Multiple versions of a non-exported dependency

Neil Bartlett njbartlett at
Wed Aug 31 18:54:44 UTC 2016


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.


> 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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