mirror of https://github.com/docker/docs.git
253 lines
11 KiB
Markdown
253 lines
11 KiB
Markdown
---
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title: "Build your Java image"
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keywords: Java, build, images, dockerfile
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description: Learn how to build your first Docker image by writing a Dockerfile
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---
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{% include_relative nav.html selected="1" %}
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## Prerequisites
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Work through the orientation and setup in Get started [Part 1](../../get-started/index.md){:target="_blank" rel="noopener" class="_"} to understand Docker concepts. Refer to the following section for Java prerequisites.
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{% include guides/enable-buildkit.md %}
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## Overview
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Now that we have a good overview of containers and the Docker platform, let’s take a look at building our first image. An image includes everything needed to run an application - the code or binary, runtime, dependencies, and any other file system objects required.
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To complete this tutorial, you need the following:
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- Docker running locally. Follow the instructions to [download and install Docker](../../get-docker.md)
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- A Git client
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- An IDE or a text editor to edit files. We recommend using [IntelliJ Community Edition](https://www.jetbrains.com/idea/download/){: target="_blank" rel="noopener" class="_"}.
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## Sample application
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Let’s clone the sample application that we'll be using in this module to our local development machine. Run the following commands in a terminal to clone the repo.
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```console
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$ cd /path/to/working/directory
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$ git clone https://github.com/spring-projects/spring-petclinic.git
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$ cd spring-petclinic
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```
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## Test the application without Docker (optional)
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In this step, we will test the application locally without Docker, before we
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continue with building and running the application with Docker. This section
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requires you to have Java OpenJDK version 15 or later installed on your machine.
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[Download and install Java](https://jdk.java.net/){: target="_blank" rel="noopener" class="_"}
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If you prefer to not install Java on your machine, you can skip this step, and
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continue straight to the next section, in which we explain how to build and run
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the application in Docker, which does not require you to have Java installed on
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your machine.
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Let’s start our application and make sure it is running properly. Maven will manage all the project processes (compiling, tests, packaging, etc). The **Spring Pets Clinic** project we cloned earlier contains an embedded version of Maven. Therefore, we don't need to install Maven separately on your local machine.
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Open your terminal and navigate to the working directory we created and run the following command:
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```console
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$ ./mvnw spring-boot:run
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```
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This downloads the dependencies, builds the project, and starts it.
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To test that the application is working properly, open a new browser and navigate to `http://localhost:8080`.
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Switch back to the terminal where our server is running and you should see the following requests in the server logs. The data will be different on your machine.
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```console
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o.s.s.petclinic.PetClinicApplication : Started
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PetClinicApplication in 11.743 seconds (JVM running for 12.364)
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```
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Great! We verified that the application works. At this stage, you've completed
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testing the server script locally.
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Press `CTRL-c` from within the terminal session where the server is running to stop it.
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We will now continue to build and run the application in Docker.
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## Create a Dockerfile for Java
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{% include guides/create-dockerfile.md %}
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Next, we need to add a line in our Dockerfile that tells Docker what base image
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we would like to use for our application.
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```dockerfile
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# syntax=docker/dockerfile:1
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FROM openjdk:16-alpine3.13
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```
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Docker images can be inherited from other images. For this guide, we use the
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official `openjdk` image from Docker Hub with Java JDK that already has all the
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tools and packages that we need to run a Java application.
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To make things easier when running the rest of our commands, let’s set the image's
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working directory. This instructs Docker to use this path as the default location
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for all subsequent commands. By doing this, we do not have to type out full file
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paths but can use relative paths based on the working directory.
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```dockerfile
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WORKDIR /app
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```
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Usually, the very first thing you do once you’ve downloaded a project written in
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Java which is using Maven for project management is to install dependencies.
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Before we can run `mvnw dependency`, we need to get the Maven wrapper and our
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`pom.xml` file into our image. We’ll use the `COPY` command to do this. The
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`COPY` command takes two parameters. The first parameter tells Docker what
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file(s) you would like to copy into the image. The second parameter tells Docker
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where you want that file(s) to be copied to. We’ll copy all those files and
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directories into our working directory - `/app`.
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```dockerfile
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COPY .mvn/ .mvn
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COPY mvnw pom.xml ./
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```
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Once we have our `pom.xml` file inside the image, we can use the `RUN` command
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to execute the command `mvnw dependency:go-offline`. This works exactly the same
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way as if we were running `mvnw` (or `mvn`) dependency locally on our machine,
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but this time the dependencies will be installed into the image.
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```dockerfile
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RUN ./mvnw dependency:go-offline
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```
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At this point, we have an Alpine version 3.13 image that is based on OpenJDK version 16, and we have also installed our dependencies. The next thing we need to do is to add our source code into the image. We’ll use the `COPY` command just like we did with our `pom.xml` file above.
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```dockerfile
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COPY src ./src
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```
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This `COPY` command takes all the files located in the current directory and copies them into the image. Now, all we have to do is to tell Docker what command we want to run when our image is executed inside a container. We do this using the `CMD` command.
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```dockerfile
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CMD ["./mvnw", "spring-boot:run"]
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```
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Here's the complete Dockerfile.
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```dockerfile
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# syntax=docker/dockerfile:1
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FROM openjdk:16-alpine3.13
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WORKDIR /app
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COPY .mvn/ .mvn
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COPY mvnw pom.xml ./
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RUN ./mvnw dependency:go-offline
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COPY src ./src
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CMD ["./mvnw", "spring-boot:run"]
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```
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### Create a `.dockerignore` file
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To increase the performance of the build, and as a general best practice, we recommend that you create a `.dockerignore` file in the same directory as the Dockerfile. For this tutorial, your `.dockerignore` file should contain just one line:
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```
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target
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```
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This line excludes the `target` directory, which contains output from Maven, from the Docker build context.
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There are many good reasons to carefully structure a `.dockerignore` file, but this one-line file is good enough for now.
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## Build an image
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Now that we’ve created our Dockerfile, let’s build our image. To do this, we use the `docker build` command. The `docker build` command builds Docker images from a Dockerfile and a “context”. A build’s context is the set of files located in the specified PATH or URL. The Docker build process can access any of the files located in this context.
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The build command optionally takes a `--tag` flag. The tag is used to set the name of the image and an optional tag in the format `name:tag`. We’ll leave off the optional `tag` for now to help simplify things. If we do not pass a tag, Docker uses “latest” as its default tag. You can see this in the last line of the build output.
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Let’s build our first Docker image.
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```console
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$ docker build --tag java-docker .
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```
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```console
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Sending build context to Docker daemon 5.632kB
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Step 1/7 : FROM java:3.7-alpine
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Step 2/7 : WORKDIR /app
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...
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Successfully built a0bb458aabd0
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Successfully tagged java-docker:latest
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```
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## View local images
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To see a list of images we have on our local machine, we have two options. One is to use the CLI and the other is to use [Docker Desktop](../../desktop/dashboard.md#explore-your-images). As we are currently working in the terminal let’s take a look at listing images using the CLI.
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To list images, simply run the `docker images` command.
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```console
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$ docker images
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REPOSITORY TAG IMAGE ID CREATED SIZE
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java-docker latest b1b5f29f74f0 47 minutes ago 567MB
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```
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You should see at least the we just built `java-docker:latest`.
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## Tag images
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An image name is made up of slash-separated name components. Name components may contain lowercase letters, digits, and separators. A separator is defined as a period, one or two underscores, or one or more dashes. A name component may not start or end with a separator.
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An image is made up of a manifest and a list of layers. Do not worry too much about manifests and layers at this point other than a “tag” points to a combination of these artifacts. You can have multiple tags for an image. Let’s create a second tag for the image we built and take a look at its layers.
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To create a new tag for the image we’ve built above, run the following command:
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```console
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$ docker tag java-docker:latest java-docker:v1.0.0
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```
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The `docker tag` command creates a new tag for an image. It does not create a new image. The tag points to the same image and is just another way to reference the image.
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Now, run the `docker images` command to see a list of our local images.
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```console
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$ docker images
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REPOSITORY TAG IMAGE ID CREATED SIZE
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java-docker latest b1b5f29f74f0 59 minutes ago 567MB
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java-docker v1.0.0 b1b5f29f74f0 59 minutes ago 567MB
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```
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You can see that we have two images that start with `java-docker`. We know they are the same image because if you take a look at the `IMAGE ID` column, you can see that the values are the same for the two images.
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Let’s remove the tag that we just created. To do this, we’ll use the `rmi` command. The `rmi` command stands for “remove image”.
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```console
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$ docker rmi java-docker:v1.0.0
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Untagged: java-docker:v1.0.0
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```
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Note that the response from Docker tells us that the image has not been removed but only “untagged”. You can check this by running the `docker images` command.
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```console
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$ docker images
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REPOSITORY TAG IMAGE ID CREATED SIZE
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java-docker latest b1b5f29f74f0 59 minutes ago 567MB
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```
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Our image that was tagged with `:v1.0.0` has been removed, but we still have the `java-docker:latest` tag available on our machine.
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## Next steps
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In this module, we took a look at setting up our example Java application that we'll use for the rest of the tutorial. We also created a Dockerfile that we used to build our Docker image. Then, we took a look at tagging our images and removing images. In the next module, we’ll take a look at how to:
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[Run your image as a container](run-containers.md){: .button .primary-btn}
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## Feedback
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Help us improve this topic by providing your feedback. Let us know what you think by creating an issue in the [Docker Docs](https://github.com/docker/docker.github.io/issues/new?title=[Java%20docs%20feedback]){:target="_blank" rel="noopener" class="_"} GitHub repository. Alternatively, [create a PR](https://github.com/docker/docker.github.io/pulls){:target="_blank" rel="noopener" class="_"} to suggest updates.
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<br />
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