Open source Java projects: Docker

Redefine Java virtualization with Docker

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A Dockerfile is a set of instructions that tells Docker how to build an image. It defines the starting point and what specific things to do to configure the image. Let's take a look at a sample Dockerfile and walk through what it is doing. Listing 1 shows the Dockerfile for the base CentOS image.

Listing 1. CentOS Dockerfile

FROM scratch
MAINTAINER The CentOS Project <> - ami_creator
ADD centos-7-20150616_1752-docker.tar.xz /
# Volumes for systemd
# VOLUME ["/run", "/tmp"]
# Environment for systemd
# ENV container=docker
# For systemd usage this changes to /usr/sbin/init
# Keeping it as /bin/bash for compatibility with previous
CMD ["/bin/bash"]

You'll note that most of this Dockerfile is comments. It has four specific instructions:

  1. FROM scratch: All Dockerfiles are derived from a base image. In this case, the CentOS image is derived from "scratch," which is the root of all images. Essentially this is a no-op, which indicates that it is one of Docker's root images.
  2. MAINTAINER ...: The MAINTAINER directive identifies the owner of the image. In this case the owner is the CentOS Project.
  3. ADD centos...tar.xz: The ADD directive tells Docker to upload the specified file to the image and, if it is compressed, to decompress it to the specified location. In this example, Docker is going to upload a GZipped TAR file of the CentOS operating system and decompress it to the root of the file system (/).
  4. CMD ["/bin/bash"]: Finally, the CMD directive tells Docker what command to execute. In this case, it is going to create a Bourne Again Shell (bash).

Now that you have a sense of what a Dockerfile looks like, let's explore the official Tomcat Dockerfile. Figure 2 shows this file's hierarchy.

Tomcat Dockerfile hierarchy

Figure 2. Tomcat Dockerfile hierarchy

This hierarchy might not be as simple as you would have anticipated, but when we take it apart piece-by-piece it is actually quite logical. You already know that the root of all Dockerfiles is scratch, so seeing that as the root is no surprise. The first meaningful Dockerfile image is debian:jessie. The Docker official images are built from build packs, or standard images. This means that Docker does not need to reinvent the wheel every time it creates a new image, but rather it has a solid base from which to start building new images. In this case, debian:jessie is simply a Debian Linux image installed just like the CoreOS that we just looked at. It includes the three lines in listing 2.

Listing 2. debian:jessie Dockerfile

FROM scratch
ADD rootfs.tar.xz /
CMD ["/bin/bash"]

On top of that we see two additional installations: CURL and Source Code Management. The Dockerfile for buildpack-deps:jessie-curl is shown in Listing 3.

Listing 3. buildpack-deps:jessie-curl Dockerfile

FROM debian:jessie
RUN apt-get update && apt-get install -y --no-install-recommends \
		ca-certificates \
		curl \
		wget \
	&& rm -rf /var/lib/apt/lists/*

This Dockerfile uses apt-get to install curl and wget so that the image will be able to download software from other servers. The RUN directive tells Docker to execute the specified command on the running instance. In this case it updates all libraries (apt-get update) and then executes the apt-get install to download and install curl and wget.

The Dockerfile for buildpack-deps:jessie-scp is shown in Listing 4.

Listing 4. buildpack-deps:jessie-scp Dockerfile

FROM buildpack-deps:jessie-curl
RUN apt-get update && apt-get install -y --no-install-recommends \
		bzr \
		git \
		mercurial \
		openssh-client \
		subversion \
	&& rm -rf /var/lib/apt/lists/*

This Dockerfile installs source code management tools, such as Git, Mercurial, and Subversion, following the same model as the jessie-curl Dockerfile we just looked at.

The Java Dockerfile is a little more complicated; it is shown in Listing 5.

Listing 5. Java Dockerfile

FROM buildpack-deps:jessie-scm
# A few problems with compiling Java from source:
#  1. Oracle.  Licensing prevents us from redistributing the official JDK.
#  2. Compiling OpenJDK also requires the JDK to be installed, and it gets
#       really hairy.
RUN apt-get update && apt-get install -y unzip && rm -rf /var/lib/apt/lists/*
RUN echo 'deb jessie-backports main' > /etc/apt/sources.list.d/jessie-backports.list
# Default to UTF-8 file.encoding
ENV JAVA_DEBIAN_VERSION 8u66-b01-1~bpo8+1
# see
# and
RUN set -x \
	&& apt-get update \
	&& apt-get install -y \
		openjdk-8-jdk="$JAVA_DEBIAN_VERSION" \
		ca-certificates-java="$CA_CERTIFICATES_JAVA_VERSION" \
	&& rm -rf /var/lib/apt/lists/*
RUN /var/lib/dpkg/info/ca-certificates-java.postinst configure
# If you're reading this and have any feedback on how this image could be
#   improved, please open an issue or a pull request so we can discuss it!

Essentially, this Dockerfile runs apt-get install -y openjdk-8-jdk to download and install Java, with some added configuration to install it securely. The ENV directive sets system environment variables that will be needed during the installation.

Finally, Listing 6 shows the Tomcat Dockerfile.

Listing 6. Tomcat Dockerfile

FROM java:7-jre
ENV CATALINA_HOME /usr/local/tomcat
# see
RUN gpg --keyserver --recv-keys \
	05AB33110949707C93A279E3D3EFE6B686867BA6 \
	07E48665A34DCAFAE522E5E6266191C37C037D42 \
	47309207D818FFD8DCD3F83F1931D684307A10A5 \
	541FBE7D8F78B25E055DDEE13C370389288584E7 \
	61B832AC2F1C5A90F0F9B00A1C506407564C17A3 \
	79F7026C690BAA50B92CD8B66A3AD3F4F22C4FED \
	9BA44C2621385CB966EBA586F72C284D731FABEE \
	A27677289986DB50844682F8ACB77FC2E86E29AC \
	A9C5DF4D22E99998D9875A5110C01C5A2F6059E7 \
	DCFD35E0BF8CA7344752DE8B6FB21E8933C60243 \
	F3A04C595DB5B6A5F1ECA43E3B7BBB100D811BBE \
RUN set -x \
	&& curl -fSL "$TOMCAT_TGZ_URL" -o tomcat.tar.gz \
	&& curl -fSL "$TOMCAT_TGZ_URL.asc" -o tomcat.tar.gz.asc \
	&& gpg --verify tomcat.tar.gz.asc \
	&& tar -xvf tomcat.tar.gz --strip-components=1 \
	&& rm bin/*.bat \
	&& rm tomcat.tar.gz*
CMD ["", "run"]

Technically, Tomcat uses a Java 7 parent Dockerfile (the default or "latest" version of Java is 8, which is shown in Listing 5). This Dockerfile sets up the CATALINA_HOME and PATH environment variables using the ENV directive. It then creates the CATALINA_HOME directory by running the mkdir command. The WORKDIR directive changes the working directory to CATALINA_HOME. The RUN command executes a host of different commands in a single line:

  1. Download the Tomcat GZipped TAR file.
  2. Download the checksum for the file.
  3. Verify that the Tomcat TAR file download was successful.
  4. Decompress the Tomcat TAR file.
  5. Remove all batch files (we're running in Linux after all).
  6. Remove the Tomcat file that we downloaded (since we've already decompressed it).

Defining all of these instructions in one command means that Docker sees a single instruction, and caches the resulting image as such. Docker has a strategy for detecting when images need to be rebuilt and each instruction is verified during the build process. It caches the result of each step as an optimization, so that if the last step in a Dockerfile changes, Docker is able to start from a mostly complete image and just apply that last step. The downside of specifying all of these commands in one line is that if any one of the commands change, the whole Docker image will need to be rebuilt.

The EXPOSE directive tells Docker to expose a particular port on the Docker container when it starts. As you saw when we launched this image earlier, we needed to tell Docker what physical port to map to the container port (the -p argument), and the EXPOSE directive is the mechanism for defining available Docker instance ports. Finally, the Dockerfile starts Tomcat by executing the command (which is in the PATH) with the run argument.

Quick recap

Building the Dockerfile from inception to Tomcat was a long process, so let me summarize all the steps so far:

  1. Install Debian Linux.
  2. Add curl and wget.
  3. Add source code management tools in case you need them.
  4. Download and install Java.
  5. Download and install Tomcat.
  6. Expose port 8080 on the Docker instance.
  7. Run Tomcat by executing run.

At this point you should be a Dockerfile expert -- or at least somewhat dangerous! Next we'll try building a custom Docker image that contains our application.

Deploying a custom application to Docker

Because this tutorial is more about deploying a Java application to Docker and less about the actual Java application, I have built a very simple Hello World servlet. You can access the project on GitHub. The source code is nothing special; it's just a servlet that outputs "Hello, World!" What's more interesting is the accompanying Dockerfile, shown in Listing 7.

Listing 7. Dockerfile for the Hello World servlet

FROM tomcat
ADD deploy /usr/local/tomcat/webapps

It might not look like much, but you should already understand what the code in Listing 7 is doing:

  • FROM tomcat indicates that this Dockerfile is based on the Tomcat image we explored in the previous section.
  • ADD deploy /usr/local/tomcat/webapps tells Docker to copy all the files in the local "deploy" directory to /usr/local/tomcat/webapps, which is the home for web applications on this Tomcat image.

Clone the project locally and build it using the Maven command:

mvn clean install

This will create the file, target/helloworld.war. Copy that file to the project's docker/deploy directory (which you will need to create). Finally, you need to build the Docker image from the Dockerfile. Execute the following command from the project's docker directory (which already contains the Dockerfile in Listing 7):

docker build -t lygado/docker-tomcat .

This command tells Docker to build a new image from the current working directory, which is indicated by the dot (.) with the tag (-t): lygado/docker-tomcat. In this case, lygado is my DockerHub username and docker-tomcat is the image name (you'll want to specify your own username). To see that the image has been built you can execute the docker images command:

$ docker images
REPOSITORY                      TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
lygado/docker-tomcat            latest              ccb455fabad9        42 seconds ago      849.5 MB

Finally, you can launch the image with the docker run command:

docker run -d -p 8080:8080 lygado/docker-tomcat

After the instance starts, you can access it through the following URL (be sure to substitute the IP Address of the VirtualBox instance on your machine):

Again, you can stop this Docker container instance with the docker stop INSTANCE_ID command.

Docker push

Once you have built and tested your Docker image, you can push that image to your DockerHub account with the following command:

docker push lygado/docker-tomcat

This makes your image available to the world (or your private DockerHub repository if you opt for privacy), as well as to any automated provisioning tools you will ultimately use to deploy containers to production.

Next we'll integrate Docker into our build process, so that it can produce a Docker image that includes our application.

Integrating Docker into your Maven build process

In the previous section we created a custom Dockerfile and deployed our WAR file to it. This meant copying the WAR file from our project's target directory to the docker/deploy directory and running docker from the command line. It wasn't much work, but if you are actively developing and want to modify your code and test it immediately afterwards, you might find this process tedious. Furthermore, if you want to run your build from a continuous integration (CI) server and output a runnable Docker image, then you are going to figure out how to integrate Docker with your CI tool.

So let's explore a more efficient process, where we build (and optionally run) a Docker image from Maven using a Maven Docker plug-in.

If you search the Internet you will find several Maven plugins that integrate with Docker, but for the purpose of this article I chose to use the following plug-in: rhuss/docker-maven-plugin. While it isn't an exhaustive comparison of all scenarios and plug-in providers, author Roland Huss provides a pretty good comparison of several contenders. I encourage you to read it, to help guide your decision about the best Maven Docker plug-in for your use case.

My use cases were:

  1. To be able to create a Tomcat-based Docker image that hosted my application.
  2. To be able to run it from my build for my own tests.
  3. To be able to integrate it with pre- and post- integration test Maven phases for automated tests.

The docker-maven-plugin accomplished these tasks, was very easy to use, and was easy to understand.

About the Maven Docker plugin

The plug-in itself is well documented, but as an overview it really consists of two main configuration components:

  • Docker image build and run configurations in your POM.xml file.
  • An assembly description of files to include in your image.

The build and run image configuration is defined in the plugins section of your POM file, shown in Listing 8.

Listing 8. POM file build section for Docker Maven plug-in configuration


As you can see, this configuration is pretty simple and consists of the following categories of elements:

Plug-in definition
The groupId, artifactId, and version identify the plug-in to use.

Global configuration
The dockerHost and certPath elements define the location of your Docker host, which is emitted when you start Docker, and the location of your Docker certificates. The Docker certificate file is available in your DOCKER_CERT_PATH environment variable.

Image configuration
All images in your build are defined as image child elements under your images element. An image element has image-specific configuration, as well as build and run configuration (explained below). The core image-specific element is the name of the image you want to build. In our case this is my DockerHub username (lygado), the name of the image (tomcat-with-my-app), and the version of the Docker image (0.1). Note that you can use a Maven property for any of these values.

Image build configuration
When you build an image, as we did with the docker build command, you needed a Dockerfile that defines how to build it. The Maven Docker plug-in allows you to use a Dockerfile, but in this example we are going to build the Docker image from a Dockerfile that is built on-the-fly and resident in memory. Therefore, we specify the parent image in the from element, which in this case is tomcat, then we reference an assembly configuration.

The maven-assembly-plugin, provided by Maven, defines a common structure for aggregating a project's output with its dependencies, modules, site documentation, and other files into a single distributable archive, and the docker-maven-plugin leverages this standard. In this example, we opt to use the dir mode, which means that the files defined in the src/main/docker/assembly.xml file should be copied directly to the basedir on the Docker image. Other modes include Tar (tar), GZipped Tar (tgz), and Zipped (zip). The basedir specifies where the files should be placed in the Docker image, which in this case is Tomcat's webapps directory.

Finally, the descriptor tells the plug-in the name of the assembly descriptor, which will be located in the src/main/docker directory. This is a very simplistic example, so I encourage you to read through the documentation. In particular, you will want to familiarize yourself with the entrypoint and cmd elements, which allow you to specify the command to start the Docker image, the env element to specify environment variables, the runCmds to execute commands just as you would in a proper Dockerfile, workdir to change the working directory, and volumes if you want to mount external volumes. In short, this plug-in exposes everything that you need to be able to build a Dockerfile, which means that all of the Dockerfile directives specified earlier in the article are all available through the plug-in.

Image run configuration
When you run a Docker image using the docker run command, you may pass a collection of arguments to Docker. In this example we start our Docker instance with a command like: docker run -d -p 8080:8080 lygado/tomcat-with-my-app:0.1, so essentially we only need to specify our port mapping.

The run element allows us to specify all of our runtime arguments, so we specify that we should map port 8080 on our Docker container to port 8080 on our Docker host. Additionally, the run section allows us to specify volumes to bind (using the volumes element) and containers to link together (using the links element). Because the docker:start command is often used with integration tests, the run section includes the wait argument, which allows us to wait for a specified period of time, for a log entry, or for a URL to become available before continuing execution. This ensures that our image is running before we launch any integration tests.

Loading dependencies

The src/main/docker/assembly.xml file defines the files (or file in this case) that we want to copy to the Docker image. It is shown in Listing 9.

Listing 9. assembly.xml

<assembly xmlns=""

In Listing 9 we see a dependency set that includes our hello-world-servlet-example artifact, and we see see that we want to output it to the . directory. Recall that in our POM file we defined a basedir, which specified the location of Tomcat's webapps directory; the outputDirectory is relative to that base directory. So in other words we want to deploy the hello-world-servlet-example artifact to Tomcat's webapps directory.

The plug-in defines a set of Maven targets:

  • docker:build: used to build an image
  • docker:start: used to start a container
  • docker:stop: used to stop a container
  • docker:push: used to push an image to a repository, such as DockerHub
  • docker:remove: used to delete an image from the local Docker host
  • docker:logs: used to show a container's logs

Build the Docker image

You can access the source code for the example application from GitHub, then build it as follows:

mvn clean install

To build a Docker image you can execute the following command:

mvn clean package docker:build

Once the image is built, you can see it in Docker using the docker images command:

$ docker images
REPOSITORY                  TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
lygado/tomcat-with-my-app   0.1                 1d49e6924d19        16 minutes ago      347.7 MB    

You can run the container with the following command:

mvn docker:start

Now you should see it running with the docker ps command. It should be accessible through the same URL defined above:

Finally, you can stop the container with the following command:

mvn docker:stop


Docker is a container technology that virtualizes processes more than it does machines. It consists of a Docker client process that communicates with a Docker daemon running on a Docker host. On Linux, the Docker daemon runs directly as a process on the Linux operating system, whereas on Windows and Mac it starts a Linux virtual machine in VirtualBox and runs the Docker daemon there. Docker images contain a very lightweight operating system footprint, in addition to whatever libraries and binaries you need to run your application. Docker images are driven by a Dockerfile, which defines the instructions necessary to configure an image.

In this Open source Java projects tutorial I've introduced Docker fundamentals, reviewed the details of the Dockerfiles for CentOS, Java, and Tomcat, and showed you how to build a Dockerfile from Tomcat. Finally, we integrated the docker-maven-plugin into a build process and we built a Docker image from Maven. Being able to build and run a Docker container as part of our build makes testing easier, and it also allows us to build Docker images from a CI server that is ready for production deployment.

The example application for this article was very simple, but the steps you've learned are applicable to more complex enterprise applications. Happy docking!

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