Haha,I'm also fan of Jiaozi,but don't like Tofu.
Anyway,I'm in Beijing now,never been to Xi'an,in Beijing,the oldest and largest mosque is Niujie mosque,founded in 996 AD,but in Xi'an,you have Great mosque of Xi'an built in 742 AD,one of the oldest in China,and in that time,Xi'an is still the capital of Imperial China.
The spread of Islam in China is because peaceful contact between China and Islamic world,not because of war and conquest,all those mosques were protected by the old imperial government,and some of those mosques were even built because of the emperors' orders