Deployment with Docker

Introduction

Docker packages your Java app + JRE into a single, reproducible image you can run on any host. It's the standard handoff between dev and prod.

Informative example

A production-grade multi-stage Dockerfile for a Spring Boot app:

# ---- build stage
FROM eclipse-temurin:21-jdk AS build
WORKDIR /app
COPY . .
RUN ./mvnw -B -DskipTests package

# ---- runtime stage (small)
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-XX:+UseZGC", "-XX:MaxRAMPercentage=75", "-jar", "app.jar"]

Best practices

• Multi-stage builds — final image holds only the JRE + jar.
• Set MaxRAMPercentage instead of Xmx so the JVM scales with the container limit.
• Use a non-root user; pin the base-image digest.
• Health-check endpoint (/actuator/health) for k8s liveness/readiness.

Purpose of this lesson

Master Deployment with Docker so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Deployment with Docker.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Useful template

FROM eclipse-temurin:21-jre-alpine
COPY target/app.jar /app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "/app.jar"]

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Use multi-stage builds — final image with only the JRE + jar.
• Layer the jar (Spring Boot's layertools) for faster rebuilds.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Deployment with Docker daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Deployment with Docker — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.