We live in a world buzzing full of cloud-native applications. Containerization has become a part and parcel of delivering software. What we need to provide for is an easy way to deploy an application–what better way than Docker images? Fortunately for us, Gradle & the gradle-docker-plugin can significantly help with this task.
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A sample Docker image that you can additionally opt to push to the Docker registry.
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A text editor
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A command prompt
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The Java Development Kit (JDK), version 1.8 or higher
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A Gradle distribution, version 5.2 or better
The first step is to create a folder for the new project and change directory into it.
$ mkdir docker $ cd docker
From inside the new project directory, run the init task using the gradle init command in the terminal.
$ gradle init Select type of project to generate: 1: basic 2: application 3: library 4: Gradle plugin Enter selection (default: basic) [1..4] 2 Select implementation language: 1: C++ 2: Groovy 3: Java 4: Kotlin 5: Scala 6: Swift Enter selection (default: Java) [1..6] 3 Split functionality across multiple subprojects?: 1: no - only one application project 2: yes - application and library projects Enter selection (default: no - only one application project) [1..2] 1 Select build script DSL: 1: Groovy 2: Kotlin Enter selection (default: Groovy) [1..2] 1 Generate build using new APIs and behavior (some features may change in the next minor release)? (default: no) [yes, no] Select test framework: 1: JUnit 4 2: TestNG 3: Spock 4: JUnit Jupiter Enter selection (default: JUnit Jupiter) [1..4] 1 Project name (default: docker): Source package (default: docker..java): > Task :init Get more help with your project: https://docs.gradle.org/7.4.1/samples/sample_building_java_applications.html BUILD SUCCESSFUL in 29s 2 actionable tasks: 2 executed
The init task generates the new project with the following structure:
docker ├── gradle │ └── wrapper │ ├── gradle-wrapper.jar │ └── gradle-wrapper.properties ├── app | ├── build.gradle │ └── src │ └── main | | └── java | | └── App.java | ├── test | | └── java | | └── AppTest.java │ └── gradle-wrapper.properties ├── gradlew ├── gradlew.bat └── settings.gradle
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Generated folder for wrapper files
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Gradle wrapper start scripts
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Settings file to define build name and subprojects
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Build script of lib project
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./app folder containing the Java application
To build the project, we levy the build command.
$ ./gradlew build BUILD SUCCESSFUL 7 actionable tasks: 7 executed
The project provides the Gradle Wrapper as part of the repository. It is a recommended practice for any Gradle project as it enables your project to built on CI without having to install the Gradle runtime.
$ ./gradlew build --scan Publishing build scan... https://gradle.com/s/xxxxxxx
We make use of the 'docker-java-application' plugin here to carry out the essential task of building the Docker image.
plugins {
id 'application'
id 'java'
id 'com.bmuschko.docker-java-application' version '7.3.0'
// mentioning a compatible version is a must
}
With this command, gradle picks up the tasks related to Docker. We can verify the same by running the tasks task of the gradle wrapper.
❯ ./gradlew tasks > Task :tasks ------------------------------------------------------------ Tasks runnable from root project 'docker' ------------------------------------------------------------ ... Docker tasks ------------ dockerBuildImage - Builds the Docker image for the application. dockerCreateDockerfile - Creates the Docker image for the application. dockerPushImage - Pushes created Docker image to the repository. dockerSyncBuildContext - Copies the distribution resources to a temporary directory for image creation. ... [listing.terminal]
As an optional step, we can generate and inspect the Dockerfile which is generated on levying the dockerCreateDockerfile command:
FROM openjdk:11-jre-slim LABEL maintainer=codespace WORKDIR /app COPY libs libs/ COPY classes classes/ ENTRYPOINT ["java", "-cp", "/app/resources:/app/classes:/app/libs/*", "docker.java.App"] EXPOSE 8080
To build the Docker image, we carry out the dockerBuildImage task. This step will auto-generate the Dockerfile and create the image for us.
$ ./gradlew dockerBuildImage > Task :app:dockerBuildImage Building image using context '/workspaces/testing-gradle/app/build/docker'. Using images 'docker-getting-started-gradle-java/app:latest'. Step 1/7 : FROM openjdk:11-jre-slim ---> d3b5993fad1f Step 2/7 : LABEL maintainer=codespace ---> Running in 4c9a751ea6c1 Removing intermediate container 4c9a751ea6c1 ---> 3d951b688e90 Step 3/7 : WORKDIR /app ---> Running in ede4f9b275d8 Removing intermediate container ede4f9b275d8 ---> cddca74203de Step 4/7 : COPY libs libs/ ---> fca50e87d768 Step 5/7 : COPY classes classes/ ---> 5a91c294d8d2 Step 6/7 : ENTRYPOINT ["java", "-cp", "/app/resources:/app/classes:/app/libs/*", "docker.java.App"] ---> Running in d1c53fba1df3 Removing intermediate container d1c53fba1df3 ---> b7ef69af5fd3 Step 7/7 : EXPOSE 8080 ---> Running in 07e2d60733b2 Removing intermediate container 07e2d60733b2 ---> f9d090080e8a Successfully built f9d090080e8a Successfully tagged docker/app:latest Created image with ID 'f9d090080e8a'.
The generated Docker image can be run via the Docker CLI command docker run by providing the image ID.
$ docker run --rm f9d090080e8a Hello World!
We can also opt to push to a registry of our choice by levying the dockerPushImage command.
That’s it! You’ve successfully built & run a Docker image with the help of the gradle-docker-plugin. You’ve learned how to:
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Initialize a project that produces a Java application
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Run the build and view the test report
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Install the
gradle-docker-plugin -
Run the Gradle tasks command to verify
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Generate the Dockerfile
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Build the Docker image
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Run the Docker image
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Push the Docker image to a registry
You can learn more about advanced Docker usage through these resources:
Building a Docker image may seem like a trivial task, but for advanced use cases you can check out: