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 <[hidden email]
</user/SendEmail.jtp?type=node&node=5713366&i=0>> 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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