Multiple Versions at Launch-Time

[Nicolai Parlog]

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 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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