How to Shrink Spring Boot Fat JARs from Hundreds MB to a Few KB

Overview

With Spring Boot, a single java -jar command can launch an application, but as projects grow the generated fat JAR often reaches 200‑300 MB, and a micro‑service architecture can easily exceed 1‑2 GB for the whole system. Deploying such large files slows down updates, especially for urgent bug fixes.

Level 0 – Regular Fat JAR Build

Project directory:

package-optimize-level0

<build>
    <finalName>${project.artifactId}</finalName>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

Build and run:

cd package-optimize-level0
mvn clean install
ls -lh package-optimize-app1/target/package-optimize-app1.jar
java -jar package-optimize-app1/target/package-optimize-app1.jar

The resulting JAR is about 16 MB for a minimal example, but real projects often produce tens or hundreds of megabytes.

Level 1 – Separate Dependencies into lib Directory

Project directory:

package-optimize-level1

<build>
    <finalName>${project.artifactId}</finalName>
    <plugins>
        <!-- copy all dependencies to lib/ -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <executions>
                <execution>
                    <id>copy-dependencies</id>
                    <phase>package</phase>
                    <goals><goal>copy-dependencies</goal></goals>
                    <configuration>
                        <outputDirectory>${project.build.directory}/lib</outputDirectory>
                        <excludeTransitive>false</excludeTransitive>
                        <stripVersion>false</stripVersion>
                        <silent>true</silent>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <!-- Spring Boot repackage without any dependencies inside the JAR -->
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <includes>
                    <include>null:null</include>
                </includes>
                <layout>ZIP</layout>
            </configuration>
            <executions>
                <execution>
                    <goals><goal>repackage</goal></goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Build and run:

cd package-optimize-level1
mvn clean install
ls -lh package-optimize-app1/target/package-optimize-app1.jar   # 149K
java -jar -Djava.ext.dirs=lib package-optimize-app1/target/package-optimize-app1.jar

The business JAR is now only a few hundred kilobytes, while all dependencies reside in lib/.

Level 2 – Merge All Services' Dependencies into a Single lib Directory

Project directory:

package-optimize-level2

<build>
    <finalName>${project.artifactId}</finalName>
    <plugins>
        <!-- add Class‑Path entry to MANIFEST -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <useUniqueVersions>false</useUniqueVersions>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
        <!-- copy all dependencies to a shared lib/ -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <executions>
                <execution>
                    <id>copy-dependencies</id>
                    <phase>package</phase>
                    <goals><goal>copy-dependencies</goal></goals>
                    <configuration>
                        <outputDirectory>${boot-jar-output}/lib</outputDirectory>
                        <excludeTransitive>false</excludeTransitive>
                        <stripVersion>false</stripVersion>
                        <silent>false</silent>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <!-- Spring Boot repackage without embedded dependencies -->
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <includes><include>null:null</include></includes>
                <layout>ZIP</layout>
            </configuration>
            <executions>
                <execution><goals><goal>repackage</goal></goals></execution>
            </executions>
        </plugin>
    </plugins>
</build>

Result:

No need for -Djava.ext.dirs=lib at runtime.

All services share a common lib/ directory; total deployment size is a few hundred megabytes instead of gigabytes.

Each service’s MANIFEST contains a precise Class-Path to avoid version conflicts.

Level 3 – Handle Non‑Official Third‑Party JARs (system scope)

Project directory:

package-optimize-level3

<build>
    <finalName>${project.artifactId}</finalName>
    <plugins>
        <!-- add custom Class‑Path entries for system‑scope JARs -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <useUniqueVersions>false</useUniqueVersions>
                    </manifest>
                    <manifestEntries>
                        <Class-Path>${jar-manifestEntries-classpath}</Class-Path>
                    </manifestEntries>
                </archive>
            </configuration>
        </plugin>
        <!-- copy all dependencies (including system JARs) to lib/ -->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <executions>
                <execution>
                    <id>copy-dependencies</id>
                    <phase>package</phase>
                    <goals><goal>copy-dependencies</goal></goals>
                    <configuration>
                        <outputDirectory>${boot-jar-output}/lib</outputDirectory>
                        <excludeTransitive>false</excludeTransitive>
                        <stripVersion>false</stripVersion>
                        <silent>false</silent>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <!-- Spring Boot repackage -->
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <includes><include>null:null</include></includes>
                <layout>ZIP</layout>
            </configuration>
            <executions>
                <execution><goals><goal>repackage</goal></goals></execution>
            </executions>
        </plugin>
    </plugins>
</build>

<properties>
    <boot-jar-output>../devops</boot-jar-output>
    <jar-manifestEntries-classpath>. lib/hik-sdk-1.0.0.jar</jar-manifestEntries-classpath>
</properties>

<dependencies>
    <dependency>
        <groupId>com.hik</groupId>
        <artifactId>hik-sdk</artifactId>
        <version>1.0.0</version>
        <scope>system</scope>
        <systemPath>${project.basedir}/lib/hik-sdk-1.0.0.jar</systemPath>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

Resulting MANIFEST Class-Path includes the custom SDK JAR followed by all Spring Boot dependencies, ensuring the system‑scope JAR is available at runtime.

Final Outcome

All services share a common lib/ directory; total size is only a few hundred megabytes, dramatically speeding up first‑time deployment.

Each micro‑service JAR is reduced to a few hundred kilobytes, allowing near‑instant updates for urgent bug fixes.

Explicit Class-Path entries prevent version‑conflict issues between services.

Non‑official third‑party SDKs can be packaged and loaded without Maven repository support.

Special Tip

If a Maven dependency version changes, remember to copy the updated JAR into the shared lib/ directory. Storing the deployment resources in a Git repository makes it easy to track which files have changed between releases, minimizing the amount of data that needs to be transferred.