The size of the Java 11 base Docker image, specifically the openjdk:11-jre-slim, can often surprise developers due to its relatively larger size compared to expectations. Understanding why this image is not as "slim" as one might hope involves exploring various technical details pertaining to Docker images, the Java environment, and the underlying Linux distribution.

Understanding Docker Images

Docker images are composed of multiple layers, each layer contributing to the overall size. The base image size is an aggregate of these layers, which can encapsulate everything from the operating system to the application runtime, and any additional software that are part of the image.

Key Layers in Java Docker Image

1. Base Linux Distribution:
   • The openjdk:11-jre-slim uses a stripped-down version of Debian. Despite being "slim," it still contains essential Linux userland utilities and libraries which can be substantial.
2. Java Runtime Environment (JRE):
   • The JRE itself is responsible for running Java applications. Even in a slim variant, it includes core libraries and components, which although stripped of optional and often-used features, still require a footprint.
3. Dependencies and Shared Libraries:
   • Java applications, including the JRE, often require shared libraries for execution. These are bundled into the image and add to the overall size.

Why "Slim" Isn't Small

The term "slim" in a Docker image doesn’t guarantee minimal size but indicates a lighter variant relative to a standard image, which includes all features. Here’s why the openjdk:11-jre-slim might still seem large:

• Essential Runtime Components: Removing all optional components might break functionality, so only genuinely non-essential components are stripped away.
• Security and Integrity: Images maintain a balance between size and security. Essential libraries and tools needed for security updates or diagnostics are retained.
• Compatibility: Some layers are necessary for broad compatibility across various environments, which increases size.

Example: Comparing Image Sizes

Below is a table that compares the sizes of different Java images:

Strategies for Size Optimization

While utilizing openjdk:11-jre-slim, there are several approaches to minimize image size:

Multistage Builds

By leveraging Docker’s multistage builds feature, developers can compile Java applications in one stage and copy only the compiled artifacts into the final image. This way, you avoid including unnecessary build tools and files in the final output.

1# Multistage Dockerfile Example
2FROM openjdk:11-jdk-slim AS build
3COPY . /app
4WORKDIR /app
5RUN ./mvnw package -DskipTests
6
7FROM openjdk:11-jre-slim
8COPY --from=build /app/target/app.jar /app/app.jar
9ENTRYPOINT ["java", "-jar", "/app/app.jar"]

Tooling and Obfuscation

Utilizing tools such as JLink or ProGuard can be effective. JLink allows the creation of custom runtime images that only include the necessary modules, significantly reducing size.

jlink --module-path $JAVA_HOME/jmods --add-modules java.base,java.logging --output custom-java-runtime

Alpine Linux Base Images

Though not natively supported for openjdk:11-jre-slim, using Alpine Linux can further reduce image size given its minimal footprint. However, compatibility with the OpenJDK may require additional configurations or adjustments for dependencies.

Consideration of Application Code

Optimization of the application itself can often yield significant savings. Minimizing external dependencies and leveraging smaller libraries can contribute to a leaner output.

Conclusion

The apparent bulkiness of the openjdk:11-jre-slim Docker image is a result of practical considerations around functionality, security, and compatibility. While slim versions attempt to minimize extraneous components, inherent constraints still demand a relatively substantial size. However, options such as multistage builds, customer runtime creation, and optimized base images can substantially alleviate size concerns.