What happens if the EXPOSE directive is not set for the ports used by the container in the Dockerfile?

If the EXPOSE directive is not specified in the Dockerfile for the ports used by the container, it will not prevent the container from working and listening on those ports. The EXPOSE directive mainly serves as documentation and a hint for users and tools that the container expects traffic on these ports.

However, without EXPOSE, when running the container with docker run -P (automatic port forwarding), Docker will not know which ports to expose externally. In such cases, port forwarding must be explicitly specified with -p.

In summary: EXPOSE does not affect the operation of the application inside the container, but it helps manage port forwarding and improves the readability of the Dockerfile.