- Open a terminal window.
- Copy and paste this:
sudo lsof -i -n -P | grep TCP
- Take note of the process id (PID) currently listening on the port your app was running on (look for something that looks like:
TCP 127.0.0.1:8080 (LISTEN)).
- Open Activity Monitor.
- Sort the processes list by PID.
- Kill the process matching the PID you noted in step 3.
- Restart your server from Google App Engine Launcher.
Thanks to irs8925 for providing the answer on Stack Overflow.