To run spring boot application in a container. We should do the following steps:

• Create a jar package with maven or gradle (mvn clean install or mvn clean package)
• Create a Dockerfile (to containerize an application)
• Build to Dockerfile to create an docker image
• Run the docker image

Finally we will look at the how to connect running postgres container from Spring Boot application

I will skip the first step: “Create a jar package”. Assuming that you are already familiar with.

Create Dockerfile

We should set the base image for our java application. I will use openjdk version 11. (You may look at the all base images related to the java via https://hub.docker.com/_/openjdk )

FROM openjdk:11
COPY target/yourJarNameInLocal.jar yourJarNameInDocker.jar
ENTRYPOINT ["java","-jar","/yourJarNameInDocker.jar"]

Build docker file

Go to your Dockerfile and run the command

$ podman build -t yourName .
...
...
COMMIT yourName
--> ea58ea6b407
Successfully tagged localhost/yourName:latest
ea58ea6b40766c62afad735d5b6918a214c6491cf56ecec81df2783133d13c1

# list the images

$ podman images
REPOSITORY                     TAG         IMAGE ID      CREATED             SIZE
localhost/yourName             latest      ea58ea6b4076  About a minute ago  719 MB
docker.io/library/openjdk      11          e273ff3d8df8  5 days ago          671 MB
docker.io/library/httpd        latest      d54056386fbb  5 days ago          142 MB
docker.io/library/postgres     latest      d191afba1bb1  5 days ago          382 MB
docker.io/library/hello-world  latest      feb5d9fea6a5  3 weeks ago         19.9 kB

Run the docker image

podman run localhost/yourName -dt -p 8000:8080

Because I didn’t specify server.port, spring application will run on the port 8080. And I am redirecting the requests on localhost:8000 to the localhost/yourName container on port 8080

That’s it. Spring application will run inside the container. If we look at the container itself:

$ podman exec -it {containerId or containerName} bash
root@10959577521a:/# ls
bin  boot  dev	etc  home  lib	lib64  media  mnt  opt	proc  root  run  sbin  srv  sys  tmp  usr  var	yourJarNameInDocker.jar

Container Spring Boot and PostgreSQL

Let’s assume that we should also need to connect postgresql and also I am assuming that we have another container for postgres. At the end SpringBoot container should connect to the Postgres container.

Finally I am also assuming that Postgres container is already running and there is a port binding such as -p 5432:5432

We have three options to connect Postgres container:

• Connect from your IDE (for instance, run button in the IntelliJ IDEA)
• Connect from jar file using the ip address of the postgres host.
• Connect from jar file using shared network between containers

Connect from IDE to Postgres Container

There is a running postgres container and you want to connect from your IDE (or in other words, you want to connect from your host)

Host means that “server or laptop that container runs on it”

In this case you don’t need to change spring.datasource.url property because you will connect the postgres container from the host itself.

spring.datasource.url=jdbc:postgresql://localhost:5432/{yourDbName}

Connect from jar file using ip address of the postgres’ host

Learn your host ip address via $ ip addr or $ ifconfig. Or simply run the following command:

$ hostname -I | awk '{print $1}'
192.168.122.1

If you run two containers on the same of host, ip address should be 192.168.... For instance , when I want to connect postgres container from my laptop (also I am running spring boot application on my laptop), ip address was the 192.168.122.1. Finally change the property to:

spring.datasource.url=jdbc:postgresql://192.168.122.1:5432/{yourDbName}

If postgres container is placed on another server, there must be firewall permission also.

To test connection from your laptop, you can use nc command:

[mehmetozanguven@fedora demo]$ nc -vz 192.168.122.1 5432
Ncat: Version 7.80 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.122.1:5432.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.

Connect from jar file using shared network between containers

This can only be done by rootful containers. (Basically run every command podman run, podman network with sudo prefix)

I am not going into detail of this. But there are awesome blog for that (and also for other network communication). Here is the link