The “mapped port can only be obtained after the container is started” is a statement related to containerization technologies like Docker. When running a containerized application, the application is encapsulated within a container, which is isolated from the host system and other containers running on the same host.
Containers often need to expose network services or applications that can be accessed from outside the container. To achieve this, containers can map ports from the host system to ports within the container. This port mapping allows incoming network traffic to be directed to the appropriate container and its associated service.
When a container is created, the port mapping configuration is defined, usually as part of the Dockerfile or Docker Compose configuration. However, the actual mapping of ports happens when the container is started or running. Until the container is running, the mapping information is not available.
Let’s consider an example to illustrate this. Suppose you have a web application running inside a Docker container. The Dockerfile or Docker Compose configuration might specify that the container’s internal port 80 should be mapped to the host system’s port 8080. This means that incoming requests to the host’s port 8080 will be routed to the container’s port 80, allowing access to the web application.
However, until the container is actually started using the “docker run” command or a similar mechanism, the host system doesn’t know about the port mapping. Only after the container is running, can you access the web application through the mapped port 8080 on the host system.