JVM microservices - JVM based application as Docker container

Continuation of Windows and Docker integration guide. The goal of this post is to show quick and easy way of creating new JVM project and run it as a Docker container on Windows.

In the previous post I’ve described how to setup Docker environment on Windows powered PC and run existing Docker container. Now it’s time to create own JVM project, create Docker image and run it.

When it comes to quickly prototype something on JVM platform - Groovy is a great language choice. Furthermore, using Groovy in this guide is a perfect example of Docker powered JVM microservices polyglot nature. For a build tool my natural choice is Gradle, that will be used in this guide as well.

Setup Gradle using sdkman

Gradle can be installed and added to PATH manually, but there exists sdkman tool that drastically simplifies installation of Gradle and other utilities. So, I’d suggest to spend some minutes to setup it and use for Gradle installation.

First of all unzip should be installed, since it is required for sdkman installer. In new bash console execute.
```
$ pacman -S unzip
```
Then install sdkman.
```
$ curl -s get.sdkman.io | bash
```
Now sdkman is ready, open new bash console and install latest Gradle version.
```
$ sdk install gradle
```
Check that Gradle was installed correctly (may require opening new bash).

$ gradle --version

------------------------------------------------------------
Gradle 2.9
------------------------------------------------------------

Build time:   2015-11-17 07:02:17 UTC
Build number: none
Revision:     b463d7980c40d44c4657dc80025275b84a29e31f

Groovy:       2.4.4
Ant:          Apache Ant(TM) version 1.9.3 compiled on December 23 2013
JVM:          1.8.0_45 (Oracle Corporation 25.45-b02)
OS:           Windows 8.1 6.3 amd64

Sample Groovy application

Project folder structure can be generated by running Gradle task.
```
$ gradle init --type groovy-library
```
After generation Library.groovy and LibraryTest.groovy could be removed.
Create main application class.
ua.eshepelyuk.blog.Hello.groovy
```
package ua.eshepelyuk.blog

println "Hello from Docker by Groovy and Gradle" (1)
```
1 declaration of public static void main could be omitted in Groovy

Although gradle init generates Gradle wrapper scripts, I won’t use them in this guide. For real-life projects I’d suggest to use wrapper and don’t rely on local Gradle installation.

Enable Docker support for project build script

When using Gradle you don’t need to deal with Dockerfile and other things for creating Docker image. There’re plugins for this :))

To enable and customize them just add some lines to build.gradle as described below.

Enable plugins

build.gradle

plugins {
    id 'groovy'
    id 'application' (1)
    id 'com.bmuschko.docker-java-application' version '2.6.1' (2)
}

1	Plugin for building runnable application that can be embedded into `Docker` image
2	Docker support plugin

Customize plugins

build.gradle

mainClassName = 'ua.eshepelyuk.blog.Hello' (1)

docker {
    url = System.env.DOCKER_HOST.replaceAll("tcp", "https") (2)
    javaApplication {
        tag = "eshepelyuk/hellodockergradle:latest" (3)
    }
}

1	Entry point for `application` plugin
2	Fixing docker machine URL for Java API
3	`Docker` image tag name

Build image and run Docker container

Must run start.sh script from Docker for windows installation before proceed to further steps. Please refer to previous post for details.

Execute command to create Docker image.
```
$ gradle dockerBuildImage
```

Check new image is available by running docker images. Command output should include new image tagged with eshepelyuk/hellodockergradle (setting from build.gradle).

$ docker images
REPOSITORY                        TAG                 IMAGE ID            CREATED                  VIRTUAL SIZE
..
eshepelyuk/hellodockergradle   latest              daa12bd8bb4f        About a minute ago   649 MB
..

Start container using docker run and inspect the output to match expected from Hello.groovy class.
```
$ docker run eshepelyuk/hellodockergradle
Hello from Docker by Groovy and Gradle
```

Full project’s code is available at My GitHub