Was China a self-contained or an isolated civilization?
The answer to this will depend on the exact way in which you define "self-contained" and "isolated." I would argue that "self-contained" is a better phrase to use to describe China.
When we speak of an isolated society, we tend to refer to one that is cut off, or practically cut off, from the outside world. A fairly good example of this would be Japan during the period when it was illegal for Japanese to leave the country or for foreigners to come to any part of it except for one small island.
China was not like this. China remained open to trade and other sorts of contact with the outside. Evidence of this can be seen in such things as the coming of Buddhism along the Silk Road. It can also be seen in the fact that China was conquered by Mongols.
Therefore, we cannot say China was isolated. It is better to say that it was self-contained because it did not need a great deal from the outside world. It did conduct some trade and got some ideas from the outside, but for the most part it had what it needed.