one invocation.
> (Compelling for reasons of "simplicity" as well as performance ...).
> If scattering .class files into unrelated binfolders is not intended,
> this seems to suggest to forget about multi-module compilation,
> and keep compiling one project/module at a time, where we can specify
> individual output folders.
>
> Is that the message?
>
> Maybe, at the end of the pipeline there will be one use case where
> multi-module compilation will indeed collect "everything" into one
> global bin folder, ready for packaging and shipping, but that would be
> the least frequently performed use case, and then it's much easier
> to just copy / merge (or link) the individual binfolders into one.
>
> Any positive reasons to consider multi-module builds after all?
>
> thanks,
> Stephan
>
> On 01/25/2017 07:38 PM, Jonathan Gibbons wrote:
>>
>>
>> On 1/25/17 12:20 AM, Nicolai Parlog wrote:
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>>>   Hi Jonathan,
>>>
>>> thanks for considering this.
>>>
>>>> If nothing else, it would require all the module-specific output
>>>> directories to be created ahead of time, so that javac can
>>>> determine which ones to use
>>> Why would that be the case? It is not necessary to create them now so
>>> why would using asterisk change that?
>>
>> OK, I missed that you were not suggesting to adopt all of the 
>> functionality
>> of module source path, including the ability to specify a path of output
>> locations, so that module classes could be written into a directory 
>> that is
>> more closely associated with the source code.
>>
>>>
>>>> It has always been the case that a single compilation for
>>>> different packages from different libraries would result in the
>>>> classes being placed in a single output directory hierarchy, and
>>>> that the classes could then be selectively packaged into different
>>>> files like .jar files.
>>> It has also always been the case that the compiler had no notion of
>>> projects/artifacts/modules but just of plain source files. ;) That
>>> changed, too, so why not do the same for class files?
>>>
>>>> If you're compiling modules together, why could you not do
>>>> something similar?
>>> I have no particular use case (except writing some demos) but I would
>>> guess that it would make it more comfortable for existing tools to
>>> move towards multi-module compilation.
>> I don't see any reason why the current design would make it harder
>> for tools to move towards multi-module compilation.
>>
>>>
>>> I also like this idea for its symmetry. You can define input the
>>> compilers input, sorted by modules, with *, so why not do the same for
>>> its output? Conceptually that should be obvious (which does not mean
>>> that there are not plenty if reasons against it).
>>
>> There is a much stronger, more compelling relationship in play.
>> The current -d option is designed so that you can put the output
>> directory on the class path or module path as appropriate. That's
>> always been the case for the classpath -- even though source
>> may come from a variety of directory hierarchies, the compiled
>> classes are put into a simple directory hierarchy that can be put
>> on the classpath. Now, with modules, that continues to be the case:
>> you can put the -d output directory on the runtime module path,
>> either as a single "exploded module" or as a directory of "exploded
>> modules". That is a compelling argument in favor of the current
>> design of the javac output directory.
>>
>> In contrast, I don't see any compelling advantage to allowing
>>     -d "./*/target/classes"
>> since that would make it significantly harder to place such a
>> directory on the runtime module path.
>>
>> -- Jon
>>
>>>
>>>   so long ... Nicolai
>>>
>>>
>>>
>>> On 23.01.2017 20:51, Jonathan Gibbons wrote:
>>>> Nicolai,
>>>>
>>>> I don't think this proposal is a good way to go. If nothing else,
>>>> it would require all the module-specific output directories to be
>>>> created ahead of time, so that javac can determine which ones to
>>>> use, which would require additional setup commands to be executed
>>>> after a "make clean" or its equivalent in other build systems.
>>>>
>>>> Also, I note that the output directory is typically never the
>>>> final location for the compiled classes; it is typically just a
>>>> "staging area". It has always been the case that a single
>>>> compilation for different packages from different libraries would
>>>> result in the classes being placed in a single output directory
>>>> hierarchy, and that the classes could then be selectively packaged
>>>> into different files like .jar files.   If you're compiling modules
>>>> together, why could you not do something similar?
>>>>
>>>> -- Jon
>>>>
>>>>
>>>>
>>>> On 01/21/2017 02:00 AM, Nicolai Parlog wrote:
>>>>> Hi!
>>>>>
>>>>> Another feature request from the trenches regarding multi-module
>>>>> compilation. (It is possible that there was a similar thread a
>>>>> couple of days/weeks (?) back but I didn't find it.)
>>>>>
>>>>> It would be nice to have the ability to specify module specific
>>>>> target folders, so they do not automatically end up in
>>>>> `<whatever-was-given-to-d>/<module-name>`.
>>>>>
>>>>> It seems obvious (which could very well make it stupid) to reuse
>>>>> the asterisk here and allow something like
>>>>>
>>>>> javac --module-path mods --module-source-path
>>>>> "./*/src/main/java" -d "./*/target/classes" -module
>>>>> initial.module
>>>>>
>>>>> I have not thought through how this might or might not work with
>>>>> multiple module source paths. It looks like the only tractable
>>>>> approach would be to not allow more than one -d element.
>>>>>
>>>>> so long ... Nicolai
>>>>>
>>>>>
>>>>>
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>>
>