How to make Spring Boot Docker Image smaller

wrap up

--Custom JRE and multi-stage builds can be used to reduce Spring Boot Docker Images. --The following is a summary of the results. You can see that it is less than 1/5 the capacity of the official AdoptOpenJDK Docker Image.

AdoptOpenJDK AdoptOpenJDK-Alpine Custom Runtime
436 MB 358 MB 85.5 MB

Sample code

--The sample code explained this time is stored below. The following explanation will be given based on this sample code.

Step ① Get the required module with jdeps

Use the jdeps command to find out which modules the Spring Boot application depends on.

However, when I check the executable jar file of SpringBoot with jdeps, only java.base and java.logging are output, and this JRE alone does not start SpringBoot. Therefore, use the Shell Script introduced at the following site to get the dependent modules.

-I want to narrow down the docker image of Spring Boot to the minimum necessary (September 2019 version)

# jdeps-spring-boot

set -eu

readonly TARGET_JAR=$1
readonly TARGET_VER=$2

#Directory to extract the jar
readonly TMP_DIR="/tmp/app-jar"
mkdir -p ${TMP_DIR}
trap 'rm -rf ${TMP_DIR}' EXIT

#Extract the jar
unzip -q "${TARGET_JAR}" -d "${TMP_DIR}"

jdeps \
    -classpath \'${TMP_DIR}/BOOT-INF/lib/*:${TMP_DIR}/BOOT-INF/classes:${TMP_DIR}\' \
    --print-module-deps \
    --ignore-missing-deps \
    --module-path ${TMP_DIR}/BOOT-INF/lib/javax.activation-api-1.2.0.jar \
    --recursive \
    --multi-release ${TARGET_VER} \
    -quiet \
    ${TMP_DIR}/org ${TMP_DIR}/BOOT-INF/classes ${TMP_DIR}/BOOT-INF/lib/*.jar

When the file name of Shell Script is, execute as follows.

./ <SpringBoot jar> <Java Version>

Execution example:

./ demo-0.0.1-SNAPSHOT.jar 11

The modules required for the execution result are output.


important point

When you run ./, run it on Java 12 or higher version.

This is because Java 11 has a bug that causes a NullPointerException when parsing a non-existent class when parsing To avoid this, you need to specify --ignore-missing-deps, which was added from Java 12.

--Reference: [Getting a NPE on Java 11 jdeps tool when scanning Spring Boot .jar files]( -scanning-spring-boot-jar-files)

Step ② Create a custom JRE

--Prepare the following Dockerfile. --In jlink --add-modules, describe the modules output in step ①.

FROM adoptopenjdk/openjdk11:alpine AS java-build
WORKDIR /jlink
RUN jlink --strip-debug --no-header-files --no-man-pages --compress=2 --module-path $JAVA_HOME \
    --add-modules java.base,java.desktop,java.instrument,,java.naming,java.prefs,java.scripting,,java.sql,jdk.httpserver,jdk.unsupported \
    --output jre-min

Step ③ Start SpringBoot on the custom JRE using multi-stage build

--Add the Dockerfile prepared in step ②.

--Added the settings required to start Java on alpine linux by referring to the following site. -Creating a lightweight Java environment that runs on Docker

FROM adoptopenjdk/openjdk11:alpine AS java-build
WORKDIR /jlink
RUN jlink --strip-debug --no-header-files --no-man-pages --compress=2 --module-path $JAVA_HOME \
    --add-modules java.base,java.desktop,java.instrument,,java.naming,java.prefs,java.scripting,,java.sql,jdk.httpserver,jdk.unsupported \
    --output jre-min

FROM alpine:3.12.0
USER root

RUN apk --update add --no-cache ca-certificates curl openssl binutils xz \
    && GLIBC_VER="2.28-r0" \
    && GCC_LIBS_URL="" \
    && GCC_LIBS_SHA256=e4b39fb1f5957c5aab5c2ce0c46e03d30426f3b94b9992b009d417ff2d56af4d \
    && ZLIB_URL="" \
    && ZLIB_SHA256=bb0959c08c1735de27abf01440a6f8a17c5c51e61c3b4c707e988c906d3b7f67 \
    && curl -Ls -o /etc/apk/keys/ \
    && curl -Ls ${ALPINE_GLIBC_REPO}/${GLIBC_VER}/glibc-${GLIBC_VER}.apk > /tmp/${GLIBC_VER}.apk \
    && apk add /tmp/${GLIBC_VER}.apk \
    && curl -Ls ${GCC_LIBS_URL} -o /tmp/gcc-libs.tar.xz \
    && echo "${GCC_LIBS_SHA256}  /tmp/gcc-libs.tar.xz" | sha256sum -c - \
    && mkdir /tmp/gcc \
    && tar -xf /tmp/gcc-libs.tar.xz -C /tmp/gcc \
    && mv /tmp/gcc/usr/lib/libgcc* /tmp/gcc/usr/lib/libstdc++* /usr/glibc-compat/lib \
    && strip /usr/glibc-compat/lib/* /usr/glibc-compat/lib/* \
    && curl -Ls ${ZLIB_URL} -o /tmp/libz.tar.xz \
    && echo "${ZLIB_SHA256}  /tmp/libz.tar.xz" | sha256sum -c - \
    && mkdir /tmp/libz \
    && tar -xf /tmp/libz.tar.xz -C /tmp/libz \
    && mv /tmp/libz/usr/lib/* /usr/glibc-compat/lib \
    && apk del binutils \
    && rm -rf /tmp/${GLIBC_VER}.apk /tmp/gcc /tmp/gcc-libs.tar.xz /tmp/libz /tmp/libz.tar.xz /var/cache/apk/*

COPY --from=java-build /jlink/jre-min /opt/jre-min
COPY ./demo-0.0.1-SNAPSHOT.jar /opt/demo/demo-0.0.1-SNAPSHOT.jar
ENV PATH /opt/jre-min/bin:$PATH

CMD ["java", "-jar", "/opt/demo/demo-0.0.1-SNAPSHOT.jar"]


--Store the SpringBoot jar file in the directory containing the Dockerfile created in step ③, and execute the following command.

docker build -t demo-official-openjdk-custom-runtime:latest .

--Confirm that the image is created

docker images

REPOSITORY                             TAG                 IMAGE ID            CREATED             SIZE
demo-official-openjdk-custom-runtime   latest              49049142374f        About an hour ago   85.5MB

--Confirm that it starts with the following command.

docker run -d -p 8080:8080 --name demo-official-openjdk-custom-runtime demo-official-openjdk-custom-runtime:latest

Other references

-OpenJDK11 dokcer image (1GB) is large, so create a small image (85MB) with alpine linux + jlink

-What is the Docker image of OpenJDK?

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