After years of negotiating, especially over how to prevent technology theft, Russia has 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.
China has long been producing unauthorized copies of the Russian Su-27, as the J-11. China has also designed a two-seat fighter bomber version as the J-16, a stealthy version (J-17), and obtained an aircraft carrier version of the Su-30 (the Su-33) from Ukraine and are producing a copy (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.
The original J-11 entered service in 1998, but production was very slow and only a hundred were produced. It was during this process that the Chinese mastered all the technical details of building and modifying the Russian aircraft. Chinese officials were dismayed with the performance of the obsolete Russian electronics. After that, at least a hundred of the 33 ton J-11A were built. This model was equipped with modern, Chinese made, electronics and is capable of hauling eight tons of radar guided air-to-air missiles and smart bombs. Then came the J-11B, which was the same size and weight as the J-11A but had a more capable AESA radar and is intended to specialize in air-to-ground missions, while also being able to take care of itself in air-to-air combat.