Play with Docker and Java

In this post, we'll play with Docker and Java. Since there are ready-to-use Java images for Docker you don't need to install or configure anything other than Docker.

I don't know Java at all

You just need to know, I've absolutely no skills in Java. Which software should be installed, how to run a script and so on. I'll just rely on a very few Docker commands and, about the installation, yeah, using Docker, it's easy: nothing to install, nothing to configure.

Please start a Linux shell and run mkdir -p /tmp/java && cd $_ to create a folder called java in your Linux temporary folder and jump in it.

Please create a new file called Main.java with this content:

public class Main
{
     public static void main(String[] args) {
        System.out.println("Hello, World");
    }
}

Now, you'll need to compile your source. For this, just run docker run -it --rm -v ${PWD}:/app -w /app -u 1000:1000 openjdk:11 javac Main.java.

Docker CLI reminder

As a reminder, the used Docker run command are (almost always the same):

• -it to start Docker interactively, this will allow the script running in the container to ask you for some prompts f.i.,
• --rm to ask Docker to kill and remove the container as soon as the script has been executed (otherwise you'll have a lot of exited but not removed Docker containers; you can check this by not using the --rm flag then running docker container list on the console),
• -v ${PWD}:/app to share your current folder with a folder called /app in the Docker container,
• -w /app to tell Docker that the current directory, in the container, will be the /app folder,
• -u 1000:1000 ask Docker to reuse our local credentials so when a file is updated/created in the container, the file will be owned by us,
• then openjdk:11 which is the name and the version of the Docker image to use, and, finally,
• javac Main.java i.e. the command line to start within the container.

As a result of this command, your Main.java source will be compiled into the Main.class file.

By running ls -alh you can verify that, yes, the java script has been compiled into a .java file.

❯ ls -alh

total 24K
drwxr-xr-x  2 christophe christophe 4.0K Nov 22 15:02 .
drwxrwxrwt 29 root       root        12K Nov 22 14:58 ..
-rw-r--r--  1 christophe christophe  414 Nov 22 15:02 Main.class
-rw-r--r--  1 christophe christophe  117 Nov 22 14:58 Main.java

And without to have to install something

Once again, you don't have install or configure something; just call the Docker image that goes well.

Last thing is to execute your Java program. Now, please run docker run --rm -v $PWD:/app -w /app openjdk:11 java Main to execute it.

❯ docker run --rm -v $PWD:/app -w /app openjdk:11 java Main

Hello, World

A little more difficult, calling a REST API

Please create a new file called API.java with this content:

package restclient;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class API {
    public static void main(String[] args) {
        try {
            URL url = new URL("https://jsonplaceholder.typicode.com/todos/1");
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept", "application/json");

            if (conn.getResponseCode() != 200) {
                throw new RuntimeException("Failed : HTTP Error code : "
                        + conn.getResponseCode());
            }

            InputStreamReader in = new InputStreamReader(conn.getInputStream());
            BufferedReader br = new BufferedReader(in);
            String output;

            while ((output = br.readLine()) != null) {
                System.out.println(output);
            }

            conn.disconnect();

        } catch (Exception e) {
            System.out.println("Exception in NetClientGet:- " + e);
        }
    }
}

Compile it by running docker run --rm -v $PWD:/app -w /app -u 1000:1000 openjdk:11 javac API.java; get the API.class file.

Finally, call docker run --rm -v $PWD:/app -w /app openjdk:11 java API.java to execute the API call and display the result on screen:

❯ docker run --rm -v $PWD:/app -w /app openjdk:11 java API.java

{
  "userId": 1,
  "id": 1,
  "title": "delectus aut autem",
  "completed": false
}

This example will use the sample https://jsonplaceholder.typicode.com/todos/1 to generate one fake TODO. The JSON will be displayed on the command line.

You're ready to start your Java's coding journey. Have fun.