Lsof

Find used port and what's using it, and kill it

lsof -i tcp:8080

# sample output
COMMAND       PID     USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
rootlessp 1234567 USERNAME    4u  IPv6 DEVICENO      0t0  TCP localhost:irdmi->localhost:49280 (ESTABLISHED)

kill -9 1234567