Async Spring Controllers vs normal Controllers
@DaveSyer is right, /api/2 is actually behaving as expected.
I assume you are testing the behavior with a web browser. At least Firefox and Chrome are preventing multiple simultaneous requests to the same URL. If you open 2 tabs with api/2, the second one will only send a request to the application after the first got the response.
Try testing it with a simple bash script, like:
curl localhost/api/2 &curl localhost/api/2 &curl localhost/api/2 &It will print 3 responses around the same time.