Multiple versions of a non-exported dependency

Alex Buckley alex.buckley at
Wed Aug 31 19:38:14 UTC 2016

In the JDK 8 java launcher, classpath hell is having two [incompatible] 
versions of the "same" JAR on the classpath and getting types from both 
of them.

In the JDK 9 java launcher, we avoid modulepath hell -- two 
[incompatible] versions of the "same" module on the modulepath -- by 
uniquifying each module on the modulepath. If someone tries to work 
around that by giving different names to the different versions of the 
"same" module, then we still avoid modulepath hell by refusing to load 
their [incompatible] packages into the same loader. The java launcher is 
not becoming a mini container that spins loaders for multiple versions 
of the same module -- code defined by the application loader on JDK 8 is 
defined by the application loader on JDK 9.


On 8/31/2016 12:21 PM, cowwoc wrote:
> Well, this is unfortunate. As I stated earlier, I fail to see how
> depending on constant version numbers (not version ranges) fall under
> the scope of "version selection". Was this case considered/discussed in
> depth?
> Not everyone is sold on version ranges (e.g. the vast majority of Maven
> artifacts I've seen depend on constant versions) and I think this would
> go a long way towards solving the original "classpath hell" problem.
> Gili
> On 2016-08-31 2:55 PM, Neil Bartlett [via jigsaw-dev] wrote:
>> 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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