Maven-assembly-plugin ignoring manifestEntries?

We’re using this Maven plugin to generate a fat jar for a utility, effectively including all library dependencies un-jarred and re-jarred into a single distribution. The first part was easy, hooking the assembly goal of the maven-assembly-plugin onto the package goal in the maven build lifecycle. Our pom.xml had this entry in it

<plugin>
  <artifactId>maven-assembly-plugin</artifactId>
  <configuration>
    <descriptorRefs>
      <descriptorRef>jar-with-dependencies</descriptorRef>
    </descriptorRefs>
    <archive>
      <manifest>
        <mainClass>com.thekua.maven.ExampleProgram</mainClass>
      </manifest>
    </archive>
  </configuration>
  <executions>
    <execution>
      <phase>package</phase>
      <goals><goal>assembly</goal></goals>
    </execution>
  </executions>
</plugin>

We tried adding in our own entry into the manifest, the CruisePipelineLabel, with a value that should be set by Cruise. We added the new section so our pom.xml now looked like this:

<plugin>
  <artifactId>maven-assembly-plugin</artifactId>
  <configuration>
    <descriptorRefs>
      <descriptorRef>jar-with-dependencies</descriptorRef>
    </descriptorRefs>
    <archive>
      <manifest><mainClass>com.thekua.maven.ExampleProgram</mainClass></manifest>
        <manifestEntries>
          <CruisePipelineLabel>
            ${env.CRUISE_PIPELINE_LABEL}
          </CruisePipelineLabel>
        </manifestEntries>
    </archive>
  </configuration>
  <executions>
    <execution>
      <phase>package</phase>
      <goals><goal>assembly</goal></goals>
    </execution>
  </executions>
</plugin>

After running the target and inspecting the manifest.mf, I couldn’t see the additional property set. I did some searching, found a bug apparently fixed in the 2.2-beta-2 version. After some debugging, I found out that the plugin apparently does not include these additional entries if the value is not set. I tested this out by changing the line to:

<manifestEntries>
  <CruisePipelineLabel>aTestValue</CruisePipelineLabel>
</manifestEntries>

So the answer to whether or not maven-assembly-plugin ignores an element in the manifestEntries is to ensure the value is set before testing it. It looks like a null value is interpreted as “don’t include”.