Docker (not only) for Java Developers

Examples for presentation 'Docker (not only) for Java Developer'

Alpine figlet example

Figlet is a program that generates text banners.

Dockerfile for alpine-figlet:

FROM alpine

RUN apk add figlet --no-cache

ENTRYPOINT ["figlet"]
CMD ["Hello World!"]

To build save into Dockerfile and run docker build . -t alpine figlet.

• FROM – starts from alpine image (with latest) tag
• RUN – installs figlet
• ENTRYPOINT – set default executable to figlet.
• CMD – set default parameters passed to figlet to Hello World

Running docker run alpine-figlet shows default "Hello World!" message. Different params can be passed and will replace the CMD - e.g. docker run alpine-figlet I like trains! will print "I like trains".

We can still replace the entrypoint and run shell: docker run -it --entrypoint "/bin/sh" alpine-figlet.

Container aware Java example

To build: docker build . -t temurin-containeraware

To run with CPU/memory limits:

docker run -it --rm --memory 300m --memory-swap 300m --cpu-period 100000 --cpu-quota 200000 temurin-containeraware

Inside run built jar with:

java ShowMxBeanCpuAndMemoryInfo

We can combine it with -XshowSettings:system or set explicit number of processors: -XX:ActiveProcessorCount=4.

Todos app

Simple kind-of todos app that allows to add and remove entries (todos):

• after adding the todo has done = false.
• after marking as done the todo has done = true.
• todo with done = true is in done section and can be deleted.

By default app is using port 7070 and can be available at http://localhost:7070/.

Used libraries:

• Javalin - https://javalin.io/
• Jte - https://jte.gg/
• MongoDB sync driver - https://www.mongodb.com/docs/languages/java/
• Bulma CSS - https://bulma.io/
• JUnit - https://junit.org/
• TestContainers - https://testcontainers.com/

Docker examples

All dockerfiles and docker compose files are placed in src/main/docker.

Dockerfiles use Java 21 image (Eclipse Temurin).

#1 Simplest docker image with app (shaded) - Dockerfile-outsidejar

Simplest image - requires built uber jar in target.

To build:

1. ./mvnw package -DskipTests
2. docker build -f src/main/docker/Dockerfile-outsidejar -t todoapp-outsidejar .

To run:

1. docker run -p 7070:7070 todoapp-outsidejar:latest -it

#2 Build app in the docker image with app (shaded, naive way) - Dockerfile-insidejar

Solves "works on my machine" problem - app is built in the container.

Requires both JDK and JRE - to build and run. There is no cache.

To build:

1. docker build -f src/main/docker/Dockerfile-insidejar -t todoapp-naive .

To run:

1. docker run -p 7070:7070 todoapp-naive:latest -it

#3 Build app in the docker image with app (shaded, multi stage) - Dockerfile-shaded

Improved version of #2 - uses multiple stages:

• download and cache dependencies (uses JDK image) in dependencies stage.
• build and package app (uses JDK image) in package stage.
• run app (uses JRE image) in final stage.

As a result the final image contains only the app (shaded, uber jar).

Uber jar is created using maven-shade-plugin - classes from all dependencies are put in the final jar.

To build:

1. docker build -f src/main/docker/Dockerfile-shaded -t todoapp-multistage1 .

To run:

1. docker run -p 7070:7070 todoapp-multistage1:latest -it

#4 Build app in the docker image with app (not shaded, multi stage) - Dockerfile-dependency

Improved version of #3 - stages are the same (dependencies, package and final), but image used is alpine (instead of Ubuntu one).

Uses maven-jar-plugin (configuration in pom.xml:92-105):

• in dependencies stage to copy dependencies to dependency directory in repository layout.
• in package stage to build jar with manifest file containing main class and classpath (for dependency directory).

In final stage dependency from dependencies are copied.

As a result the final image contains only the app (shaded, uber jar).

To build:

1. docker build -f src/main/docker/Dockerfile-dependency -t todoapp-multistage2 .

To run:

1. docker run -p 7070:7070 todoapp-multistage2:latest -it

To debug:

1. docker run -p 7070:7070 -p 5005:5005 -e JVM_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005" todoapp-multistage2:latest -it
2. Run remote debug on port 5005.

Docker Compose examples

#1 Running dependencies docker-compose.deps.yml

Running only dependencies:

• MongoDB - exposed at 27017, with data stored in host (src/main/docker/mongo-data)
• Mongo Express - simple GUI Mongo, exposed at 8081.

#2 Running app (memory DAO) docker-compose.memory.yml

Running the Todos app (memory version) only - available on 7070 port.

#3 Running app (build from Dockerfile) docker-compose.dockerfile.yml

Running the Todos app (memory version) only - built from the Dockerfile. App is available on port 7070.

#4 Running app (Mongo DAO) docker-compose.mongo.yml

Running the Todos app (mongo version) and Mongo + Mongo Express.

Data is stored within Mongo container only.