While they technically didn't "attack" Japan, conducting a ballistic missile test over another sovereign nation is a serious act. What is interesting to note is that this isn't the first time North Korea has launched missiles over Japan. Kim Jong-un's father did it twice before: http://abcnews.go.com/International/north-korean-missile-tests-crossed-japan/story?id=49497769

And here's a list of all of the missile tests they've done over the years: https://www.wikiwand.com/en/List_of_North_Korean_missile_tests

But the fact that this is Kim's first shot across Japan's bow, so to speak, is interesting in and of itself. It's hard to know about the internal power dynamics of that country, but there were reports several years ago of a failed coup against Kim: http://www.telegraph.co.uk/news/2017/08/24/did-kim-jong-un-kill-uncle-brother-coup-plot-involving-china/

Robert Kelly, the Southeast Asia analyst who got famous for his kid barging in during a BBC skype interview, told Bloomberg yesterday that he figured the tests were potentially part of factional infighting within the NK government: https://twitter.com/business/status/902391696557727744

So it's quite possible that the missile tests are more about internal power struggles than they are regarding anything external.

But who knows.