Why do I need JDK on my router?

[Lev Serebryakov]

 I need to have 'net-mgmt/unifi5' on my router. It is Java port, which
works with native JRE, and doesn't need build at all. But this port
depends on 'archivers/snappy-java', which needs build (and JDK and maven
for this).

  But at *runtime* neither of these ports need JDK! They works perfectly
Ok with only JRE! And still, RUN_DEPENDS includes 'java/openjdk8', WHY?!

  I've added 'JAVA_RUN=jre' to 'archivers/snappy-java/Makefile' but it
doesn't help (WHY?).

 Size difference between JDK and JRE is substantial for small
embedded-like system with "only" 512MiB of flash for "firmware".

-- 
// Lev Serebryakov