elsewhere eg. it is explained that when you install the java source in
ubuntu, it is put here: /usr/lib/jvm/java-6-sun-220.127.116.11/src.zip
The problem is that it does not contain the javax.activation package
and thus not classes like: javax.activation.DataSource
Why isn't it there?
Must I download the source manually to get that?
 Where to find Java JDK Source Code?
The package is
javax.activation, not java.activation - and typically, source code for
javax... packages isn't included.
The various components of
javax use different licenses, so this could be the reason. I've taken the liberty to ask your platform of choice's maintainers here.
I will update this answer as soon as I get an answer.