UPX IMAGE

3 min readSep 10, 2021

Do you want smaller images with a faster startup time? Use UPX to compress your executable inside your image. Use a UPX image!

For more information on the internals of UPX take a look at their website here. Or at the source code of UPX here.

We will use an existing project from a previous post on native images. We will take this small image and make it even smaller.

Starting image

The project contains a maven build that produces an executable jar. In the Dockerfile we create a native image from this jar and copy it into the image.

# Copy the shaded application jar file to the build directory
COPY target/grpc-cat-api-*.jar /home/build/# Create the native image executable
RUN native-image -jar grpc-cat-api-*.jar && mv grpc-cat-api api

From the previous post we saw that we have an image of 77.2MB with a startup time of 27ms.

UPX

To use UPX we first have to install it and compress the executable after it is copied into the image.

# Install UPX
RUN microdnf update -y oraclelinux-release-el8 && microdnf --enablerepo ol8_codeready_builder install xz && microdnf clean all && \
    curl --location --output upx-3.96-amd64_linux.tar.xz "https://github.com/upx/upx/releases/download/v3.96/upx-3.96-amd64_linux.tar.xz" && \
    tar -xJf upx-3.96-amd64_linux.tar.xz && \
    cp upx-3.96-amd64_linux/upx /bin/# Copy the shaded application jar file to the build directory
COPY target/grpc-cat-api-*.jar /home/build/# Create the native image executable
RUN native-image -jar grpc-cat-api-*.jar && mv grpc-cat-api api# Compress executable with UPX
RUN upx --best --lzma api

Notice that we install the dependencies, download UPX and extract it. This is just in the building image and will not effect the final image.