Multiple versions of a non-exported dependency

Remi Forax forax at
Wed Aug 31 19:28:50 UTC 2016

The other solution is to statically link the right version of slf4j inside guava and jsoup.
A tool like jarjar can be updated to merge two modular jars (merge two module-info).


----- Mail original -----
> De: "Neil Bartlett" <njbartlett at>
> À: cowwoc at, "Alex Buckley" <alex.buckley at>
> Cc: "ZML-OpenJDK-Jigsaw-Developers" <jigsaw-dev at>
> Envoyé: Mercredi 31 Août 2016 20:54:44
> Objet: Re: Multiple versions of a non-exported dependency

> Gili,
> As Alex points out: your use-case can be supported in Java 9 but only with the
> addition of custom ClassLoaders, or by using an existing ClassLoader-based
> module system such as OSGi.
> The same is also true of Java 8, and Java 7, etc.
> Regards,
> Neil
>> On 31 Aug 2016, at 19:29, Alex Buckley <alex.buckley at> wrote:
>> On 8/31/2016 10:56 AM, cowwoc wrote:
>>> I recently became aware of the fact that the Jigsaw specification declared
>>> "version-selection" as a non-goal. While I understand how we ended up here,
>>> I am hoping that you were able to support the following (very common)
>>> use-case:
>>> * Module "HelloWorld" depends on modules "Guava" and "JSoup".
>>> * Module "Guava" depends on module slf4j version 1 (requires but does not
>>> export it).
>>> * Module "JSoup" depends on module slf4j version 2 (requires but does not
>>> export it).
>>> * slf4j version 2 and is not backwards-compatible with version 1.
>>> What happens at runtime? Will Jigsaw (out of the box, without 3rd-party
>>> tools like Maven or OSGI) be smart enough to provide different versions of
>>> slf4j to "Guava" and "JSoup"?
>> (You mean Guava/JSoup requires slf4j version 1/2 and does not "re-export" it
>> a.k.a. 'requires public'.)
>> This use case isn't possible on JDK 8 for JARs on the classpath, and it's not
>> supported on JDK 9 for modular JARs on the modulepath:
>> - If you have two versions of a modular JAR slf4j.jar in different directories
>> on the modulepath, then the first one to be found will dominate, and that's
>> what will be resolved for both Guava and JSoup.
>> - If you have two modular JARs slf4j_v1.jar and slf4j_v2.jar on the modulepath,
>> and Guava requires slf4j_v1 and JSoup requires slf4j_v2, then launching 'java
>> -m HelloWorld' will fail. The boot layer will refuse to map the "same" packages
>> from different slf4j_v* modules to the application class loader.
>> The use case _is_ supported on JDK 9 for modular JARs loaded into custom loaders
>> of custom layers. That is, the Java Platform Module System is perfectly capable
>> of supporting the use case -- please see any of my "Jigsaw: Under The Hood"
>> presentations. The use case just isn't supported "out of the box" by the 'java'
>> launcher for JARs on the modulepath.
> > Alex

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