(Wasn’t able to comment on the given answer, so have to make another answer)
I realized that when launching Ant from Eclipse, you’ll have to add fork=”true” to the <java> task. Also, it was first not clear to me how to write nested jvmargs, so here goes an example:
<java classname="..." fork="true">
<jvmarg value="-Xdebug" />
<jvmarg value="-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5432" />
...
</java>