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https://issues.apache.org/jira/browse/MNG-8015?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17817641#comment-17817641
]
ASF GitHub Bot commented on MNG-8015:
-------------------------------------
desruisseaux commented on PR #1401:
URL: https://github.com/apache/maven/pull/1401#issuecomment-1945902723
As a real world use case for testing the Maven improvements, there is the
following scenario: a project depends on JUnit 5 with `compile` scope (because
the project is a conformance test kit). Everything is JPMS: the project, JUnit
5 and their dependencies. JUnit 5 is itself splitted in many modules, including
the `junit-jupiter-api`, `junit-jupiter-engine` and `junit-platform-commons`
modules.
For a mysterious reason, Maven 3 places `junit-platform-commons` dependency
on the module path, but `junit-jupiter-engine` on the class-path (verified with
`mvn test --debug`). The consequence is that JUnit 5 cannot be launch. The JVM
refuses to start with the following error message: _"class
org.junit.platform.engine.discovery.DiscoverySelectors (in unnamed module
@0x3b2c72c2) cannot access class org.junit.platform.commons.util.Preconditions
(in module org.junit.platform.commons) because module
org.junit.platform.commons does not export org.junit.platform.commons.util to
unnamed module @0x3b2c72c2"_. Translation: **JUnit 5 cannot access its own
internal classes!**
The reason why JUnit 5 cannot access its own classes is because
`junit-platform-commons` allows access to its packages only to other JUnit 5
modules. It can be seen with the following command:
```bash
jar --describe-module -f junit-platform-commons-1.10.2.jar
```
Output snippet (reformatted):
```
qualified exports org.junit.platform.commons.util to
org.junit.jupiter.api
org.junit.jupiter.engine
etc...
```
Because Maven puts `junit-platform-commons` on the module-path, access
restrictions to that JAR are enforced. But because Maven puts
`junit-jupiter-engine` on the class-path, that module is unnamed and
consequently not recognized as a module in above list of modules allowed to
access the `junit-platform-commons` internal packages. Thus the failure to
launch JUnit.
A workaround is to add the following lines in Surefire configuration for
breaking module encapsulation:
```xml
<argLine>
--add-exports
org.junit.platform.commons/org.junit.platform.commons.util=ALL-UNNAMED
--add-exports
org.junit.platform.commons/org.junit.platform.commons.logging=ALL-UNNAMED
</argLine>
```
We should not be forced to apply such workaround. This real use case is
another demonstration of the need to improve JPMS handling compared to what
Maven 3 does. For testing if this pull request achieves that goal in Maven 4,
we can test if it allows us to remove the above hack from [GeoAPI conformance
pom.xml
file](https://github.com/opengeospatial/geoapi/blob/9e4ff919ef56e52e05767cc85c88e5795081c883/geoapi-conformance/pom.xml#L133).
> Control the type of path where each dependency can be placed
> ------------------------------------------------------------
>
> Key: MNG-8015
> URL: https://issues.apache.org/jira/browse/MNG-8015
> Project: Maven
> Issue Type: Improvement
> Components: Core
> Affects Versions: 4.0.0-alpha-12
> Reporter: Martin Desruisseaux
> Priority: Major
>
> Make possible to declare where each dependency can be placed: on the
> module-path, class-path, agent path, doclet path, taglet path, annotation
> processing path, _etc._ The proposed improvement consists in adding a new
> {{PATH_TYPES}} property that can be associated to dependencies. The property
> value is an array of {{PathType}}, a new enumeration-like class with values
> such as {{CLASSES}}, {{MODULES}}, {{DOCLET}}, _etc._ Contrarily to real Java
> enumerations, this enumeration-like class is extensible: plugins can add
> their own enumeration values. This is required at least for the
> {{--patch-module}} option, where a new {{PathType}} enumeration value need to
> be created for each module to patch.
> Users can control indirectly the {{PathType}} of a dependency by specifying
> the dependency type. Note that there is no direct mapping between the
> dependency type and where the dependency will be placed, but only an indirect
> mapping caused by the fact that using a dependency type implies implicit
> values of some properties such as classifier, and (with this proposal) path
> types:
> * {{<type>jar</type>}} implies {{PathType.CLASSES}} and {{PathType.MODULES}}.
> * {{<type>modular-jar</type>}} implies {{PathType.MODULES}} only.
> * {{<type>classpath-jar</type>}} implies {{PathType.CLASSES}} only.
> * _etc._
> When a plugin requests the paths of dependencies, the plugin specifies the
> types of path it is interested in. For example, a Java compiler plugin can
> specify that it is interested in {{PathType.CLASSES}} and
> {{PathType.MODULES}}, but not {{PathType.DOCLET}}. If a dependency declared
> that it can be placed on the class-path or the doclet-path, only the
> class-path is left after intersection with plugin's request. This is
> important for the next step.
> If, after all filtering such as above paragraph are applied, a dependency has
> only one {{PathType}} left, then there is no ambiguity and we are done.
> Combined with above-cited dependency types like {{modular-jar}} or
> {{classpath-jar}}, this rule allows users to control where the dependency
> will be placed. But if there are two or more {{PathType}} left after
> filtering, then a choice needs to be done. For example if there are both
> {{PathType.CLASSES}} and {{PathType.MODULES}} (which may happen when
> {{<type>jar</type>}} is used), then an heuristic rule similar to Maven 3 can
> be applied: check if a {{module-info.class}} file or an {{Automatic-Name}}
> manifest attribute is present, and base the decision on that.
> This proposal aims to fix MNG-7855.
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