Minimal JRE 21 Docker Image for Java/Spring Boot Applications

Welcome to the Minimal JRE 21 Docker Image project! 🎉

This project demonstrates how to create a slim, secure Docker image for Java 21/Spring Boot applications by building a custom minimal Java Runtime Environment (JRE) using jlink. The goal is to optimize the Docker image for size, security, and performance.

As a working example, the demo application leverages JRE 21's Virtual Threads to achieve concurrent I/O while retrieving simple weather reports for the Nordic cities: Copenhagen, Stockholm, Oslo, and Helsinki. The weather forecasts are fetched concurrently and combined using Java's CompletableFuture API.

🚀 Goals

• Java 21 Compatibility: Ensure the ability to run applications using JRE 21.
• Non-Root Execution: Run the application as a non-root user for enhanced security.
• Minimal JRE: Create a custom JRE that includes only the necessary modules to reduce image size and memory footprint.
• Spring Boot 3 Demo: Showcase a sample Spring Boot 3 app utilizing app that applies the JRE 21 feature of Virtual Threads.

🔧 How It Works

The Dockerfile uses a multi-stage build:

1. Builder Stage: Uses eclipse-temurin:21-jdk-alpine to build a minimal JRE with jlink, including only the modules required by the application.
2. Final Stage: Based on alpine:3, it sets up a non-root user, copies the minimal JRE and the application JAR, and runs the application with optimized JVM parameters.

📊 Results

By building a minimal JRE and optimizing the Docker image, we achieved significant reductions in image size:

Image	Dockerfile	Image Size
Reference Image (Full JRE)	JRE21.Dockerfile	261 MB
Minimal JRE Image	MinimalJRE21.Dockerfile	99 MB

Image Size Reduction: We saved a total of 62% on the final Docker image size! 🎉

🛠️ Usage

Build the application and create the Docker image using Maven:

mvn clean install docker:build

To run the Docker application while exposing port 8080, do the following:

docker run --rm -p 8080:8080 ghcr.io/mrgraversen/minimal-jre-21-spring-boot-3-demo

To call the API, use the following curl command:

curl http://localhost:8080/forecast

Example Response

Copenhagen: Temperature: 14.4°C, Wind Speed: 19.1 km/h
Stockholm: Temperature: 13.1°C, Wind Speed: 8.3 km/h
Oslo: Temperature: 11.7°C, Wind Speed: 7.9 km/h
Helsinki: Temperature: 12.1°C, Wind Speed: 9.4 km/h

🤔 Why a Customised, Minimal JRE?

There are several reasons to select this approach when developing Java and Spring Boot applications:

• Reducing Image Size: Smaller images are faster to download and deploy. By applying these principles to this demo project, we saved a total of 62% on the final Docker image size.
• Lower Memory Footprint: Including only necessary modules reduces RAM usage at runtime, as the JVM loads fewer classes and resources.
• Improved Security: Fewer components reduce the attack surface, and avoiding the use of the root user further enhances security.