why is "test-jar" dependency required for "mvn compile"

In my case the root cause was that the module which should be used as a dependency in test scope with type test-jar did not include the required maven-jar-plugin configuration. Without the snippet below no test jar will be deployed when you call mvn deploy on the respective module.

<build>
<plugins>
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <executions>
      <execution>
        <goals>
          <goal>test-jar</goal>
        </goals>
      </execution>
    </executions>
  </plugin>
</plugins>

See https://maven.apache.org/guides/mini/guide-attached-tests.html for more details.

So I did some serious debugging, and discovered that the problem seems to be an interaction between the exec:java plugin, test-jar dependencies and mvn compile.

In short, if you attach exec:java to an execution phase, mvn compile starts looking for test-jar dependencies at compile time. If you remove the <executions> element from the exec:java plugin declaration, mvn compile works fine again.

I filed a bug report for the exec:java plugin here, though I can't really tell whether the bug is in exec:java, test-jar or mvn compile so perhaps the bug will be moved somewhere else if/when someone figures that out:

http://jira.codehaus.org/browse/MEXEC-91

Update: It's not really a bug, the maven-exec-plugin is documented as requiring test dependencies here:

http://mojo.codehaus.org/exec-maven-plugin/java-mojo.html

That doesn't mean it wouldn't make a great feature. ;-)

This looks like a definite bug to me.

I have the same problem and tested Maven 3.0.1 and 3.0.2. Validate doesn't fail, only the compile step fails. With Maven 3 mvn compile breaks but mvn test-compile works.

It appears that the compile phase is looking for test-jar artifacts in the reactor and then repo, but it shouldn't since the dependency is in test scope. Test scope artifacts should be resolved during test-compile, not compile.

As a result, I thought this could be worked around by mapping the maven-compiler-plugin's testCompile goal to the compile phase, instead of the default test-compile phase.

I added this to my pom, right next to the part that adds the test-jar creation in the upstream pom:

  <!-- there is a bug in maven causing it to resolve test-jar types
       at compile time rather than test-compile. Move the compilation 
       of the test classes earlier in the build cycle -->
  <plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <executions>
      <execution>
        <id>default-testCompile</id>
        <phase>compile</phase>
        <goals>
          <goal>testCompile</goal>
        </goals>
      </execution>
    </executions>
  </plugin>

But that won't work either because the five phases between compile and test-compile haven't run and set up things like the test classpath.

I guess the real workaround until this bug is fixed is to use test-compile in place of compile.