Multiple versions of a non-exported dependency

Neil Bartlett njbartlett at
Thu Sep 1 00:45:17 UTC 2016


Actually I don’t think that statically linking will work. This would produce modules that have overlapping private (non-exported) packages, and such modules also cannot be used in Java 9 on the modulepath.

I tested this in build 9-ea+126-jigsaw-nightly-h5280-20160713 by creating two modules both containing a private package org.example.util. The following exception resulted:  java.lang.reflect.LayerInstantiationException: Package org.example.util in both module a and module b.

Again this could be “solved” by using custom ClassLoaders or a ClassLoader-based module system like OSGi on Java 9.


> On 31 Aug 2016, at 20:28, Remi Forax <forax at> wrote:
> 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).
> cheers,
> Rémi
> ----- 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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