Japan probably has, at least on paper. They have all the know-how.
If you have all the formula, the rest of the process is just physics.
Japan has the know-how. And the highly-enriched plutonium. And most likely the components on a shelf, hidden away for a threatening enemy like China or North Korea. Problem with North Korea is few valuable targets. Not a problem with China, a vast quantity of high-value targets to strike with nukes, in the event China attacks Japan.
I read somewhere that the Hiroshima Fat-Man bomb was crude and only converted 2 percent of it's fissile material to energy. The physics is better now on creating small nukes with less fissile material, and Japan has many tons of it. Bye-bye China, it'll get split up into a dozen countries if they attack Japan.