Multiple Versions at Launch-Time

[Eric Johnson]

Hi Nicolai,

On Tue, Oct 20, 2015 at 9:56 AM, Nicolai Parlog <nipa at codefx.org> wrote:

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>  Hi Eric.
>
> > Why not use OSGi? Or is that what you mean by a "container"?
> I assume that the quote I gave uses "container" to mean systems like
> OSGi (it's from Alan, maybe he will correct me if I'm wrong). So yes,
> I mean that.


> I am trying to find a way without OSGi because it looks like it is not
> used for many projects (relatively speaking). But most projects
> routinely run into problems with conflicting versions and it would be
> great if Jigsaw could solve this for them out of the box.
>

OSGi is used for far more projects than Jigsaw is!

I think you're far, far better off if you can avoid this problem. Even with
OSGi, which is fully capable of supporting many different versions of a JAR
file, to get it right requires some skill at designing for that type of
modularity. You're unlikely to take any off-the-shelf library and get
multiple versions loaded into a JVM successfully. Java is *very* leaky with
interfaces crossing ostensible package boundaries.

As I understand it, since what you're asking for is specifically out of
scope, I don't think you're going to get a great answer here.

Eric.


>
>  so long ... Nicolai
>
>
>
> On 20.10.2015 18:10, Eric Johnson wrote:
> > Why not use OSGi? Or is that what you mean by a "container"?
> >
> > Eric.
> >
> > On Tue, Oct 20, 2015 at 6:30 AM, Nicolai Parlog <nipa at codefx.org>
> > wrote:
> >
> > Hi!
> >
> > There have been a number of threads regarding loading multiple
> > versions of the same module. The requirements say:
> >
> > "It is not necessary to support more than one version of a module
> > within a single configuration."[1]
> >
> > The threads hence usually end like this:
> >
> > "The module system isn't suggesting any solutions, it is instead
> > leaving this problem to the build tools and containers."[2]
> >
> > I can see how we can use class loaders (and now layers) to load
> > multiple versions at runtime. This is essentially what containers
> > do.
> >
> > Now I wonder, is there anything a build tool (or a dedicated
> > developer for that matter) can do to launch the JVM with multiple
> > versions of the same module?
> >
> > Running Example:
> >
> > App requires LibA App requires LibB version 2 LibA requires LibB
> > version 1
> >
> > One way I hoped to do this was to treat both versions as two
> > different modules, i.e. renaming them and update the dependencies
> > on them.
> >
> > App requires LibA App requires LibB_v2 LibA requires LibB_v1
> >
> > This does not work because both modules would export the same
> > packages. That's a little disappointing - I hoped it wouldn't
> > matter because no module is reading from both versions.
> >
> > Since this failed I see no way to solve this aspect of JAR hell
> > without containers. Does anybody else seen one?
> >
> > so long ... Nicolai
> >
> >
> > [1]
> > http://openjdk.java.net/projects/jigsaw/spec/reqs/#multiple-versions
> >
> >
> [2]
> > http://mail.openjdk.java.net/pipermail/jigsaw-dev/2015-October/004787.
> ht
> >
> >
> ml
> >
> >
> >
> >>
> >>
> >
>
> - --
>
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