Setup JRebel with Tomcat and Docker

It’s fairly straightforward to install JRebel to run on a local instance of Tomcat, here is one way of installing it on Tomcat running in a docker container instead. This article assumes a basic knowledge of using docker.

For this particular example I’m using:

  • the Eclipse IDE installation of JRebel
  • the ‘official’ Tomcat 8 image from the Docker hub

Install JRebel in the IDE

I’m using the Eclipse IDE, but there are instructions on the ZeroTurnaround website on using a different IDE or for installing it standalone.

1. For Eclipse, follow these instructions just to install and activate JRebel for the IDE:!/server-configuration

2. We need the JRebel agent (jrebel.jar) to install into Tomcat.

You can either get this from the JRebel plugin you have just installed into Eclipse (look for the section titled ‘Where do I find jrebel.jar?’);

OR you can get it from an archive

(Note that for Tomcat 8, please use the legacy version of jrebel.jar which is found in the lib sub-directory of the zip archive.)

Install JRebel in the Application Server

1. Get the base Tomcat docker image from the docker hub.

docker pull tomcat:xxx

Here xxx is the specific version of Tomcat you want to use as the base image, e.g. 8.0.23-jre7, 8-jre8, etc. You can find the list in the Tomcat docker repository:

2. Since we are using docker to run the application server, then we will need to run JRebel in remote mode. There are generic instructions on JRebel remoting, which we can adapt to do it in a docker environment. So what we want to do is to create a custom docker image, based on the Tomcat image, which incorporates the JRebel configuration.

2.1 Create an empty directory and copy the JRebel agent jrebel.jar to it.

2.2 Create a Dockerfile to build your custom Tomcat image, for example:

Note that for simplicity, I have just added the JRebel agent to the directory /jrebel. You can use a different directory, as long as the -javaagent configuration can find it.

Also you can take this opportunity to do further customizations on the Tomcat server, e.g. if you want to add your list of users, then copy the your version of tomcat-users.xml to the Tomcat config directory by adding this line to the Dockerfile:

ADD tomcat-users.xml /usr/local/tomcat/conf/

2.3 Build and run the customized Tomcat server (using your own repository name, image name and container name to replace the values in this example).

docker build -t your_repository/tomcat-jrebel .

docker run -i -t -d --name mytomcat -p 8080:8080 your_repository/tomcat-jrebel

We can verify that the JRebel configuration has been included in Tomcat by checking the startup logs.

docker logs mytomcat

We should be able to see the JRebel version and licensing information.

2015-05-22 10:38:40 JRebel:  #############################################################
2015-05-22 10:38:40 JRebel:  
2015-05-22 10:38:40 JRebel:  JRebel Legacy Agent 6.2.0 (201505201206)
2015-05-22 10:38:40 JRebel:  (c) Copyright ZeroTurnaround AS, Estonia, Tartu.
2015-05-22 10:38:40 JRebel:  
2015-05-22 10:38:40 JRebel:  Over the last 1 days JRebel prevented
2015-05-22 10:38:40 JRebel:  at least 0 redeploys/restarts saving you about 0 hours.
2015-05-22 10:38:40 JRebel:  
2015-05-22 10:38:40 JRebel:  Server is running with JRebel Remoting.
2015-05-22 10:38:40 JRebel:  
2015-05-22 10:38:40 JRebel:  
2015-05-22 10:38:40 JRebel:  #############################################################

Tip: Build Your Own
Of course you can combine these 2 steps for creating a custom image into 1, by creating your own Tomcat image from scratch instead of using the ‘official’ Tomcat image as a base.

 Configure the IDE

Finally we need to configure Eclipse to work with the Tomcat server that we have running in docker. You can do that by following these instructions.

This is a brief summary of the steps:

  1. In Eclipse, right-click on your project, select JRebel -> Add JRebel Nature
  2. Right-click on your project again, select JRebel -> Enable remote server support
  3. Right-click on your project again, select JRebel -> Advanced Properties
  4. In the dialog that pops up, click on “Edit” button next to the “Deployment URLs” text box
  5. Click on “Add” and enter the URL of the application, it will be something like “http://your_docker_host:8080/app_name”
  6. Click on “Continue”, “Apply”, and then “OK”.

Once the app is deployed, any changes you make in the IDE should now be reflected in the server running in the docker container.

No restarts, no redeploys, just code.

Pluggable Tools with Docker Data Containers

There are some apps that have a simple installation process. When using them with other applications in Docker, they may be able to be installed in their own data volume container and used in a pluggable way.

The kind of apps I’m talking about are some Java apps (and in fact, Java itself) which follow this installation process:

  1. Install the contents of the app into a single directory
  2. Set an environmental variable to point to the installation directory, e.g. XXX_HOME
  3. Add the executables of the app to the PATH environmental


That’s it.

An example of an app installation that follows this pattern is Gradle:

  1. Uncompress the Gradle files from an archive to a directory.
  2. Set the enviromental variable GRADLE_HOME to point to the gradle installation directory
  3. Add GRADLE_HOME/bin to the PATH



Using Gradle as an example, here is a Dockerfile that installs it in a data volume container:

# Install Gradle as a data volume container. 
# The app container that uses this container will need to set the Gradle environmental variables.
# - set GRADLE_HOME to the gradle installation directory
# - add the /bin directory under the gradle directory to the PATH

FROM mini/base


# setup location for installation

# install Gradle version required

RUN curl -L -O${GRADLE_VERSION} && \
    unzip -qo gradle-${GRADLE_VERSION} && \
    rm -rf gradle-${GRADLE_VERSION}
# to make the container more portable, the installation directory name is changed from the default
# gradle-${GRADLE_VERSION} to just gradle, with the version number stored in a text file for reference
# e.g. instead of /opt/gradle-2.2.1, the directory will be /opt/gradle

RUN mv gradle-${GRADLE_VERSION} gradle && \
    echo ${GRADLE_VERSION} > gradle/version

# echo to make it easy to grep
CMD /bin/sh -c '/bin/echo Data container for Gradle'

(From github

Build the image and container from the Dockerfile. Here I’ve tagged the image with the version number of the Gradle installation, and named the container gradle-2.2.1.

docker build -t yourrepo/gradle:2.2.1 .

docker run -i -t --name gradle-2.2.1 yourrepo/gradle:2.2.1

A few things to note about this installation:

  • I have changed the directory name where Gradle is installed from the default, by removing the version number in order to make it generic.
  • no environmental variables have been set, that will be done later
  • you can use any minimal image as the basis for the container, it justs need curl or wget in order to download the Gradle archive file

Now we have the Gradle installation in a docker data volume that can be persisted and shared by other containers.

You can then repeat this process with difference versions of Gradle to create separate data containers for each version (of course giving the containers different names, e.g. gradle-2.2.1, gradle-1.9, etc).

Use Case

I originally got this idea when I was running my Jenkins CI docker container. Some of the Jenkins builds required Gradle 2.x while others were using Gradle 1.x.

So instead of building multiple Jenkins + Gradle images for the different versions of Gradle required, I can now just run the Jenkins container with the appropriate Gradle data container. This is done by using –volumes-from to get access to the Gradle installation directory and setting the require environmental variables.

To use the data container with Gradle 2.2.1 installed:

docker run -i -t --volumes-from gradle-2.2.1 -e GRADLE_HOME=/opt/gradle -e PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/gradle/bin myjenkins</pre>

To use the one with Gradle 1.9:

docker run -i -t --volumes-from gradle-1.9 -e GRADLE_HOME=/opt/gradle -e PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/opt/gradle/bin myjenkins</pre>

Of course there are limitations to this technique since Docker data volume containers were designed to share persistant data rather than application installs. In particular they do not allow sharing of environmental variables.

However this work around that can be useful in some circumstances.

Backup a Docker Data Container with Fig

I have been using data volume containers to persist data in docker containers.  There are various reasons why this tends to be a better option than just using data volumes, but probably the most important is portability.

Of course now we have to backup the data in the data containers. This can be for archiving, or when the containers that use the data need to be upgraded or recreated. If your backup requirements are simple you can simply use the docker cp command or something like tar.

A Jenkins example

As a simple example, let’s run a Jenkins server in a docker container and use a data volume container to persist its data.

1. Pull or build a Jenkins image from the official repository.

2. The Jenkins images uses the directory /var/jenkins_home as the volume to store it’s data, so we need a data volume container for that volume. Here is a sample of a Dockerfile to build the data container:

Build and tag the image from the Dockerfile.

docker build -t your_repository:jenkins-data .

You can now create the data container, giving it a name for convenience. Optionally we can run the docker ps command afterwards to check that the container has been created, it should be in a stopped state.

docker run -i -t --name jenkins-data your_repository:jenkins-data
docker ps -a

3. Run the Jenkins server with the data container attached and make some changes, e.g. create a job, etc. The Jenkins data volume should have your changes in it now.

docker run --name=jenkins-sample -p 8080:8080 --volumes-from=jenkins-data jenkins

4. For this example we will use tar to backup the data container, using this command to create a temporary container to access the data container.

docker run -rm --volumes-from jenkins-data -v $(pwd):/backup busybox tar cvf /backup/jenkins_backup.tar /var/jenkins_home

There should now be a file jenkins_backup.tar in the current directory. Of course for real usage, we would probably run this command from a script and make it generic to be able to backup any data volume container.

I do give a fig …

Something else I use for development with Docker is the orchestration tool Fig (this has saved me a lot of typing!). So here is an example of  doing the same backup on the Jenkins data container using Fig.

1. Create Fig YAML file, using the same information that we used in the backup command.

2. Run Fig, that’s it!

fig up

This is a simple example that has only scratched the surface of what can be done with Docker (and Fig). If the backup requirements for the data is more complex, then you could also consider creating a dedicated container just for doing backups, with all the required tools installed in it.

The great thing about Docker is that once everything has been setup, you can get applications such as Jenkins up and running very quickly.