Containerisation of Java applications

Containerisation is the process of creating a bundle of application code and all its dependancies.

This blog post explains various benefits of containerisation.

Let’s say, we have basic java application called java-app with following directory structure and we want to containerise it.

java-app (repository root directory)
├── build.gradle
└── src
    └── main
        └── java
            └── org.example
                └── HelloWorld.java

Content of HelloWorld.java

package org.example;
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello world!");
    }
}

Content of build.gradle

buildscript {
    repositories {
        maven {
            url "https://plugins.gradle.org/m2/"
        }
    }
    dependencies {
        classpath "gradle.plugin.com.github.johnrengelman:shadow:7.1.2"
    }
}
plugins {
    id 'java'
    id 'com.github.johnrengelman.shadow' version '7.1.2'
}
group 'org.example'
version '1.0-SNAPSHOT'
apply plugin: "com.github.johnrengelman.shadow"
repositories {
    mavenCentral()
}

In order to create docker image, we need to define Dockerfile and a script to build the docker image. We can keep both the files in repository root directory like this:

java-app
├── build.gradle
├── build_image.sh
├── Dockerfile
└── src
    └── main
        └── java
            └── org.example
                └── HelloWorld.java

Content of Dockerfile

FROM amazoncorretto:11
COPY . /opt/java-app/
WORKDIR /opt/java-app/
ENTRYPOINT []

Content of build_image.sh

gradle clean shadowjar
docker buildx build --platform linux/amd64 . -t java-app:latest --load

We can build the image using:

[root@DEV-BOX java-app]# sh -e build_image.sh
[root@DEV-BOX java-app]# docker images
REPOSITORY      TAG         IMAGE ID       CREATED          SIZE
java-app        latest      a258ee1690c4   3 minutes ago    447MB

Lets run our Java app using docker run command:

[root@DEV-BOX java-app]# docker run -it java-app:latest java -cp build/libs/java-app-1.0-SNAPSHOT-all.jar org.example.HelloWorld
Hello world!

The process remains same for more complex Java applications with multiple dependancies.