You can use the
lsof utility to find what process is using a port. For example, to see what process is using port 8000:
lsof -i :8000
You'll get something like this:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME java 2501 dan 54u IPv4 0x[some numbers here] 0t0 TCP *:irdmi (LISTEN)
See this stack exchange question for more info.