Russia and China jointly developed the two seat version of the Su-30, as the Su-30MKK in the late 1990s, and, a decade ago, an upgraded version (the Su-30MK2). China has received about a hundred legal Su-30MK2s but now nearly identical J-16s are showing up. Russia is not amused or fooled.
This kind of blatant technology theft is nothing new. The Chinese J-11 jet fighter is an illegal Chinese copy of the Russian Su-27. This plagiarism has been a source of friction between Russia and China for nearly a decade. It all began, legally, in 1995, when China paid $2.5 billion for the right to build 200 Su-27s. Russia would supply engines and electronics, with China building the other components according to Russian plans and specifications. But after 95 of the Chinese built aircraft were built Russia cancelled the agreement. They claimed that China was using the knowledge acquired with this Su-27 program to build their own copy of the Su-27, the J-11. Russia kept the piracy issue quiet for as long as it could and warned the Chinese that simply copying Russian technology would produce an inferior aircraft. Apparently the Chinese did not agree and are continuing their work on the J-11, using only, what they claim, is Chinese technology.
The J-11 is believed to now include better electronics and some other Chinese design modifications. China can manufacture most of the components of the J-11, the one major element it must import are the engines. China believes it will be free from dependence on Russia for military jet engines within the next 5-10 years. Currently, China imports two Russian engines, the $3.5 million AL-31 (for the Su-27/30, J-11, J-10) and the $2.5 million RD-93 (a version of the MiG-29s RD-33) for the JF-17 (an F-16 type aircraft developed in cooperation with Pakistan). Despite the ongoing technology theft dispute, Russia still sells jet engines to China for its illegal copies of Russian aircraft. China agreed, in 2008, to stop stealing Russian military tech but went on to ignore that agreement and deny that it had reneged on its promise to stop the tech theft.
The Su-30MK2 is a 34 ton fighter-bomber similar to the American F-15E. The Su-30MK2 can carry 8 tons of smart bombs and missiles. It can be refueled in the air and is equipped to operate over land and open water. The Chinese Navy is operating 24 Su-30MK2s and some of the J-16s that have already been built.
China also has a stealthy version (J-17) of the Su-27. There is also an aircraft carrier version of the Su-30 (the Su-33, obtained from Ukraine) that is now being produced as the J-15. China insists these are all Chinese designs that just happen to bear some resemblance to Russian fighters. In response, Russia had halted (for nearly a decade) combat aircraft sales to China but still sold jet engines for these aircraft. So far China has been unsuccessful in building copies of these engines. The engine sales are too lucrative to pass up, as they enable the Russian engine manufacturers to continue developing new designs. The Chinese plan to steal these as soon as they figure out how to handle the exotic manufacturing methods and skills required to build these engines.
Recently, after years of negotiating, especially over how to prevent technology theft, Russia agreed to sell China 48 Su-35BM fighter-bombers. This aircraft is the latest version of the Su-27 design, with a more robust airframe (good for 6,000 flight hours) and better maneuverability and reliability. It’s unclear if the Chinese are also going to get all the improved electronics.
The long negotiations were the result of Russia seeking ways to halt unauthorized Chinese copying and production of the Su-35BM. This proved very difficult, especially since Russia and China are supposed to be allies these days. Earlier this year this deal was stalled because China refused to allow a "no unauthorized duplication" clause in the contract. The Chinese wanted to buy the Su-35s but were not willing to sign a binding agreement to not copy the Russian design. That appears to have changed, although just how enforceable this is remains to be seen.