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.buck...@oracle.com> 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
