How to Shrink Docker Images from 16 MB to 5 KB with Spring Boot Layered Builds

Background

In containerized deployment we often encounter large Docker images and slow releases.

Three main factors affect image size:

Base image size – prefer Alpine to reduce OS packages.

Number of Dockerfile layers – merge commands when possible.

Application JAR size – focus of this article.

helloworld image

Start with a simple Spring Boot 2.3.0 web hello world and build an image.

FROM adoptopenjdk:11-jre-hotspot as builder
WORKDIR application
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} application.jar
ENTRYPOINT ["java", "-jar", "application.jar"]

docker build --build-arg JAR_FILE=./demo-layer-0.0.1-SNAPSHOT.jar . -t demo:v1.0

View image layer information

Use docker inspect demo:v1.0 to see layer digests.

// demo:v1.0 layer digests
"Layers": [
    "sha256:b7f7d2967507ba709dbd1dd0426a5b0cdbe1ff936c131f8958c8d0f910eea19e",
    "sha256:a6ebef4a95c345c844c2bf43ffda8e36dd6e053887dd6e283ad616dcc2376be6",
    "sha256:838a37a24627f72df512926fc846dd97c93781cf145690516e23335cc0c27794",
    "sha256:28ba7458d04b8551ff45d2e17dc2abb768bf6ed1a46bb262f26a24d21d8d7233",
    "sha256:55c91231ac46fdd63c3cf84b88b11f8a04c1870482dcff033029a601bc50e1ab",
    "sha256:9816c2d488754509f6024a267738b1e5fe33a7cd33bd25c5a9cdf6d4d7bfed1d",
    "sha256:f5fb3f91797d57a92f3f7e033398b8edd094df664db849a4950eabf2f5474535",
    "sha256:b87d2ff74819f83038ea2f89736a19cfcf99bfa080b8017d191c900a09a7524f"
]

helloworld upgrade and rebuild

Modify the hello world program, rebuild image.

docker build --build-arg JAR_FILE=./demo-layer-0.0.1-SNAPSHOT.jar . -t demo:v1.1

Inspect v1.1 layers with docker inspect demo:v1.1.

// demo:v1.1 layer digests
"Layers": [
    "sha256:b7f7d2967507ba709dbd1dd0426a5b0cdbe1ff936c131f8958c8d0f910eea19e",
    "sha256:a6ebef4a95c345c844c2bf43ffda8e36dd6e053887dd6e283ad616dcc2376be6",
    "sha256:838a37a24627f72df512926fc846dd97c93781cf145690516e23335cc0c27794",
    "sha256:28ba7458d04b8551ff45d2e17dc2abb768bf6ed1a46bb262f26a24d21d8d7233",
    "sha256:55c91231ac46fdd63c3cf84b88b11f8a04c1870482dcff033029a601bc50e1ab",
    "sha256:9816c2d488754509f6024a267738b1e5fe33a7cd33bd25c5a9cdf6d4d7bfed1d",
    "sha256:f5fb3f91797d57a92f3f7e033398b8edd094df664db849a4950eabf2f5474535",
    "sha256:c1b6350d545fea605e0605c4bfd7f4529cfeee3f6759750d6a5ddeb9c882fc8f"
]

Compare v1.0 and v1.1 images

Only the last layer changed; using dive demo:v1.0 shows the 16 MB JAR caused the rebuild.

Spring Boot default packaging

By default the Spring Boot JAR is a single layer containing both application classes and third‑party dependencies.

META-INF/
  MANIFEST.MF
org/
  springframework/
    boot/
      loader/
BOOT-INF/
  classes/
  lib/

Layered JAR

Splitting the JAR into separate layers for application classes and third‑party libraries allows only changed layers to be rebuilt, speeding up image creation.

Enable layered packaging

<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <configuration>
    <layers>
      <enabled>true</enabled>
    </layers>
  </configuration>
</plugin>

Write Dockerfile that uses layertools

Use Spring Boot’s layertools to extract layers and copy them separately.

FROM adoptopenjdk:11-jre-hotspot as builder
WORKDIR application
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} application.jar
RUN java -Djarmode=layertools -jar application.jar extract
FROM adoptopenjdk:11-jre-hotspot
WORKDIR application
COPY --from=builder application/dependencies/ ./
COPY --from=builder application/spring-boot-loader/ ./
COPY --from=builder application/snapshot-dependencies/ ./
COPY --from=builder application/application/ ./
ENTRYPOINT ["java", "org.springframework.boot.loader.JarLauncher"]

Build new image and view layers

docker build --build-arg JAR_FILE=./demo-layer-0.0.1-SNAPSHOT.jar . -t demo:v2.0

"Layers": [
    "sha256:b7f7d2967507ba709dbd1dd0426a5b0cdbe1ff936c131f8958c8d0f910eea19e",
    "sha256:a6ebef4a95c345c844c2bf43ffda8e36dd6e053887dd6e283ad616dcc2376be6",
    "sha256:838a37a24627f72df512926fc846dd97c93781cf145690516e23335cc0c27794",
    "sha256:28ba7458d04b8551ff45d2e17dc2abb768bf6ed1a46bb262f26a24d21d8d7233",
    "sha256:55c91231ac46fdd63c3cf84b88b11f8a04c1870482dcff033029a601bc50e1ab",
    "sha256:9816c2d488754509f6024a267738b1e5fe33a7cd33bd25c5a9cdf6d4d7bfed1d",
    "sha256:f5fb3f91797d57a92f3f7e033398b8edd094df664db849a4950eabf2f5474535",
    "sha256:06fe18cf8ae7384f120f2c6a3a33b31999dd0460cf1edae45e8f13adeab35942",
    "sha256:16cf814564b8a667fcc9f07314b6084cbef8dc8c0a6565c7a2d91d74faf7e7de",
    "sha256:94be40f716016b68cdd6b99d2cb8154acf8475c3a170a898a22f95a8ef40ffd3",
    "sha256:427d87d6a5fe6da13cb4233939c3a1ff920bc6b4d2f14b5d78af7aef98fda7de"
]

Modify business code and rebuild.

docker build --build-arg JAR_FILE=./demo-layer-0.0.1-SNAPSHOT.jar . -t demo:v2.1

"Layers": [
    "sha256:b7f7d2967507ba709dbd1dd0426a5b0cdbe1ff936c131f8958c8d0f910eea19e",
    "sha256:a6ebef4a95c345c844c2bf43ffda8e36dd6e053887dd6e283ad616dcc2376be6",
    "sha256:838a37a24627f72df512926fc846dd97c93781cf145690516e23335cc0c27794",
    "sha256:28ba7458d04b8551ff45d2e17dc2abb768bf6ed1a46bb262f26a24d21d8d7233",
    "sha256:55c91231ac46fdd63c3cf84b88b11f8a04c1870482dcff033029a601bc50e1ab",
    "sha256:9816c2d488754509f6024a267738b1e5fe33a7cd33bd25c5a9cdf6d4d7bfed1d",
    "sha256:f5fb3f91797d57a92f3f7e033398b8edd094df664db849a4950eabf2f5474535",
    "sha256:06fe18cf8ae7384f120f2c6a3a33b31999dd0460cf1edae45e8f13adeab35942",
    "sha256:16cf814564b8a667fcc9f07314b6084cbef8dc8c0a6565c7a2d91d74faf7e7de",
    "sha256:94be40f716016b68cdd6b99d2cb8154acf8475c3a170a898a22f95a8ef40ffd3",
    "sha256:8a20c60d361696a4e480fb6fbe1daf8b88bc54c579a98e209da1fb76e25de5aa"
]

Inspect changed layer

The last layer changed only 5 KB.

Summary

Image size reduced from 16 MB to 5 KB when only the JAR changes.

Spring Boot Maven plugin can enable layered builds; see official documentation.

Official docs: https://docs.spring.io/spring-boot/docs/2.3.0.RELEASE/maven-plugin/reference/html