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In this blog, you will learn some Docker best practices mainly focussed on Spring Boot applications. You will learn these practices by applying them to a sample application. Enjoy! 1. Introduction This blog continues where the previous blog about Docker Best Practices left off. However, this blog can be read independently from the previous one. The goal is to provide some best practices that can be applied to Dockerized Spring Boot applications. The Dockerfile that will be used as a starting point is the following: Dockerfile FROM eclipse-temurin:17.0.5_8-jre-alpine@sha256:02c04793fa49ad5cd193c961403223755f9209a67894622e05438598b32f210e WORKDIR /opt/app RUN addgroup --system javauser && adduser -S -s /usr/sbin/nologin -G javauser javauser ARG JAR_FILE COPY target/${JAR_FILE} app.jar RUN chown -R javauser:javauser . USER javauser ENTRYPOINT ["java", "-jar", "app.jar"] This Dockerfile is doing the following: FROM: Take eclipse-temurin:17 Java Docker image as base image; WORKDIR: Set /opt/app as the working directory; RUN: Create a system group and system user; ARG: provide an argument JAR_FILE so that you do not have to hard code the jar file name into the Dockerfile; COPY: Copy the jar file into the Docker image; RUN: Change the owner of the WORKDIR to the previously created system user; USER: Ensure that the previously created system user is used; ENTRYPOINT: Start the Spring Boot application. In the next sections, you will change this Dockerfile to adhere to best practices. The resulting Dockerfile of each paragraph is available in the git repository in the directory Dockerfiles. At the end of each paragraph, the name of the corresponding final Dockerfile will be mentioned where applicable. The code being used in this blog is available on GitHub. 2. Prerequisites The following prerequisites apply to this blog: Basic Linux knowledge Basic Java and Spring Boot knowledge Basic Docker knowledge 3. Sample Application A sample application is needed in order to demonstrate the best practices. Therefore, a basic Spring Boot application is created containing the Spring Web and Spring Actuator dependencies. The application can be run by invoking the following command from within the root of the repository: Shell $ mvn spring-boot:run Spring Actuator will provide a health endpoint for your application. By default, it will always return the UP status. Shell $ curl http://localhost:8080/actuator/health {"status":"UP"} In order to alter the health status of the application, a custom health indicator is added. Every 5 invocations, the health of the application will be set to DOWN. Java @Component public class DownHealthIndicator implements HealthIndicator { private int counter; @Override public Health health() { counter++; Health.Builder status = Health.up(); if (counter == 5) { status = Health.down(); counter = 0; } return status.build(); } } For building the Docker image, a fork of the dockerfile-maven-plugin of Spotify will be used. The following snippet is therefore added to the pom file. XML com.xenoamess.docker dockerfile-maven-plugin 1.4.25 mydeveloperplanet/dockerbestpractices ${project.version} ${project.build.finalName}.jar The advantage of using this plugin is that you can easily reuse the configuration. Creating the Docker image can be done by a single Maven command. Building the jar file is done by invoking the following command: Shell $ mvn clean verify Building the Docker image can be done by invoking the following command: Shell $ mvn dockerfile:build Run the Docker image: Shell $ docker run --name dockerbestpractices mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT Find the IP-address of the running container: Shell $ docker inspect dockerbestpractices | grep IPAddress "SecondaryIPAddresses": null, "IPAddress": "172.17.0.3", "IPAddress": "172.17.0.3" In the above example, the IP-address is 172.17.0.3. The application also contains a HelloController which just responds with a hello message. The Hello endpoint can be invoked as follows: Shell $ curl http://172.17.0.3:8080/hello Hello Docker! Everything is now explained to get started! 4. Best Practices 4.1 Healthcheck A healthcheck can be added to your Dockerfile in order to expose the health of your container. Based on this status, the container can be restarted. This can be done by means of the HEALTHCHECK command. Add the following healthcheck: Dockerfile HEALTHCHECK --interval=30s --timeout=3s --retries=1 CMD wget -qO- http://localhost:8080/actuator/health/ | grep UP || exit 1 This healthcheck is doing the following: interval: Every 30 seconds the healthcheck is executed. For production use, it is better to choose something like five minutes. In order to do some tests, a smaller value is easier. This way you do not have to wait for five minutes each time. timeout: A timeout of three seconds for executing the health check. retries: This indicates the number of consecutive checks which have to be executed before the health status changes. This defaults to three which is a good number for in-production. For testing purposes, you set it to one, meaning that after one unsuccessful check, the health status changes to unhealthy. command: The Spring Actuator endpoint will be used as a healthcheck. The response is retrieved and piped to grep in order to verify whether the health status is UP. It is advised not to use curl for this purpose because not every image has curl available. You will need to install curl in addition to the image and this enlarges the image with several MBs. Build and run the container. Take a closer look at the status of the container. In the first 30 seconds, the health status indicates starting because the first health check will be done after the interval setting. Shell $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ddffb5a9cbf0 mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT "java -jar /opt/app/…" 8 seconds ago Up 6 seconds (health: starting) dockerbestpractices After 30 seconds, the health status indicates healthy. Shell $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ddffb5a9cbf0 mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT "java -jar /opt/app/…" 33 seconds ago Up 32 seconds (healthy) dockerbestpractices After 2-5 minutes, the health status indicates unhealthy because of the custom health indicator you added to the sample application. Shell $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ddffb5a9cbf0 mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT "java -jar /opt/app/…" 2 minutes ago Up 2 minutes (unhealthy) dockerbestpractices Again, 30 seconds after the unhealthy status, the status reports healthy. Did you notice that the container did not restart due to the unhealthy status? That is because the Docker engine does not do anything based on this status. A container orchestrator like Kubernetes will do a restart. Is it not possible to restart the container when running with the Docker engine? Yes, it can: you can use the autoheal Docker image for this purpose. Let’s start the autoheal container. Shell docker run -d \ --name autoheal \ --restart=always \ -e AUTOHEAL_CONTAINER_LABEL=all \ -v /var/run/docker.sock:/var/run/docker.sock \ willfarrell/autoheal Verify whether it is running. Shell $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ddffb5a9cbf0 mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT "java -jar /opt/app/…" 10 minutes ago Up 10 minutes (healthy) dockerbestpractices d40243eb242a willfarrell/autoheal "/docker-entrypoint …" 5 weeks ago Up 9 seconds (healthy) autoheal Wait until the health is unhealthy again or just invoke the health actuator endpoint in order to speed it up. When the status reports unhealthy, the container is restarted. You can verify this in the STATUS column where you can see the uptime of the container. Shell $ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ddffb5a9cbf0 mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT "java -jar /opt/app/…" 12 minutes ago Up 6 seconds (health: starting) dockerbestpractices You have to decide for yourself whether you want this or whether you want to monitor the health status yourself by means of a monitoring tool. The autoheal image provides you the means to automatically restart your Docker container(s) without manual intervention. The resulting Dockerfile is available in the git repository with the name 6-Dockerfile-healthcheck. 4.2 Docker Compose Docker Compose gives you the opportunity to start multiple containers at once with a single command. Besides that, it also enables you to document your services, even when you only have one service to manage. Docker Compose used to be installed separately from Docker, but nowadays it is part of Docker itself. You need to write a compose.yml file that contains this configuration. Let’s see what this looks like for the two containers you used during the healthcheck. YAML services: dockerbestpractices: image: mydeveloperplanet/dockerbestpractices:0.0.1-SNAPSHOT autoheal: image: willfarrell/autoheal:1.2.0 restart: always environment: AUTOHEAL_CONTAINER_LABEL: all volumes: - type: bind source: /var/run/docker.sock target: /var/run/docker.sock Two services (read: containers) are configured. One for the dockerbestpractices image and one for the autoheal image. The autoheal image will restart after a reboot, has an environment variable defined, and has a volume mounted. Execute the following command from the directory where the compose.yml file can be found: Shell $ docker compose up In the logging, you will see that both containers are started. Open another terminal window and navigate to the directory where the compose.yml can be found. A lot of commands can be used in combination with Docker Compose. E.g. show the status of the running containers. Shell $ docker compose ps NAME COMMAND SERVICE STATUS PORTS mydockerbestpracticesplanet-autoheal-1 "/docker-entrypoint …" autoheal running (healthy) mydockerbestpracticesplanet-dockerbestpractices-1 "java -jar /opt/app/…" dockerbestpractices running (healthy) Or stop the containers: Shell $ docker compose stop [+] Running 2/2 ⠿ Container mydockerbestpracticesplanet-autoheal-1 Stopped 4.3s ⠿ Container mydockerbestpracticesplanet-dockerbestpractices-1 Stopped 0.3s Or easily remove the containers: Shell $ docker compose rm ? Going to remove mydockerbestpracticesplanet-dockerbestpractices-1, mydockerbestpracticesplanet-autoheal-1 Yes [+] Running 2/0 ⠿ Container mydockerbestpracticesplanet-autoheal-1 Removed 0.0s ⠿ Container mydockerbestpracticesplanet-dockerbestpractices-1 Removed As you can see, Docker Compose provides quite some advantages and you should definitely consider using it. 4.3 Multi-Stage Builds Sometimes it can be handy to build your application inside a Docker container. The advantage is that you do not need to install a complete development environment onto your system and that you can interchange the development environment more easily. However, there is a problem with building the application inside your container. Especially when you want to use the same container for running your application. The sources and the complete development environment will be available in your production container and this is not a good idea from a security perspective. You could write separate Dockerfiles to circumvent this issue: one for the build and one for running the application. But this is quite cumbersome. The solution is to use multi-stage builds. With multi-stage builds, you can separate the building stage from the running stage. The Dockerfile looks as follows: Dockerfile FROM maven:3.8.6-eclipse-temurin-17-alpine@sha256:e88c1a981319789d0c00cd508af67a9c46524f177ecc66ca37c107d4c371d23b AS builder WORKDIR /build COPY . . RUN mvn clean package -DskipTests FROM eclipse-temurin:17.0.5_8-jre-alpine@sha256:02c04793fa49ad5cd193c961403223755f9209a67894622e05438598b32f210e WORKDIR /opt/app RUN addgroup --system javauser && adduser -S -s /usr/sbin/nologin -G javauser javauser COPY --from=builder /build/target/mydockerbestpracticesplanet-0.0.1-SNAPSHOT.jar app.jar RUN chown -R javauser:javauser . USER javauser HEALTHCHECK --interval=30s --timeout=3s --retries=1 CMD wget -qO- http://localhost:8080/actuator/health/ | grep UP || exit 1 ENTRYPOINT ["java", "-jar", "app.jar"] As you can see, this Dockerfile contains two FROM statements. The first one is used for building the application: FROM: A Docker image containing Maven and Java 17, this is needed for building the application; WORKDIR: Set the working directory; COPY: copy the current directory to the working directory into the container; RUN: The command in order to build the jar file. Something else is also added to the FROM statement. At the end, AS builder is added. This way, this container is labeled and can be used for building the image for running the application. The second part is identical to the Dockerfile you used to have before, except for two lines. The following lines are removed: Dockerfile ARG JAR_FILE COPY target/${JAR_FILE} app.jar These lines ensured that the jar file from our local build was copied into the image. These are replaced with the following line: Dockerfile COPY --from=builder /build/target/mydockerbestpracticesplanet-0.0.1-SNAPSHOT.jar app.jar With this line, you indicate that you want to copy a file from the builder container into the new image. When you build this Dockerfile, you will notice that the build container executes the build and finally, the image for running the application is created. During building the image, you will also notice that all Maven dependencies are downloaded. The resulting Dockerfile is available in the git repository with the name 7-Dockerfile-multi-stage-build. 4.4 Spring Boot Docker Layers A Docker image consists of layers. If you are not familiar with Docker layers, you can check out a previous post. Every command in a Dockerfile will result in a new layer. When you initially pull a Docker image, all layers will be retrieved and stored. If you update your Docker image and you only change for example the jar file, the other layers will not be retrieved anew. This way, your Docker images are stored more efficiently. However, when you are using Spring Boot, a fat jar is created. Meaning that when you only change some of your code, a new fat jar is created with unchanged dependencies. So each time you create a new Docker image, megabytes are added in a new layer without any necessity. For this purpose, Spring Boot Docker layers can be used. A detailed explanation can be found here. In short, Spring Boot can split the fat jar into several directories: /dependencies /spring-boot-loader /snapshot-dependencies /application The application code will reside in the directory application, whereas for example, the dependencies will reside in directory dependencies. In order to achieve this, you will use a multi-stage build. The first part will copy the jar file into a JDK Docker image and will extract the fat jar. Dockerfile FROM eclipse-temurin:17.0.4.1_1-jre-alpine@sha256:e1506ba20f0cb2af6f23e24c7f8855b417f0b085708acd9b85344a884ba77767 AS builder WORKDIR application ARG JAR_FILE COPY target/${JAR_FILE} app.jar RUN java -Djarmode=layertools -jar app.jar extract The second part will copy the split directories into a new image. The COPY commands replace the jar file. Shell FROM eclipse-temurin:17.0.4.1_1-jre-alpine@sha256:e1506ba20f0cb2af6f23e24c7f8855b417f0b085708acd9b85344a884ba77767 WORKDIR /opt/app RUN addgroup --system javauser && adduser -S -s /usr/sbin/nologin -G javauser javauser COPY --from=builder application/dependencies/ ./ COPY --from=builder application/spring-boot-loader/ ./ COPY --from=builder application/snapshot-dependencies/ ./ COPY --from=builder application/application/ ./ RUN chown -R javauser:javauser . USER javauser HEALTHCHECK --interval=30s --timeout=3s --retries=1 CMD wget -qO- http://localhost:8080/actuator/health/ | grep UP || exit 1 ENTRYPOINT ["java", "org.springframework.boot.loader.JarLauncher"] Build and run the container. You will not notice any difference when running the container. The main advantage is the way the Docker image is stored. The resulting Dockerfile is available in the git repository with the name 8-Dockerfile-spring-boot-docker-layers. 5. Conclusion In this blog, some best practices are covered when creating Dockerfiles for Spring Boot applications. Learn to apply these practices and you will end up with much better Docker images.

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