Hi,
Ant package (http://packages.debian.org/unstable/devel/ant.html) do not depend on any Java Compiler.
This way, <javac/> ant task is unusable without some help (JAVA_HOME environment variable, build.compiler property). It is a core Ant task (http://jakarta.apache.org/ant/manual/index.html).
On the opposite, Ant package depends on junit. So <junit/> task is usable. But it is an optionnal Ant task.
What is the logic behind?