October 30, 2022

Build and run Clojure with Multi-stage Dockerfile

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Deployment of Clojure is very simple, only an Uberjar archive file (Clojure Project and Clojure run-time) and the Java Run-time Environment (JRE) are required.

A Clojure service rarely works in isolation and although many services are access via a network connection (defined in Environment Variables), provisioning containers to build and run Clojure along with any other services can be valuable as complexity of the architecture grows.

A Multi-stage Dockerfile is an effective way to build and run Clojure projects in continuous integration pipelines and during local development where multiple services are required for testing.

Docker Hub provides a wide range of images, supporting development, continuous integration and system integration testing.

Multi-stage Dockerfile

A multi-stage Dockerfile contains builder stage and an unnamed stage used as the run-time. The builder stage can be designed optimally for building the Clojure project and the run-time stage optimised for running the service efficiently and securely.

The uberjar created by the builder image is copied over to the run-time image to keep that image as clean and small as possible (to minimise resource use).

Example Multi-stage Dockerfile for Clojure projects derived from the configuration currently used for commercial and open source work. The example uses make targets, which are Clojure commands defined in the example Makefile

Official Docker images

Docker Hub contains a large variety of images, using those tagged with Docker Official Image is recommended.

Ideally a base image should be used where both builder and run-time images share the same ancestor, this helps maintain consistency between build and run-time environments.

The official Eclipse OpenJDK image is used by the official Clojure docker image, so they implicitly use the same base image without needed to be specified in the project Dockerfile. The Eclipse OpenJDK image could be used as a base image in the Dockerfile but it would mean repeating (and maintaining) much the work done by the official Clojure image)

Alternative Docker images CircleCI Convenience Images => Clojure - an optimised Clojure image for use with the CircleCI service

Amazon Corretto is an alternative version of OpenJDK

Clojure image as Builder stage

Practicalli uses the latest Clojure CLI release and the latest Long Term Support (LTS) version of Eclipse Temurin (OpenJDK). Alpine Linux is used to keep the image file size as small as possible, reducing local resource requirements (and image download time).

FROM clojure:temurin-17-alpine AS builder

CLOJURE_VERSION will over-ride the version of Clojure CLI in the Clojure image (which defaults to latest Clojure CLI release). Or choose an image that has a specific Clojure CLI version, e.g. temurin-17-tools-deps-

FROM clojure:temurin-17-alpine AS builder

Create directory for building the project code and set it as the working directory within the Docker container to give RUN commands a path to execute from.

RUN mkdir -p /build
WORKDIR /build

Use Clojure CLI to download dependencies for the project and any other tooling that will be used during the build stage, e.g. test runners, packaging tools to create an uberjar.

deps.edn file is first copied to the builder stage and then clojure is called with the -P prepare execution option, along with aliases that represent any tooling that also needs dependencies downloading

deps.edn is copied before copying all the other project files to minimise the changes that would trigger the dependencies command to run.

After the first run of the builder stage, if the deps.edn file has not changed, then the docker cache of dependencies is used rather than downloading all dependencies again.

COPY deps.edn /build/
RUN clojure -P -X:env/test:package/uberjar

If using make for the build, also copy the Makefile to the builder stage and call the deps target to download the dependencies (ensure deps target in the Makefile depends on the deps.edn file so the target is skipped if that file has not changed)

COPY deps.edn Makefile /build/
RUN make deps

Maximising the docker cache by careful consideration of command order and design in a Dockerfile can have a significant positive affect on build speed. Each command is effectively a layer in the Docker image and if its respective files have not changed, then the cached version of the command will be run

Now the dependencies are downloaded (and cached), copy the project files to the docker builder working directory and run the command to generate an Uberjar.

COPY ./ /build
RUN clojure -X:package/uberjar

See the section on Docker Ignore Patters to make the COPY command more efficient

:package/uberjar is an alias in the deps.edn project that includes the dependencies and tools to build the Clojure project into an Uberjar. The latest approach for building an Uberjar is to use Clojure tools.build, optionally with seancorfield/build-clj.

If using make for the build call the all target to run CI tests, download dependencies (if not cached) and run the dist target to build the Uberjar. Or call make dist if the CI pipeline has already run tests.

COPY ./ /build
RUN RUN make all

Docker Ignore patterns

.dockerignore file in the root of the project defines file and directory patterns that Docker will ignore with the COPY command. Use .dockerignore to avoid copying files that are not required for the build

Keep the .dockerignore file simple by excluding all files with * pattern and then use the ! character to explicitly add files and directories that should be copied

# Ignore all files

# Include Clojure code and config

Makefile and test-data directories are commonly used by Practicalli, although in general are not widely needed.

The classic approach would be to specify all files and directories to exclude in a Clojure project, although this typically means more maintenance

OpenJDK for Run-time stage

The Alpine Linux version of the Eclipse Temurin image is used as it is around 5Mb in size, compared to 60Mb or more of other operating system images.

FROM eclipse-temurin:17-alpine

Run-time containers are often cached in a repository, e.g. AWS Container Repository (ECR). LABEL adds metadata to the container helping it to be identified in a repository or in a local development environment.

LABEL org.opencontainers.image.authors="nospam+dockerfile@practicall.li"
LABEL io.github.practicalli.service="Gameboard API Service"
LABEL io.github.practicalli.team="Practicalli Engineering Team"
LABEL version="1.0"
LABEL description="Gameboard API service"

Use docker inspect to view the metadata

Optionally, add packages to support running the service or helping to debug issue in the container when it is running. For example, add dumb-init to manage processes, curl and jq binaries for manual running of system integration testing scripts

apk is the package tool for Alpine Linux and --no-cache option ensures the install file is not part of the resulting image, saving resources. Alpine Linux recommends setting versions to use any point release with the ~= approximately equal version, so any same major.minor version of the package can be used.

RUN apk add --no-cache \
    dumb-init~=1.2.5 \
    curl~=7.83.1 \

Create Non-root group and user to run service securely

Docker runs as root user by default and if a container is compromised the root permissions and could lead to a compromised system. Add a user and group to the run-time image and create a directory to contain service archive, owned by the non-root user. Then instruct docker that all future commands should run as the non-root user

RUN addgroup -S practicalli && adduser -S practicalli -G practicalli
RUN mkdir -p /service && chown -R practicalli. /service
USER practicalli

Copy Uberjar to run-time stage

Create a directory to run the service or use a known existing path that will not clash with any existing files from the image.

Set the working directory and copy the uberjar archive file from Builder image

RUN mkdir -p /service
WORKDIR /service
COPY --from=builder /build/practicalli-service.jar /service/practicalli-service.jar

Optionally, add system integration testing scripts to the run-time stage for testing from within the docker container.

RUN mkdir -p /service/test-scripts
COPY --from=builder /build/test-scripts/curl--* /service/test-scripts/

Set Service Environment variables

Define values for environment variables should they be required (usually for debugging), ensuring no sensitive values are used. Environment variables are typically set by the service provisioning the containers (AWS ECS / Kubernettes) or on the local OS host during development (Docker Desktop).

# optional over-rides for Integrant configuration

Optimising the container for Java Virtual Machine

Clojure Uberjar runs on the Java Virtual Machine which is a highly optimised environment that rarely needs adjusting, unless there are noticeable performance or resource issue use. The most likely option to set is the minimum and maximum heap sizes, i.e. -XX:MinRAMPercentage and -XX:MaxRAMPercentage.

java -XshowSettings -version displays VM settings (vm), Property settings (property), Locale settings (locale), Operating System Metrics (system) and the version of the JVM used. Add the category name to show only a specific group of settings, e.g. java -XshowSettings:system -version.

JDK_JAVA_OPTIONS can be used to tailor the operation of the Java Virtual Machine, although the benefits and constraints of options should be well understood before using them (especially in production).

Example: show system settings on startup, force container mode and set memory heap maximum to 85% of host memory size.

ENV JDK_JAVA_OPTIONS "-XshowSettings:system -XX:+UseContainerSupport -XX:MaxRAMPercentage=85"

Relative heap memory settings (-XX:MaxRAMPercentage) should be used for containers rather than the fixed value options (-Xmx) as the provisioning service for the container may control and change the resources available to a container on deployment (especially a Kubernettes system).

Options that are most relevant to running Clojure & Java Virtual Machine in a container include:

  • -XshowSettings:system display (container) system resources on JVM startup
  • -XX:InitialRAMPercentage Percentage of real memory used for initial heap size
  • -XX:MaxRAMPercentage Maximum percentage of real memory used for maximum heap size
  • -XX:MinRAMPercentage Minimum percentage of real memory used for maximum heap size on systems with small physical memory
  • -XX:ActiveProcessorCount specifies the number of CPU cores the JVM should use regardless of container detection heuristics
  • -XX:┬▒UseContainerSupport force JVM to run in container mode, disabling container detection (only useful if JVM not detecting container environment)
  • -XX:+UseZGC low latency Z Garbage collector (read the Z Garbage collector documentation and understand the trade-offs before use) - the default Hotspot garbage collector is the most effective choice for most services

Without performance testing of a specific Clojure service and analysis of the results, let the JVM use its own heuristics to determine the most optimum strategies it should use

Provide Access to running Clojure service

If Clojure service listens to network requests when running, then the port it is listening on should be exposed so the world outside the container can communicate to the Clojure service.

e.g. expose port of HTTP Server that runs the Clojure service


Command to run the service

Finally define how to run the Clojure service in the container. The java command is used with the -jar option to run the Clojure service from the Uberjar archive.

The java command will use arguments defined in JDK_JAVA_OPTIONS automatically.

ENTRYPOINT directive defines the command to run the service

ENTRYPOINT ["java", "-jar", "/service/practicalli-service.jar"]

ENTRYPOINT is the recommended way to run a service in Docker. CMD can be used to pass additional arguments to the ENTRYPOINT command, or used instead of ENTRYPOINT.

jshellis the default ENTRYPOINT for the Eclipse Temurin image, so jshell will run if an ENTRYPOINT of CMD directive is not included in the run-time stage of the Dockerfile.

The ENTRYPOINT command runs as process id 1 (PID 1) inside the docker container. In a Linux system PID 1 should respond to all TERM and SIGTERM signals.

dump-init provides a simple process supervisor and init system, designed to run as PID 1 and manage all signals and child processes effectively.

Use dumb-init as the ENTRYPOINT command and CMD to pass the java command to start the Clojure service as an argument. dumb-init ensures TERM signals are sent to the Java process and all child processes are cleaned up on shutdown.

ENTRYPOINT ["/usr/bin/dumb-init", "--"]
CMD ["java", "-jar", "/service/practicalli-service.jar"]

Alternatively, run dumb-jump and java within the ENTRYPOINT directive, ENTRYPOINT ["/usr/bin/dumb-init", "--", "java", "-jar", "/service/practicalli-service.jar"]

Build and Run with docker

Ensure docker services are running, i.e. start docker desktop.

Build the service and create an image to run the Clojure service in a container with docker build. Use a --tag to help identify the image and specify the context (in this example the root directory of the current project, .)

docker build --tag practicalli/service-name:1.1 .

After the first time building the docker image, any parts of the build that havent changed will use their respecitve cached layers in the builder stage. This can lead to very fast, even zero time builds.

Docker build image optomised to use docker layer cache for build stage

Run the built image in a docker container using docker run, publishing the port number so it can be used from the host (developer environment or deployed environment). Use the name of the image created by the tag in the docker build command.

docker run --publish 8080:8080 practicalli/service-name

Consider creating a docker-compose.yml file that defines all the services that should be run to support local development, then run docker compose up to start all the services.


A Multi-stage Dockerfile is an effective way of building and running Clojure projects, especially as the architecture grows in complexity.

Organising the commands in the Dockerfile to maximise the use of docker cache will speed up the build time by skipping tasks that would not change the resulting image.

Consider creating a docker-compose.yaml file to orchestrate services that are required for development of the project and local system integration testing.

Thank you

practicalli GitHub profile I @practical_li

Tags: clojure java docker jvm