How to use a multi-stage Dockerfile for Java EE app packaging

Introduction

Docker's multi-stage Dockerfiles offer a powerful solution for streamlining the packaging process of Java EE applications. This tutorial will guide you through the steps to build and optimize your Java EE app using a multi-stage Dockerfile, ensuring efficient deployment and distribution.

Skills Graph

Understanding Multi-Stage Dockerfiles

Multi-stage Dockerfiles are a powerful feature introduced in Docker 17.05 that allows you to create optimized Docker images by using multiple stages in a single Dockerfile. This approach helps to reduce the final image size and improve the build process by separating the build environment from the runtime environment.

What is a Multi-Stage Dockerfile?

A multi-stage Dockerfile is a Dockerfile that contains multiple FROM statements, each representing a different stage in the build process. Each stage can use a different base image, and the final stage is used to create the final Docker image.

Benefits of Using Multi-Stage Dockerfiles

1. Reduced Image Size: By separating the build environment from the runtime environment, you can create a smaller final Docker image that only contains the necessary components for your application to run.
2. Improved Build Process: Multi-stage Dockerfiles can simplify the build process by allowing you to use different base images for different stages, reducing the need for complex build scripts or separate Dockerfiles.
3. Increased Security: By reducing the final image size, you can minimize the attack surface and reduce the risk of security vulnerabilities.

How to Use Multi-Stage Dockerfiles

To use a multi-stage Dockerfile, you need to define multiple FROM statements in your Dockerfile, each representing a different stage. The final stage is used to create the final Docker image.

## Build stage
FROM maven:3.8.2-openjdk-11 AS build
COPY . /app
WORKDIR /app
RUN mvn clean package

## Runtime stage
FROM openjdk:11-jdk-slim
COPY --from=build /app/target/*.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

In this example, the first stage uses the maven:3.8.2-openjdk-11 image to build the Java EE application, and the second stage uses the openjdk:11-jdk-slim image to create the final Docker image, which only contains the necessary runtime components.

Building a Java EE App with Multi-Stage Dockerfiles

In this section, we'll walk through the process of building a Java EE application using a multi-stage Dockerfile.

Preparing the Java EE Application

Let's assume you have a Java EE application that you want to package using a multi-stage Dockerfile. The application is structured as follows:

my-java-ee-app/
├── pom.xml
└── src/
    ├── main/
    │   ├── java/
    │   │   └── com/
    │   │       └── example/
    │   │           └── MyApp.java
    │   └── webapp/
    │       └── WEB-INF/
    │           └── web.xml
    └── test/
        └── java/
            └── com/
                └── example/
                    └── MyAppTest.java

Creating the Multi-Stage Dockerfile

Here's an example of a multi-stage Dockerfile for building and packaging the Java EE application:

## Build stage
FROM maven:3.8.2-openjdk-11 AS build
COPY . /app
WORKDIR /app
RUN mvn clean package

## Runtime stage
FROM tomcat:9.0-jdk11-openjdk-slim
COPY --from=build /app/target/*.war /usr/local/tomcat/webapps/app.war

In this Dockerfile, we have two stages:

1. Build stage: This stage uses the maven:3.8.2-openjdk-11 image to build the Java EE application. The COPY and RUN commands are used to copy the application source code and build the application using Maven.
2. Runtime stage: This stage uses the tomcat:9.0-jdk11-openjdk-slim image as the base image. The COPY --from=build command is used to copy the built WAR file from the previous stage to the Tomcat webapps directory.

Building and Running the Docker Image

To build the Docker image, run the following command in the directory containing the Dockerfile:

docker build -t my-java-ee-app .

Once the image is built, you can run the container:

docker run -p 8080:8080 my-java-ee-app

This will start the Tomcat server and deploy the Java EE application. You can then access the application at http://localhost:8080/app.

Optimizing Multi-Stage Dockerfiles for Java EE Apps

When building Java EE applications with multi-stage Dockerfiles, there are several optimization techniques you can use to further reduce the size of your final Docker image and improve the build process.

Leveraging Build Dependencies

One common optimization is to leverage build dependencies only in the build stage and exclude them from the final runtime stage. For example, you can use a separate build image that includes all the necessary build tools, such as a Maven or Gradle image, and then copy the built artifacts to a smaller runtime image.

## Build stage
FROM maven:3.8.2-openjdk-11 AS build
COPY . /app
WORKDIR /app
RUN mvn clean package

## Runtime stage
FROM openjdk:11-jdk-slim
COPY --from=build /app/target/*.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

Utilizing Multi-Stage Cache

Docker's multi-stage build feature also allows you to leverage the build cache to speed up subsequent builds. By organizing your Dockerfile in a way that maximizes cache reuse, you can significantly reduce the build time.

## Base stage
FROM maven:3.8.2-openjdk-11 AS base
WORKDIR /app

## Copy dependencies
COPY pom.xml .
RUN mvn dependency:go-offline

## Build stage
FROM base AS build
COPY . .
RUN mvn clean package

## Runtime stage
FROM openjdk:11-jdk-slim
COPY --from=build /app/target/*.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

In this example, the pom.xml file is copied and the dependencies are downloaded in the base stage. This ensures that the dependency resolution step is cached, speeding up subsequent builds.

Optimizing the Runtime Image

Another optimization technique is to use a smaller base image for the runtime stage. For example, you can use the openjdk:11-jdk-slim image instead of the full openjdk:11-jdk image, which can significantly reduce the final image size.

## Runtime stage
FROM openjdk:11-jdk-slim
COPY --from=build /app/target/*.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

Additionally, you can use techniques like multi-stage builds, Alpine-based images, or distroless images to further optimize the runtime image size.

By applying these optimization techniques, you can create highly optimized Docker images for your Java EE applications, reducing the image size and improving the build process.

Summary

By the end of this tutorial, you will have a solid understanding of how to leverage multi-stage Dockerfiles to package your Java EE applications effectively. You will learn techniques to optimize the build process, reduce image size, and create a reliable and reproducible deployment pipeline using Docker. This knowledge will empower you to enhance the efficiency and scalability of your Java EE application deployments.