Comprehensive Analysis of Docker OpenJDK Image Variants: From Alpine to Slim

In the Docker ecosystem, OpenJDK images offer multiple variants, each designed for specific use cases. Understanding the differences between these variants is crucial for optimizing containerized deployment of Java applications. This article provides a detailed analysis based on official documentation and community best practices.

Standard Image Variants

The openjdk:<version> is the de facto standard image. If you are unsure about your needs, this version is generally recommended. It is designed to be used both as a throwaway container (mounting source code and starting the container to run the application) and as a base for building other images. Starting with OpenJDK 12, the default image along with -oracle and -oraclelinux7 variants are based on the official Oracle Linux 7 image. The OpenJDK binaries in these images are built by Oracle and sourced from the OpenJDK community.

Debian-Based Variants

The openjdk:<version>-buster, openjdk:<version>-stretch, and openjdk:<version>-jessie variants include buster, jessie, or stretch in their names, which are code names for Debian releases, indicating which Debian version the image is based on. These variants provide a complete Debian environment, suitable for applications requiring standard Linux tools and libraries.

Alpine Linux Variant

The openjdk:<version>-alpine is based on the Alpine Linux project, a very small base image (approximately 5MB), resulting in generally smaller final images. This variant is highly recommended when minimizing final image size is a priority. The main caveat is that it uses musl libc instead of glibc, which may cause issues for certain software depending on their libc requirements. However, most software does not encounter problems, making this variant usually a safe choice.

To minimize image size, Alpine-based images typically exclude additional tools such as git or bash. When using this image as a base, necessary components must be added in the Dockerfile. For example, a Dockerfile snippet for installing basic packages:

FROM openjdk:8-alpine
RUN apk add --no-cache bash git
COPY . /app
WORKDIR /app
CMD ["java", "-jar", "app.jar"]

Slim Variant

The openjdk:<version>-slim image contains only the minimal packages required to run Java, lacking many UI-related Java libraries and some common packages included in the default tag. Unless deploying in an environment with space constraints where only the openjdk image will be used, the default image of this repository is strongly recommended. The Slim variant is particularly suitable for headless server applications where graphical interface components are unnecessary.

Windows Server Core Variant

The openjdk:<version>-windowsservercore is based on the Windows Server Core image. Consequently, it only works in environments where that image is available, such as Windows 10 Professional/Enterprise (Anniversary Edition) or Windows Server 2016. For scenarios requiring Java applications to run in Windows environments, this variant provides native support.

Selection Guidelines

Choosing the appropriate OpenJDK image variant involves considering multiple factors:

1. Image Size: If final image size is a critical concern, the Alpine variant is the best choice, followed by the Slim variant.
2. Compatibility: For applications requiring standard glibc environments, Debian-based or standard variants are more appropriate.
3. Functional Requirements: If the application needs complete Java UI libraries or additional system tools, the standard variant offers the most comprehensive environment.
4. Deployment Environment: When deploying on Windows servers, the Windows Server Core variant is the only option.

In practice, it is advisable to start with the standard variant and consider others if image size or specific environmental needs arise. Using Docker multi-stage builds allows leveraging full environments during build stages while using minimized images for runtime, balancing functionality and efficiency.