DevOps Classroom notes 08/Dec/2025

Lets try to containerize a java application

• The java application package (jar) is available Refer Here
• Refer Here for code

Building image from a Java Package

• Steps:
  • Ensure Java 21 is installed
  • This application uses port 8080
  • Command to run this application is java -jar spring-petclinic-3.3.0-SNAPSHOT.jar
• Context:
  • Java comes in two forms
    • JDK
    • JRE
• We need to find a base image with java 21 installed. lets choose eclipsetemurin or amazon correto
• Dockerfile-version1

FROM eclipse-temurin:21-alpine
LABEL author=shaikkhajaibrahim
LABEL project=learning
LABEL version=3.3.0
EXPOSE 8080
RUN wget https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar
CMD ["java", "-jar", "spring-petclinic-3.3.0-SNAPSHOT.jar"]

• Dockerfile instructions
  • LABEL
  • RUN: Run instruction is used to execute any command during image building
  • CMD: This instruction will be used to execute while container is starting (PID 1)
  • ADD: Add is similar to copy and it also allows downloding from URLs
  • WORKDIR
• Commands in Dockerfile have two forms
  • Shell form:
    • we directly write the command
    • example ping -c 4 google.com
    • In RUN Instruction we prefer using shell form
  • Exec form:
    • we enclose the command in square brackets
    • example: [ "ping", "-c", "4", "google.com" ]
    • IN CMD/ENTRYPOINT we use Exec form
• Dockerfile-version2

FROM eclipse-temurin:21-alpine
LABEL author=shaikkhajaibrahim
LABEL project=learning
LABEL version=3.3.0
EXPOSE 8080
ADD https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar /spring-petclinic-3.3.0-SNAPSHOT.jar
CMD ["java", "-jar", "/spring-petclinic-3.3.0-SNAPSHOT.jar"]

• It is not a good idea to copy files in root directory

FROM eclipse-temurin:21-alpine
LABEL author=shaikkhajaibrahim
LABEL project=learning
LABEL version=3.3.0
EXPOSE 8080
ADD https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar /app/spring-petclinic-3.3.0-SNAPSHOT.jar
WORKDIR /app
CMD ["java", "-jar", "spring-petclinic-3.3.0-SNAPSHOT.jar"]

• Its a security risk to run as root user inside a docker image. This means we need to create a user in a image and run our application as that user
• Exercise:
  • how to create a linux user
    • with a home directory /app
    • with no prompts and passwords set
• Dockerfile

FROM eclipse-temurin:21-alpine
LABEL author=shaikkhajaibrahim
LABEL project=learning
LABEL version=3.3.0
RUN adduser -D -h /app -s /bin/sh spc
USER spc
EXPOSE 8080
ADD --chown=spc:spc https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar /app/spring-petclinic-3.3.0-SNAPSHOT.jar
WORKDIR /app
CMD ["java", "-jar", "spring-petclinic-3.3.0-SNAPSHOT.jar"]

• Exercise:
  • Try redoing all of this amazon correto 21 as base image.
  • Find the slimmest jre iamge to run spc.