We've started adding Java Code Coverage numbers to PRs using the jacoco gradle plugin.
When we ran this in Maven, we got very low coverage reported for things like the Java SDK core. This was likely because we test the bulk of the SDK not in the SDK module, but in fact in the DirectRunner and other similar modules.
This is the still the case in gradle. So we'll need to make sure to watch for it.
This is a good "random improvement" issue for anyone to pick up.