In this instance, you want to run the executable JAR you copied to the /app directory. Subsequent builds will use a cached version of this image layer which means that the dependencies will be referenced locally and won't have to be pulled down again. The image is defined in the Dockerfile below and although it's compact, there's plenty going on. The next time you build the image, the Maven dependencies and the application JAR will be taken from the cached layer and won't have to be downloaded and built again. The -t option allows you to specify a name and optionally a tag. We also looked at the limitations of building Java apps with Docker and a potential workaround. This in itself is fine as we want to rebuild the layer. Alpine has a package manager so you can install whatever you need, but the important point is you're starting with an image that is very small. Now that you have a Docker image defined, it's time to build it. So choosing a lightweight JRE image is important if you want to keep the final image as small as possible. While the Maven image will be discarded, openjdk:8-jre-alpine will form part of the final image. COPY pom.xml /build/ creates a build directory in the image and copies the pom.xml into it. This is where multi-stage builds come into play. WORKDIR /build/ sets build as the working directory. The -p 8080:8080 option maps the containers internal port 8080 to port 8080 on the host machine. We talked about the importance of keeping images as light as possible and you did that by using Alpine base images and a multi-stage build to discard build. Over 2 million developers have joined DZone. As you might expect, this can make the first build quite slow. Now that the image is image built you can run a container with the following command docker container run -p 8080:8080 docker-boot-intro. FROM maven:3.5.2-jdk-8-alpine AS MAVEN_BUILD tells Docker to use the Maven imagemaven:3.5.2-jdk-8-alpine as the base image for the first stage of the build. If you were to retain everything from the MAVEN_BUILD stage you'd end up with an image that contains Maven, a local Maven repo. You can grab the full source for this post from GitHub. When you run a container, Docker will run the executable JAR and the application will start on port 8080 inside the container. On the downside, your Docker image has lost some of its autonomy. This makes it a great fit for building lean images. Any further commands will run from the build directory. A layer is created for each step along with a unique ID. The project structure is very straight forward. Let's take a look a closer look at the docker-boot-intro image and see how we arrived at 105 MB. The image you'll define later will compile, package, and run a Spring Boot application. From a Java developer's perspective, the typical Docker use case is running your application inside a Docker container. Unfortunately using Docker means that when the application layer is rebuilt, you lose the local Maven cache. Although Maven will be discarded from the final image (see COPY --from command later) I've used the Alpine-flavored Maven image as its faster to download. The workaround for this issue is to use a local Maven repository on the host machine as the source of your Maven dependencies. This is a nice lightweight image with only the bare essentials needed to run the application. On the upside, you can change the application source and rebuild without sacrificing quick build times because you're using cached Maven dependencies. This means that once you've built an image, subsequent builds become much faster. As part of the RUN mvn package command, Docker pulls all POM dependencies from the public Maven repo, builds an executable JAR, and stores all of this in layer c48659e0197e. If a cached version of the layer is available, Docker uses it instead of building the layer from scratch. If you look on DockerHub you'll see that many popular images have an Alpine version. Remember, one of the main reasons you're using Docker is so that you don't have to worry about configuring software on the environment it's running on. a base image that contains the bare essentials and nothing else. It's best to choose a base image that contains the bare essentials and nothing else. MAINTAINER Brian Hannaway isn't essential but improves maintainability by providing a point of contact for the image author. A Start to Finish Guide to Docker With Java, Part 1. This obviously slows the build considerably and will become a real pain during development. WORKDIR /app tells Docker to create a new working directory in the image called /app. This is what's happening in step 6 above. Build, Package, and Run Spring Boot Apps With Docker, Building Microservices Using Spring Boot and Docker. and all the class files generated in the target directory. The image is a deployable artefact and can be used to run containers on any virtual or physical machine with Docker installed. Every time you make a change to the application source or POM, Docker will see that layer as changed and disregard the cached copy. All further commands will run from this directory. When you're building Java apps without Docker, you pull down the Maven dependencies from the remote repository the first time you build and then reference them in your local Maven cache after that. The super-slim 5.53 MB Alpine base image is listed as the first layer. When I run this command I can see that my Linux VM IP is 192.168.99.100. I'm planning to use the default actuator health endpoint to test the application later. Open a directory containing the Dockerfile (project root). I mentioned earlier that Docker caches layers so that they can be reused to reduce build times. So when you rebuild the application layer with the mvn package command, all Maven dependencies will be pulled from the remote repository again. COPY --from=MAVEN_BUILD /build/target/docker-boot-intro-0.1.0.jar /app/ tells Docker to copy docker-boot-intro-0.1.0.jar from the /build/target directory in the MAVEN_BUILD stage to the /app directory of the current stage. Ideally, your Docker image should be self-contained and have everything it needs to build and run without any dependencies on the host. During the build, Docker checks its cache to see if it already has a cached version of each layer before attempting to build it. Why Building an External Data Product Is So Hard, Create a Minimal Web API With ASP.NET Core and Publish To Azure API Management With Visual Studio, Copies the application source code from the host machine into a temporary build directory in the image, Uses Maven to compile and package the app as an executable JAR. This is the first part of the multi-stage build finished. If you run the docker image history boot-docker-intro command you'll see a breakdown of the various layers in the image. That's great, but wouldn't it be even better if you could use Docker to build the application, too? The final Docker image is your deployable artefact so it needs to contain only the application and runtime dependencies. Published at DZone with permission of Brian Hannaway, DZone MVB. I've recently started playing around with Docker again and have decided to put together a few posts to share what I've learned. I'll cover Docker volumes in my next post and show you how they can be used to access a Maven repo on the host machine. If you're familiar with Spring Boot, you'll probably know that by default a Boot app starts on port 8080. You'll see a few examples of this later when we use Alpine-flavored Maven and Open JDK JRE images. The first time you build an image, Docker will pull whatever external images it needs from DockerHub and build new layers from scratch. You can see cached layers being used in the build output above where Docker outputs --> Using Cache and then the hash of the layer used. You'll see the docker-boot-intro image listed with a size of 105 MB. If everything works as expected you should see the Boot app start on port 8080 as follows. FROM openjdk:8-jre-alpine tells Docker you want to use the openjdk:8-jre-alpine base image for the next stage of the multi-stage build. I'll be taking this approach in the sample app later by using Alpine images. To build the image run the following commanddocker image build -t docker-boot-intro . I mentioned earlier that it's good practice to keep your images as light as possible. To keep your images slim there are a few fundamental things you need to consider. If you see output similar to that shown above, your container has started and you should be able to test the app. Docker will look for this image locally and if it isn't available it will be pulled from DockerHub. By using a Maven cache on the host you lose that autonomy. I'll explain each line in detail below. Docker is a containerization technology that allows you to build an image containing your application and all the dependencies required to run it. They allow you to split your Docker build into distinct steps and copy specific items between steps, discarding everything else. As mentioned earlier, multi-stage builds are great because they allow you to copy specific artefacts from one build stage to another and discard everything else. Join the DZone community and get the full member experience. Run the docker image ls command to list all your local images. See the original article here. A number of environment variables are configured in the next few layers and then the 79.4 MB JRE is added. In this post, I'll show you how to compile, package and run a simple Spring Boot app in a Docker container. Smaller images make for quicker build cycles, faster downloads, and lower storage costs. At this point, I've defined the image up to the point where it builds an executable JAR. ENTRYPOINT ["java", "-jar", "app.jar"] tells Docker what command to run when a container is started from this image. If you want to get things up and running quickly you can grab the full source for this post on GitHub. This will allow you to discard build tools and anything else that isn't essential to run the app. In order to access the application running in the container you need to map the internal container port to a port on the host machine. For example, step 1 created a layer with ID 293423a981a7. If you don't specify a tag Docker will automatically tag the image as latest. The next stage will take the JAR and run it. All you really want in the final image is the executable JAR and a Java JRE to run it. If you have any comments or questions please leave a note below. The first time the image is built, Maven will pull all required dependencies from the public Maven repo and cache them locally in the image. The problem is the Maven dependencies that were saved in the cached layer are lost. While this stands true, there's something you need to consider when building Java apps. What you want to do is build the application and then discard anything you don't need from the final image. 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spring boot docker maven example