Us Citizen, Visiting Japan, then go to Shanghai

Hey as I said, I'm a US Citizen, I live in South Korea, but because I less than 6 months on my korean visa, I can't apply to go to shanghai from here. So if I go to Japan, can I get a rush 1 day visa, then hop over to Shanghai from there? Do i do it in tokyo at the chinese embassy? or do I have to look for a travel agency and does it take longer?