MwRYum
Major
If Japan is afraid not to have security treaty, it should have one with China. Then they wouldn't have foreign forces in their country, especially important to Okinawa, and not have to pay for those forces.
Without the US stationed there, Japan will have to spend far more on defence than they're now, bridging the gap between what the US is offering and their own. In fact, a good part of the economic miracle Japan enjoyed came from such alliance, which Japan could concentrate on economy despite such a hot spot the region is.
In short, Japan and US have a love-hate relationship that Japan find themselves can't live without at the end of the day. If not, the whole Okinawa base re-location issue won't be stuck there for so long.
And signing a security treaty with China? Now, you either from another planet or you need to have your brain examined, braincell by braincell...not only there're too much bad blood between the two nations, China can't even hope to offer a fraction of what the US is doing as we speak. Like I've said before, alliance with the US is the reason that Japan don't have to spend more than they are now...the "otherwise" scenario would have Japan develop up to and including nuclear arms...
Plus, most importantly, China IS still the enemy, encircling China is still within Japan's own agenda, which we can see in their diplomacy to essentially caging China behind a ring of sort, how nervous they get whenever PLAN making fleet movements past the island chain, and how hard they tried to create legal precedence in jurisdiction in Diaoyu Islands area.