Dockerize Java Application

How to dockerize the Java application

Create Hello World Program in Java

class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, it's great to be running inside docker image");
    }
}

Prepare Dockerfile to build the image.

Create Dockerfile without any extension and populate with below contents

FROM java:8
COPY . /var/www/java
WORKDIR /var/www/java
RUN javac Hello.java
CMD ["java", "Hello"]

Build the image and run it.

Run this command to build the image.

$ docker build -t java-app .

This command will build the image with the name java-app

Run the image

$ docker run java-app

You should see the console output.

Using docker-compose to build the image.

Create a file with name docker-compose.yaml Update its content to this.

version: '2'
services:
  java:
    build: .

The build parameter informs about the location of Dockerfile. Here . signifies, it’s in the current directory.

Build the image

docker-compose build

Run the image

docker-compose up

You should see the same console output again.

Introducing environment variables in docker-compose.yaml

We can pass or set environment variables in docker-compose file. Update docker-compose.yaml file with one more detail.

version: '2'
services:
  java:
    build: .
    environment:
    - ENV_TYPE = "test"

Now update the java code to read this environment variable.

class Hello {
    public static void main(String[] argss){
        System.out.println("Hello, it is nice to be running inside docker image");
        if(System.getenv("ENV_TYPE").equals("prod")){
            System.out.println("Running on prod like env");
        } else if (System.getenv("ENV_TYPE").equals("test")){
            System.out.println("Running on test like env");
        } else {
            System.out.println("This is something other than prod or test");
        }
    }
}

Build the image again and then run it.

$ docker-compose build
$ docker-compose up

The console should print the value set in the compose file.

Passing the environment variable from outside

$ docker-compose run -e ENV_TYPE=test java
$ docker-compose run -e ENV_TYPE=prod java
$ docker-compose run -e ENV_TYPE=stag java

Now, you should see the console output depending upon the environment variable passed.