Hi I am a passport holder of the U.S and will be visiting japan soon.

i will transit through Beijing coming and going.i was wondering if i would need to apply for a visa for entry back into china so i could transit.both are under 24 hours.i heard if you don't have an entry permit to enter china from japan you wont be aloud entry.also do i need to apply for the 24 hour direct transit before hand?or is it done when you land in china?thanks.