Building Docker images for multi-module Java projects

22 Sep 2023

Docker

In modern software development, containerization has become a popular approach for building and deploying applications. Docker is one of the widely used containerization platforms that enables developers to package their applications and dependencies into containers, ensuring consistency across different environments.

In this blog post, we will explore how to build docker images for multi-module Java projects. A multi-module project is a project that consists of multiple modules or sub-projects, each with its own set of source code, configuration, and dependencies. Managing such projects can be challenging, but Docker simplifies the process by allowing you to create separate containers for each module while maintaining the overall project structure.

Prerequisites

Before we begin, make sure you have the following prerequisites:

Docker installed on your machine
A multi-module Java project, with each module having its own Dockerfile and dependencies defined

Dockerizing individual modules

To build docker images for each module, we need to create individual Dockerfiles for each module. The Dockerfile is a text file that contains a set of instructions to build a Docker image.

Navigate to each module directory within your multi-module Java project.
Create a Dockerfile in each module directory. This file will define the instructions to build the Docker image for that particular module.
In each Dockerfile, specify the base image, which is an existing Docker image that forms the starting point for your image. For Java projects, you can use the OpenJDK image as the base.
```
FROM openjdk:11
```
Copy the module’s source code and any required configuration files into the Docker image.
```
COPY . /app
```
Define the working directory and any necessary environment variables.
```
WORKDIR /app
ENV JAVA_OPTS="-Xmx256m"
```
Build the JAR file or compile the source code, depending on your project structure.
```
RUN ./gradlew build
```
or
```
RUN mvn package
```
Expose any required ports.
```
EXPOSE 8080
```
Specify the command to run when the container starts.
```
CMD ["java", "-jar", "your-module.jar"]
```
Repeat these steps for each module in your multi-module Java project.

Building the Docker images

Once you have created the Dockerfile for each module, you can build the docker images by following these steps:

Open a terminal or command prompt.
Navigate to the root directory of your multi-module Java project.
Build the Docker image for each module using the docker build command.
```
docker build -t your-image-name:tag module-directory
```
Replace your-image-name with the desired name for your Docker image and tag with the version or tag you want to assign to the image. Replace module-directory with the directory of the module for which you are building the image.
Repeat the above command for each module in your multi-module Java project.

Conclusion

Containerizing multi-module Java projects using Docker offers several benefits, including simplified deployment and scalability. By building separate Docker images for each module, you can ensure encapsulation and maintainability for your project. Docker’s ease of use and flexibility make it an ideal choice for containerizing complex Java projects.

#Java #Docker