Possibly you could use Directshow from java using Java Native Interface (JNI). But I would rather use Java Media Framework (JMF). If you know directshow is not very difficult to learn; the arquitecture is also based on connected filters.
You would also need a DVB-T/S/C capture card to access the content. JMF will probably present audio and video correctly; anyway I would check this before buying the card. I'm not sure about next, but probably to access MHP data you would have to use JNI, and things would be easier if you buy a card provided with an SDK.
Note that if you use JNI, that application will NOT be platform independent.
You could also build all your application in Directshow, parse MHP data and send it to a Java Program that executes the Xlet..
In orther to execute the Xlets you could use XletView, which will save you lots of work; but you should have a look at the license.