How to Shrink Docker Images: Reduce a 743 MB Java Image to 536 MB

1. Introduction

We want Docker images as small as possible; this article shows how to reduce a simple Java image from 743 MB to 536 MB.

2. Example

Goal

Build a Java image based on a CentOS base and a downloaded jdk-8u101-linux-x64.tar.gz.

Process

1) First version

Dockerfile content:

FROM centos

COPY jdk-8u101-linux-x64.tar.gz /usr/local/
WORKDIR /usr/local
RUN tar -zxf /usr/local/jdk-8u101-linux-x64.tar.gz && rm /usr/local/jdk-8u101-linux-x64.tar.gz

ENV JAVA_HOME /usr/local/jdk1.8.0_101
ENV PATH $JAVA_HOME/bin:$PATH

Resulting image size: 743 MB . The problem is that the COPY and RUN are in separate layers, so the tar file remains in the image.

2) Second version

Use ADD to unpack directly:

FROM centos

ADD jdk-8u101-linux-x64.tar.gz /usr/local/

ENV JAVA_HOME /usr/local/jdk1.8.0_101
ENV PATH $JAVA_HOME/bin:$PATH

Image size: 562 MB . If the archive must be downloaded, combine download, extract, and delete in one RUN layer:

RUN wget http://xxx.com/app && tar -xzf app.tar.gz && rm app.tar.gz

3) Third version

Further reduce by removing unnecessary files with a .dockerignore file.

Dockerfile:

FROM centos

COPY jdk1.8.0_101 /usr/local/jdk1.8.0_101

ENV JAVA_HOME /usr/local/jdk1.8.0_101
ENV PATH $JAVA_HOME/bin:$PATH

.dockerignore: */*.zip Directory layout:

├── .dockerignore
├── Dockerfile
└── jdk-8u101-linux-x64.tar.gz

Final image size: 536 MB , a reduction of 207 MB . .dockerignore can also exclude docs, .git, .svn, tmp, etc.

3. Summary

The optimization principle is to put as little as possible into the image and choose the smallest suitable base image, such as Alpine (≈5 MB) instead of larger CentOS or Ubuntu images, or create custom base images tailored to each service.