IntelliJ Platform has built-in server, so, you don't need (and should not) create your own.
Default port is 63342. If default port is occupied, 63342 + n will be used (n < 20). If still not possible to bind, any free port will be used.
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Vladimir Krivosheev
Socket lock port (6942, range 6942 — 6992) could not be used to communicate using port unification handler due to backward compatibility: on connect we write list of locked paths. So, we cannot use incoming data to route.