jigsaw-dev Digest, Vol 37, Issue 29

[Dr. James J. Hunt]

On 02/25/2012 09:00 PM, jigsaw-dev-request at openjdk.java.net wrote:
> Message: 1
> Date: Fri, 24 Feb 2012 22:54:26 +0100
> From: R?mi Forax <forax at univ-mlv.fr>
> Subject: Re: Still driving off the cliff
> To: jigsaw-dev at openjdk.java.net
> Message-ID: <4F480712.8050404 at univ-mlv.fr>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> On 02/24/2012 07:50 PM, David M. Lloyd wrote:
>> > On 02/24/2012 12:15 PM, R?mi Forax wrote:
>>> >> On 02/24/2012 06:26 PM, David M. Lloyd wrote:
>>>>> >>>> With Jigsaw then it's usually a 1-1 mapping between module and class
>>>>> >>>> loaders but we do have complications in the platform and it remains to
>>>>> >>>> be seen where that goes. In addition I'm not sure where the class 
>>>>> >>>> loader
>>>>> >>>> relationship will be specified or not.
>>>> >>>
>>>> >>> The *only* reason I can see that Jigsaw doesn't mandate that a module
>>>> >>> is a class loader is a module is so that platform classes can return
>>>> >>> "null" to getClassLoader and yet still be called "modules".
>>>> >>>
>>>> >>> This is such a very thin foundation on which to base decisions such as
>>>> >>> redefining what a module *is*, and consequently requiring two *modes*
>>>> >>> of operation (doesn't that seem really awful to anyone else?). If the
>>>> >>> platform *must* be considered a single class loader then it shouldn't
>>>> >>> be considered to be multiple modules - more like a single module with
>>>> >>> some optionally available "features". With this simple change you
>>>> >>> don't need to have a "compatibility mode" to run old-style
>>>> >>> applications, and you can consequently allow users to leverage a
>>>> >>> *huge* body of prior work.
>>> >>
>>> >> There are lot of program like profilers, debuggers, agents etc
>>> >> that relies on the fact that there is a way to separate the platform
>>> >> classes from
>>> >> the application classes, this is usually done by checking if
>>> >> getClassLoader returns null
>>> >> on a class.
>>> >> If you break this assumption a lot of code will just break.
>>> >> So even in a module world, you need a way to separate platform code from
>>> >> application code
>>> >> (it should be better to even have a system which is able to separate the
>>> >> application, from the container,
>>> >> from the platform).
>>> >>
>>> >> I agree with David that we should not have a compatibility mode (at
>>> >> runtime).
>>> >> Instead of having getClassLoader() returning a module class loader,
>>> >> getClassLoader() should still
>>> >> return null for the platform class and jigsaw should add a new method
>>> >> called getModuleClassLoader()
>>> >> that returns the module class loader.
>> >
>> > Honestly I don't even see the need to actually divide the platform 
>> > into actual modules.  If the desire is to be able to express 
>> > dependencies on certain features of the platform which would be made 
>> > optional, then this can be done with module aliases or views or 
>> > feature dependencies or any number of other ways which don't carry 
>> > this cost.
>> >
> I see at least 3 advantages:
> - ease development of JDK (make integration easier)
> - push incremental update (module == distrib packet)
> - get ride of stupid legacy code (CORBA?)
>
> R?mi
>
Dear Colleagues,

Perhaps the biggest reason is to support dynamic programming platforms
on systems with limited resources.  Modules provide a means of defining
clearly what is supported and what not on such a platform.  There are
even systems where one would like to leave out a seemingly central part
of Java, such as file access.

Sincerely,
James

P.S. I do not think CORBA is stupid legacy code, but being able to leave
it out or easily provide a better implementation would be helpful.

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