How to Create Spring Boot Docker Images

Docker is a container engine that makes it possible to bundle, ship, and distribute applications across distributed servers. <!--more-->

Prerequisites #

1. Have Java Development Kit 11 or newer installed on your computer.
2. Make sure Docker is installed on your computer.

To get started with Docker, visit Getting started with Docker.

Project setup #

We will bundle and containerize the application that we created here. The application allowed us to perform CRUD functionalities on the Todo items via a REST API.

To check if JDK is installed and configured correctly on your computer, execute the command below.

$ java --version
java 15.0.2 2021-01-19

Execute the command below to create a working directory for the project.

$ mkdir todo
$ cd todo

In the directory created above, execute the command below to clone the project to your computer.

$ git clone https://github.com/paulodhiambo/springbootcrud.git

Execute the command below to move into the project folder.

$ cd springbootcrud

Run the application to verify if the project is correctly set up and the application runs without errors.

$ gradle build

The project structure is shown below.

└── springbootcrud         # < project root package
    ├── src
    │   ├── main
    │   ├── test
    ├── build.gradle
    ├── gradlew
    ├── Dockerfile # Dockerfile where we write all the container build instructions
    └── gradlew.bat    
    └── settings.gradle

Creating a Dockerfile #

A Dockerfile is a plain text file that contains instructions from which a Docker image is built. A Dockerfile contains the commands below.

• FROM: directive sets the image on which the container will be based on.
• RUN: executes commands in the container.
• COPY: creates a copy of files from the file system in the container.
• CMD: sets the executable commands within the container.

Create a Dockerfile in the project's root folder.

Add the code snippets below to the Dockerfile created above.

FROM openjdk:8-jdk-alpine
RUN addgroup -S springdocker && adduser -S springdocker -G springdocker
USER springdocker:springdocker
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} app.jar
ENTRYPOINT ["java","-jar","/app.jar"]

• FROM openjdk:8-jdk-alpine sets the base image from which the Docker container will be created.
• RUN addgroup -S springdocker && adduser -S springdocker -G springdocker add a user profile to the container.
• USER springdocker:springdocker sets the user in the container making it possible to execute commands in the container as an administrator.
• ARG JAR_FILE=target/*.jar sets the path to jar generated by gradle build command to ARG_FILE variable.
• COPY ${JAR_FILE} app.jar copies the jar file created after running gradle build command to the app.jar file inside the docker container.
• ENTRYPOINT ["java","-jar","/app.jar"] indicates that the container has an executable file.

Building the Docker image #

To build a Docker image from the Dockerfile we have created above, execute the command below.

$ docker build --build-arg JAR_FILE=build/libs/\*.jar -t spring-boot-docker:latest .

• --build-arg JAR_FILE=build/libs/\*.jar sets the path to the jar file generated by running gradle build command.
• -t sets the tag for the image.
• . indicates that the Dockerfile is in the current directory.

Execute the command below to list all the Docker images available on your computer.

$ docker image ls
REPOSITORY    TAG            IMAGE ID       CREATED         SIZE
spring-boot-docker               latest              b7a1056c3eb2   31 minutes ago   143MB
springio/gs-spring-boot-docker   latest              b7a1056c3eb2   31 minutes ago   143MB
<none>                           <none>              4f8c8633df3e   2 days ago       105MB
latest                           latest              6c802830c996   4 weeks ago      414MB

From the list above, we see the spring-boot-docker image that we have created.

Creating and running the Docker Container #

Execute the command below to create a Docker container from the Docker image we have created above.

$ docker run --name spring-boot-docker -d -p 8080:8080 spring-boot-docker:latest

• --name flag sets the Docker container name.
• -d flag makes the image run in the background.
• -p 8080:8080 maps port 8080 in the Docker container to port 8080 in localhost.
• spring-boot-docker:latest specifies the image from which the Docker container is built.

Execute the command below to list all the running Docker containers.

$ docker container ps
CONTAINER ID   IMAGE                   COMMAND                  CREATED       STATUS       PORTS                        NAMES
792fce534e3a   spring-boot-docker   "java -jar /app.jar"   31 minutes ago   Up 31 minutes   0.0.0.0:8080->8080/tcp   spring-boot-docker

1. Visit localhost on port 8080 to confirm if the spring-boot-crud application is running in the container.

Pushing the Docker image to Docker Hub #

Docker Hub is an online repository of Docker container images. Docker hub stores Docker containers and also makes it easier to share Docker images with other developers.

Create a Docker Hub account here if you don't have one.

After creating an account, log in to Docker Hub, create a repository with the name spring-boot-crud and the description Spring Boot Docker Image.

Execute the command below to push the Docker image to the repository that we have created in the Docker Hub.

$ docker login

$ docker tag spring-boot-docker:latest <username>/spring-boot-docker:latest

$ docker push <username>/spring-boot-docker:latest

• The docker login command authenticates you into your Docker Hub account on the command line, making it possible to push Docker images to Docker Hub.
• docker tag spring-boot-docker: latest <username>/spring-boot-docker: latest this command modifies the tag of our local Docker image to include the Docker Hub username.
• docker push <username>/spring-boot-docker: latest pushes the image to Docker Hub repository we created earlier.

Note: Replace <username> with your actual Docker Hub username.

The source code to the application is in this GitHub repository.

Conclusion #

Docker is a great tool for building and packaging applications with the required dependencies to run them both locally and in production. Docker playground is a browser-based environment that you can use to test various Docker commands without installing Docker locally on your computer.

Happy coding!

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Peer Review Contributions by: Miller Juma