Hi Jun Rao, even the ./contrib directory has an almost empty .jar file in ./contrib/target/scala-2.8.0. Aggregated projects in Build.scala (i.e. kafka and contrib) are exhibiting this behavior. This is because they don't have any src folders as immediate children of those base directories (root or contrib) and hence they are generating a manifest only .jar file.
If the name as 'kafka_<version> seems confusing, we can either 1) add a build.sbt to the root folder that changes the name of the root target folder's jar to root_2.8.0-0.8-SNAPSHOT.jar. OR 2) we can change the core/target folder's output jar to core_2.8.0-0.8.0-SNAPSHOT.jar instead of kafka.
I have added a patch for (1) just in case.