What should I do if my application is not listening on the TCP/IP socket I expect it to?

Issue

• When starting an application on bare-metal, in a virtual machine, or in a containerized environment, I cannot connect to the application externally or internally (over loopback).
• If I expect my application to listen on TCP port 8080, if I try to connect to it locally it may fail like so:

$ curl localhost:8080
curl: (7) Failed to connect to localhost port 8080: Connection refused

• I do not see the application listening in socket statistics, such as that provided by netstat:

$ netstat -ltnp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      1744/cupsd
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1746/sshd: /usr/sbi
tcp6       0      0 :::22                   :::*                    LISTEN      1746/sshd: /usr/sbi

• I would expect to see my application listening in the above on local address 0.0.0.0:8080 but it does not appear.