A quick glance at a map tells the story; if China gains control of Taiwan, it will have a foot on the throat of the Japanese. The Japanese presumably prefer not to become a Chinese vassal state, but any efforts to beef up their defensive capabilities will run afoul of both public opinion and their constitution. What, then can the Japanese government do?
Provide America with enhanced intelligence and cyberwarfare assets. They are relatively cheap, consistent with traditional Japanese areas of expertise, and easy to conceal from both the Chinese and the voting public. They would also be very helpful in the event of an attack on Taiwan.