There are some rules if you want to re implement Java, but it is possible since we have many alternatives to chose from. T^he main point is to pass all the needed tests to confirm your re-implementation is compatible and not do what MS tried to do (embrace,extend,extinguish)

You missed that you will be arbitrarily denied the ability to actually run those tests if you're trying to make a true open source implementation - see Apache Harmony.

The TCK has been open sourced, since then. However, I think only for GPL implementations, see details at http://openjdk.java.net/groups/conformance/JckAccess/

That doesn't sound like an open source license at all - you have to submit a form to Oracle and they'll decide whether you get a copy or not.

That is old news, do you have an uptodate link?

What's changed? There still isn't an independent implementation of Java, Harmony is still shut down because they were arbitrarily denied a TCK license.