Certainly if the US essentially decides not to help out, Japan is in serious trouble.
Trump recently asked why it is that if Japan gets attacked the US are supposed to help Japan, but Japan doesn't seem to have to help the US if it gets attacked.
Is it too much Donald Trump to have heard of World War Two and the way that the US actually wrote the constitution of Japan following its defeat to commit it to renouncing war and a military? Of course, as everyone knows, Japan DOES have a military, and one that has been growing in recent years to include an aircraft carrier for F-35s. But of course, now Japan is in a similar quandary to other countries that had assumed the US was an ally. What happens when they suddenly decide not to be?
They have recently been deepening ties with other East Asian countries such as South Korea (but of course there is some lingering hostility there), and South-East Asian countries such as Vietnam, the Philippines, Indonesia, etc... and there is Australia.
Ultimately, I think Japan may have to develop nuclear weapons. It almost certainly has all the components lying around that only have to be assembled. They have rockets, nuclear power stations and plenty of technologically advanced companies.