Maven: Error whan setting local log4j jar as dependency - maven

Question

For some technical reasons, I have to include log4j in the pom file as a local jar. I've tried:
<dependency>
<groupId>log4j</groupId>
<artifactId>apache-log4j-extras</artifactId>
<scope>system</scope>
<systemPath>path_to_jar/apache-log4j-extras-1.1.jar</systemPath>
</dependency>
With and without:
<version>1.1</version>
Either way, I get the cryptic message Failed to validate POM for project, without reference to the specific failure reason. When log4j is fetched from maven repository, this is working well.
[INFO] Trace
org.apache.maven.reactor.MavenExecutionException: Failed to validate POM for project ... at ...hudson_checkout/vespa_qa/pom_local.xml
at org.apache.maven.DefaultMaven.getProjects(DefaultMaven.java:404)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:272)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:138)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)
at org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:60)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
Caused by: org.apache.maven.project.InvalidProjectModelException: Failed to validate POM for project ... at /....xml
at org.apache.maven.project.DefaultMavenProjectBuilder.processProjectLogic(DefaultMavenProjectBuilder.java:1077)
at org.apache.maven.project.DefaultMavenProjectBuilder.buildInternal(DefaultMavenProjectBuilder.java:880)
at org.apache.maven.project.DefaultMavenProjectBuilder.buildFromSourceFileInternal(DefaultMavenProjectBuilder.java:508)
at org.apache.maven.project.DefaultMavenProjectBuilder.build(DefaultMavenProjectBuilder.java:200)
at org.apache.maven.DefaultMaven.getProject(DefaultMaven.java:604)
at org.apache.maven.DefaultMaven.collectProjects(DefaultMaven.java:487)
at org.apache.maven.DefaultMaven.getProjects(DefaultMaven.java:391)
... 12 more
[INFO] ------------------------------------------------------------------------
[INFO] Total time: < 1 second
The jar is located in the specified folder and has read permissions. Any idea what's wrong with this dependency?

Answer 1

I've executed maven with "-X", and got a more helpful message:
[0] 'dependencies.dependency.version' is missing for log4j:apache-log4j-extras:jar
So I added the version again. The project still fails, with strange error messages. I searched a little further, and found that the common way to fetch og4j from the system path requires tis group id:
<groupId>org.apache.log4j</groupId>
Instead of:
<groupId>log4j</groupId>