Run your image as a container

Estimated reading time: 9 minutes

Prerequisites

Work through the steps to build a Java image in Build your Java image.

Overview

In the previous module, we created our sample application and then we created a Dockerfile that we used to build an image. We created our image using the command docker build. Now that we have an image, we can run that image and see if our application is running correctly.

A container is a normal operating system process except that this process is isolated and has its own file system, its own networking, and its own isolated process tree separated from the host.

To run an image inside a container, we use the docker run command. The docker run command requires one parameter which is the name of the image. Let’s start our image and make sure it is running correctly. Run the following command in your terminal:

$ docker run java-docker

After running this command, you’ll notice that we did not return to the command prompt. This is because our application is a REST server and runs in a loop waiting for incoming requests without returning control back to the OS until we stop the container.

Let’s open a new terminal then make a GET request to the server using the curl command.

$ curl --request GET \
--url http://localhost:8080/actuator/health \
--header 'content-type: application/json'
curl: (7) Failed to connect to localhost port 8080: Connection refused

As you can see, our curl command failed because the connection to our server was refused. It means that we were not able to connect to the localhost on port 8080. This is expected because our container is running in isolation which includes networking. Let’s stop the container and restart with port 8080 published on our local network.

To stop the container, press ctrl-c. This will return you to the terminal prompt.

To publish a port for our container, we’ll use the --publish flag (-p for short) on the docker run command. The format of the --publish command is [host port]:[container port]. So, if we wanted to expose port 8000 inside the container to port 8080 outside the container, we would pass 8080:8000 to the --publish flag.

Start the container and expose port 8080 to port 8080 on the host.

$ docker run --publish 8080:8080 java-docker

Now, let’s rerun the curl command from above.

$ curl --request GET \
--url http://localhost:8080/actuator/health \
--header 'content-type: application/json'
{"status":"UP"}

Success! We were able to connect to the application running inside of our container on port 8080.

Now, press ctrl-c to stop the container.

Run in detached mode

This is great so far, but our sample application is a web server and we don’t have to be connected to the container. Docker can run your container in detached mode or in the background. To do this, we can use the --detach or -d for short. Docker starts your container as earlier, but this time, it will “detach” from the container and return you to the terminal prompt.

$ docker run -d -p 8080:8080 java-docker
5ff83001608c7b787dbe3885277af018aaac738864d42c4fdf5547369f6ac752

Docker started our container in the background and printed the Container ID on the terminal.

Again, let’s make sure that our container is running properly. Run the same curl command from above.

$ curl --request GET \
--url http://localhost:8080/actuator/health \
--header 'content-type: application/json'
{"status":"UP"}

List containers

As we ran our container in the background, how do we know if our container is running, or what other containers are running on our machine? Well, we can run the docker ps command. Just like how we run the ps command in Linux to see a list of processes on our machine, we can run the docker ps command to view a list of containers running on our machine.

$ docker ps
CONTAINER ID   IMAGE            COMMAND                  CREATED              STATUS              PORTS                    NAMES
5ff83001608c   java-docker      "./mvnw spring-boot:…"   About a minute ago   Up About a minute   0.0.0.0:8080->8080/tcp   trusting_beaver

The docker ps command provides a bunch of information about our running containers. We can see the container ID, the image running inside the container, the command that was used to start the container, when it was created, the status, ports that exposed and the name of the container.

You are probably wondering where the name of our container is coming from. Since we didn’t provide a name for the container when we started it, Docker generated a random name. We’ll fix this in a minute, but first we need to stop the container. To stop the container, run the docker stop command which does just that, stops the container. We need to pass the name of the container or we can use the container ID.

$ docker stop trusting_beaver
trusting_beaver

Now, rerun the docker ps command to see a list of running containers.

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

Stop, start, and name containers

You can start, stop, and restart Docker containers. When we stop a container, it is not removed, but the status is changed to stopped and the process inside the container is stopped. When we ran the docker ps command in the previous module, the default output only shows running containers. When we pass the --all or -a for short, we see all containers on our machine, irrespective of their start or stop status.

$ docker ps -a
CONTAINER ID   IMAGE               COMMAND                  CREATED          STATUS                        PORTS                    NAMES
5ff83001608c   java-docker         "./mvnw spring-boot:…"   5 minutes ago    Exited (143) 18 seconds ago                            trusting_beaver
630f2872ddf5   java-docker         "./mvnw spring-boot:…"   11 minutes ago   Exited (1) 8 minutes ago                               modest_khayyam
a28f9d587d95   java-docker         "./mvnw spring-boot:…"   17 minutes ago   Exited (1) 11 minutes ago                              lucid_greider

You should now see several containers listed. These are containers that we started and stopped, but have not been removed.

Let’s restart the container that we just stopped. Locate the name of the container we just stopped and replace the name of the container below using the restart command.

$ docker restart trusting_beaver

Now, list all the containers again using the docker ps command.

$ docker ps -a
CONTAINER ID   IMAGE         COMMAND                  CREATED          STATUS                      PORTS                    NAMES
5ff83001608c   java-docker   "./mvnw spring-boot:…"   10 minutes ago   Up 2 seconds                0.0.0.0:8080->8080/tcp   trusting_beaver
630f2872ddf5   java-docker   "./mvnw spring-boot:…"   16 minutes ago   Exited (1) 13 minutes ago                            modest_khayyam
a28f9d587d95   java-docker   "./mvnw spring-boot:…"   22 minutes ago   Exited (1) 16 minutes ago                            lucid_greider

Notice that the container we just restarted has been started in detached mode and has port 8080 exposed. Also, observe the status of the container is “Up X seconds”. When you restart a container, it starts with the same flags or commands that it was originally started with.

Now, let’s stop and remove all of our containers and take a look at fixing the random naming issue. Find the name of your running container and replace the name in the command below with the name of the container on your system.

$ docker stop trusting_beaver
trusting_beaver

Now that our container is stopped, let’s remove it. When you remove a container, the process inside the container will be stopped and the metadata for the container will be removed.

To remove a container, simply run the docker rm command passing the container name. You can pass multiple container names to the command using a single command. Again, replace the container names in the following command with the container names from your system.

$ docker rm trusting_beaver modest_khayyam lucid_greider
trusting_beaver
modest_khayyam
lucid_greider

Run the docker ps --all command again to see that all containers are removed.

Now, let’s address the random naming issue. The standard practice is to name your containers for the simple reason that it is easier to identify what is running in the container and what application or service it is associated with.

To name a container, we just need to pass the --name flag to the docker run command.

$ docker run --rm -d -p 8080:8080 --name springboot-server java-docker
2e907c68d1c98be37d2b2c2ac6b16f353c85b3757e549254de68746a94a8a8d3
$ docker ps
CONTAINER ID   IMAGE         COMMAND                  CREATED         STATUS         PORTS                    NAMES
2e907c68d1c9   java-docker   "./mvnw spring-boot:…"   8 seconds ago   Up 8 seconds   0.0.0.0:8080->8080/tcp   springboot-server

That’s better! We can now easily identify our container based on the name.

Next steps

In this module, we took a look at running containers, publishing ports, and running containers in detached mode. We also took a look at managing containers by starting, stopping, and, restarting them. We also looked at naming our containers so they are more easily identifiable.

In the next module, we’ll learn how to run a database in a container and connect it to our application. See:

Use containers for development

Feedback

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Java, run, image, container