"Module path" needs additional documentation!

Trying to run a JavaFX program with the exec-maven-plugin, rather than the javafx-maven-plugin.

The latter works flawlessly, for anyone interested the declaration is

   <plugin>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-maven-plugin</artifactId>
            <version>0.0.8</version>

            <configuration>
                 <mainClass>org.openjfx.App</mainClass>
            </configuration>

   </plugin>

But I thought, why not try exec-maven-plugin too, maybe you can get it to work, too.

So I need to inject the module path into the java command line.

So I achieved this:

<plugin>

     <groupId>org.codehaus.mojo</groupId>
     <artifactId>exec-maven-plugin</artifactId>
     <version>3.4.1</version>

     <configuration>
         <executable>java</executable>
         <arguments>
             <argument>--module-path</argument> <!-- inject string into the command line -->
             <modulepath/> <!-- inject the module path based on what's in the local maven .m2 repo  -->
             <argument>--add-modules</argument> <!-- inject string into the command line -->
             <argument>javafx.controls,javafx.fxml,javafx.web,javafx.graphics,javafx.media</argument> <!-- inject string into the command line -->
             <argument>-classpath</argument> <!-- inject string into the command line -->
             <classpath/> <!-- inject dependencies from the local maven .m2 repo  -->
             <argument>org.openjfx.App</argument> <!-- the program -->
          </arguments>
     </configuration>

</plugin>

Running mvn -X exec:exec reveals that the generated command line is

java
@/home/aloy/IdeaProjects/compile_javafx/target/modulepath 
--add-modules
javafx.controls,javafx.fxml,javafx.web,javafx.graphics,javafx.media
-classpath [correct classpath]
org.openjfx.App

This means the --module-path string was not injected.

Instead java is told to look for file @/home/aloy/IdeaProjects/compile_javafx/target/modulepath which has indeed been created.

This behavior is not documented at https://www.mojohaus.org/exec-maven-plugin/examples/example-exec-for-java-programs.html

But the JavaFX program can actually run!

Note that if one removes

<argument>--module-path</argument> <!-- inject string into the command line -->

then the module path is injected into the command line directly (instead of the reference to the file modulepath) but without option -p prepended, leading to failure. Really weird, it would be much better for the plugin to bail out at once with a clear message here.

Start with the exec-for-Java-programs documentation page linked in the issue and reproduce the behavior using the shown exec-maven-plugin configuration and mvn -X exec:exec. Document how <modulepath/> is rendered when used with --module-path, including the observed @.../modulepath output and the behavior without that option. Done means the example or reference clearly explains the generated command line and its limitations.