Build small docker images for Java (jlink)

1. Story

  • As a Java developer I package my project into a runnable immage, for example package it into Docker images as explained here and here. So I have runtime images in which there is a JRE(Java Runtime Environment) to execute my code. But is it performant enaugh to have entire JRE there?
  • Is it possible to select only parts of JRE that my program needs them?
  • Smaller JRE means smaller image size and also prevents wasting memory. Wow..
  • So in this post we will see how to reduce JRE size to and fits it on our project/module/code.

Please note that this all commands in this post are executable on linux and if you are using Windows or IOS, then little fitnesses is needed.

2. Solution idea by using jlink

Before Java 9, JRE was a monolithic software system. But in Java 9 a new feature comes in as a game changer with huge benefits. Java modules is the feature provided by Java Platform Module System(JPMS) in Java 9.

The general idea with Java modules is that it makes it possible to remove parts of a program which your application not be using.

With this new feature, JRE as a software system also restructured into a modular system, and now we want to recruit only JRE modules wich our application needs them.

In the other side, as the main player of our goal in this post, jlink is a tool that comes with JDK and makes it possible to assemble a set of Java modules and their dependencies into a custom and optimised runtime image.

jlink can be called via command like this:

In comming steps we use jlink in the process of packaging our application.

3. Sample java project

You can recruit any Java project that you prefer to carry out with this tutorial, but a recommendation would be using this Github repository as a very simple hello world Spring-Boot project.

4. What was the state before using jlink

At first we build a Docker image straight forward and without using jlink to see its size and in next step we will show how it will be reduced by using jlink. So in this step we built a Docker image as it explained here as "Simple Java + Docker". And we can see that its size is 238MB.

But let's see how we can make it even smaller than 238MB by using jlink.

5. Using jlink for our application

At first check your Java version by "java -version" and if it is above 9 then it would be OK for this step.

Jlink tool operates on jar files and our application is a jar file. But if your application is a war file you can copy it into a jar file by a simple command as below:

This is the main command we call for compose our custom JRE by using jlink:

In above command there is a call for jdeps which is a tool in JDK which can list dependencies of our jar file.

So in jlink command we use --add-modules to add Java modules which our jar file depends on them and also we add some other Java modules that we know Spring needs them.

6. Spring dependencies for custom JRE

I couldn't find a straight forward way for knowing Spring depends on which Java modules. But with some knowledge and also some try and errors this is the list that I found at last: jdk.unsupported,java.xml,java.sql,java.naming,java.desktop,,,java.instrument

7. Build docker image

Now we have both our application jar file and our custom JRE in a directory named myjre. So let's:

  1. create a Dockerfile for building final Docker image.
  2. Put this file in the root of your project

  3. as the final step we build the Docker image and show its size:
  4. Now we can call "docker image ls | grep amir/hello" and see that the image file is 180MB and much less than 238MB. Also we can think about the performance as we already have a JRE without unwanted modules in running memory when we run it.
  5. For final testing we can run the container by calling "docker run -p 8080:8080 amir/hello" and then go to browser and bring up "http://localhost:8080", then you could see a "Hello" in response.

8. Multi-Stage Docker image

As you see everything is done and we built our optimised Docker image for our Java application. But how we can simplify these steps by using multi-stage Docker file. For this purpose we can easily have a Dockerfile in our project root which would be able to:

  1. build the project
  2. and then build needed custom JRE
  3. and at last wrap them all up into a Docker image

You can see such a Dockerfile below, Also it's available as a whole project in Github.


CI/CD with Jenkins

Recently I had a nice experience with Jenkins in which I worked with many different facilities CI/CD and concepts via Jenkins. In this article I want to share this experience as short as not being boring.

Jenkins is The leading open source automation server, Jenkins provides hundreds of plugins to support building, deploying and automating any project. (cited from

1. CI/CD (Continuous Integration/Delivery/Deployment)

Jenkins is a tool for continuous integration and continuous deployment (CI/CD) which is widely used in many projects because of its simplicity and independence. I don't want to make this part long so below picture shows you the meaning of three expressions of CI/CD and their differences in a big-picture.

2. Install Jenkins

Based on our conditions we decided to install Jenkins as a Docker container. Docker provides the ability to package and run an application in a loosely isolated environment called a container. The isolation and security allow you to run many containers simultaneously on a given host.

Jenkins describes how to install it as a docker container in the formal website but there is a point to make it much better by a small change about not using docker-in-docker(dind). So I write all steps on macOS and Linux here without more description and at the end describe just about the point.

As you can see we have "-v /var/run/docker.sock:/var/run/docker.sock" which makes "dind" needless. Dind is a docker engine running as a container on host's docker engine. And instead of dind we used host's docker engine itself by socket solution. At the moment In Jenkins formal website there is one more step before running Jenkins container which is running docker-in-docker container and use it in Jenkins container with adding this "--env DOCKER_HOST=tcp://docker:2376". But we don't need it any more.

The main idea which caused me thinking about removing dind is that it will be unstable in some situations in which it would be probable to loose our dind image and all Jenkins needed utilities installed on that dind. So I googled and found some hints and between them also found an article written by one of dind contributors, "Using Docker-in-Docker for your CI or testing environment? Think twice"

Our project code is written by Java and for some reasons I also mount JDK and Gradle from our host into the Jenkins container. Also some unnecessary parts removed. So at last I had run below command instead of last one:

Now we can browse http://localhost:8080 or http://your-host-ip:8080 to access Jenkins UI, and follow its instruction for defining admin user/pass.

3. Install JDK on Jenkins

I know that JDK is not the only kit you need but here I tell about my experience just about JDK and Gradle and you can map this into other needs like nodeJS, Maven, etc.

Jenkins lets install multi JDKs (for example you need different versions). As described in above section, we extract JDK on host and mount its root into our Jenkins docker container. Also it would be possible to install JDK on Jenkins image/container itself which I don't want to do that here. Now that we already have JDK mounted in our container:

  1. login in Jenkins panel
  2. go to "Manage Jenkins"
  3. "Global Tool Configuration"
  4. click on "JDK Installations..."
  5. Type a name in "name" text-box
  6. Type JDK path based on mounted path in prev section in "JAVA_HOME" text-box (/opt/jdk/jdk1.8.0_241 in our case)
  7. click on "save"

As mentioned we did such work for Gradle and you can do it for other toolkits.

4. Create Simplest Project in Jenkins

On Jenkins UI click on "New Item" then type a name for this project and select "Freestyle Project", and then click on "OK", Now you have a very simple project for first try.

In configure project go to "Build" section and click on "Add Build Step">"Execute shell". This is a very simple try for moving our hands on the main work. Write some shell scripts there for example: echo $JAVA_HOME , java -version . Click on "Save" and at last click on "Build now". At last you can select your build number and click on "Console Output" to see what Jenkins cooked for you and how things matched together.

5. Jenkins Pipeline

Now that's time to move one step toward more advanced features and I think working with "Pipeline" would be a very main step. Repeat previous part steps to add a project but this time select "Pupeline" instead of "Freestyle". Pipeline script is a groovy-based syntax which Jenkins provides it and you can code it while configuring your project.

6. Pipeline as Code

I know that two steps are missed lets say "Installing plugins" and "Connect project to Git". And I would add them here or on another article soon. But for completing the puzzle I'm eager to write here that we made a file in our Java source code (in git) named "Jenkinsfile" and code our desired Pipeline steps there. After that we told our Pipeline Project in Jenkins to follow that file as a Pipeline-code. Sorry for lack of my time and notes here, all these steps would be added or linked here soon.


Spring Framework

1. What is Spring

Spring is an open-source Framework who simplifies development complexities of java-based enterprise applications. It provides infrastructural support at application level so that developers can focus only on business logic.

2. Framework of frameworks

Also Spring can be thought as a framework for frameworks" because Spring supports and facilitate using a wide range of frameworks such as Hibernate, Struts, etc.

3. Spring Main Approach

Spring at its core, is a dependency injection container which is a pattern for developing decoupled applications. For example if you want to has an object of class ClassA which implements interface MyInterface in class ClassB one option is to new it (ClassA a = new(ClassA);) in class ClassB. This way we would couple class B with implementation of MyInterface and loose the flexibility of managed switching implementation from ClassA into another one. In the other hand Spring manage lifecycle(creation and destroy) of objects (beans) itself, and so developer needs only to work with POJOs and you do not need an EJB container such as an application server. So what happens if somebody, say Spring, can wire these dependencies?

4. Spring Magic

Now it's time to understand how Spring simplifies development process. Spring introduces a descriptive and conventional way in which developer can determine how things must be wired together. For better understanding below is an example for inject dependency using XML while using Spring:
Or using JAVA simply in another way you can annotate a variable of ClassA in ClassB:

5. Spring Features

Spring provides many features such as MVC, Aspec-Oriented-Programming, Batch, Integration, etc. That their explanation is beyond the scope of this article. This article aims to make a short but well understanding of Spring framework essence.