Actually I doubt that J-XX will enter service sooner than PAK-FA, You are right that Chinese are experiencing a tech boom atm. but I would say China still havn't catched up to russia yet, and it will take more than 2-3years for china to do so, Russian stil have more experience in weapons system incorperation, material science(although china's heavy investment in basic science the last 15years are starting to yield results), and a clear edge in engine technology, fighterborn AESA is also something that china still lack behind russia currently, as well as super-sonic cruise missle tech. and space tech., these are basicly everything where Russia still have a advantage over china, some of which are not just surpassing china but also still have a edge over the West as well, and for china to fully surpass russia in military tech, I'd say will take at least another 10years.
China are putting more and more resources in education and R&D recent 2decades, today chinese universities pour out more graduated students than any other country in the world, surpassing even USA since several years ago, so I would expect nothing less than a technological boom that will continuie indefinitely.
But this doesn't mean the military technological cooperation between china and russia will end anytime soon, I would expect a new form of cooperation between the two nation such as co-development of systems, as arms purchase and co-development doesn't only mean money and equipment, but also a military/geopolitical co-operation and alliance, although for this to happen you need wait until china to show the ability to develop systems that are equally or more advanced than any russian weapons system development.
Take the Pak-FA, project for example, reason why china firmly declined to join this program is that China alrdy have a J-XX program up and running well, but anoher important reason is because of Russian's plan for this program, basicly, neither china nor India is to be treated as equal partner, this program is thought to be planned as US JSF/F35 program, all the "partner" that join the development of this fighter are basicly just there to give money and some limited expertise, in turn Russia gets all the copyrights and tech while other get the right to buy this fighter and proberbly at a lower cost ofc, this however is obiously not acceptable for china, which are capable of developing it's own program now.
For the co-operation between the 2 countries to continuie, Russia needs to start treat China as a equal partner, and this soon, or the risk is that military equipment co-operation might really break off into a stop someday.
The co-operation neeeds to become equally beneficial to both side, not only oneside spend cash and the other side make product, it needs to become 2 directional.
