If you have tried to create a Docker image for a Java application, you know it tends to be large because:
- Official Java base Docker images are relatively large. For example, the
openjdk:12-alpinehas 339MB. - There are many classes that might not be used by your application, but they are bundled anyway by the JRE (for example, application servers that run in headless mode still bundle graphical packages such as
javax.swing). Even for a small application we have to maintain a complete JRE, which is simply a waste of memory.
In this repository we use the following approaches to have a smaller Docker image:
- Java modularity. Java 9 introduced a new level of packages abstraction known as modules, making possible to eliminate unnecessary modules that are part of the JDK as they are (most of them) decoupled.
- Create a custom JRE using jlink and jdeps. We want a JRE that contains only the platform modules that are required by the application.
- Multi-stage builds. Creating a custom JRE allows us to leverage smaller final images like alpine - a minimal Docker image based on Alpine Linux with a complete package index and only 5 MB in size!
For development purposes:
- Install Java JDK 9 or later
- Install Maven 3.x or later
For building and running containers:
- Install Docker
This repository contains Maven and Spring Boot samples, structured in the following way:
| Folder | Description |
|---|---|
| maven | The maven-app-v1 folder contains a Maven project using the default JRE, while the maven-app-v2 is similar but using a custom JRE. |
| spring-boot | The spring-boot-default folder contains a Spring Boot project using the default JRE, while the spring-boot-custom is similar but using a custom JRE. |
You can build all Docker images for Maven and Spring Boot applications to compare image sizes.
Follow the instructions described here to compare results of maven-app-v1 and maven-app-v2 projects.
The image size was reduced from 339MB to 53.9MB, representing ~84.1% reduction in the image size.
Follow the instructions described here to compare results of spring-boot-default and spring-boot-custom projects.
The image size was reduced from 358MB to 113MB, representing ~68.4% reduction in the image size.