How to Slim Down SpringBoot JARs: Remove Unused Libraries for Faster Deployments

Introduction

Deploying a SpringBoot application is straightforward, but when the service runs on a public cloud the generated JAR can become huge because it bundles all external dependencies (e.g., Spring Cloud). Adjusting the running application then becomes painful.

Jar size before slimming

In a typical build the JAR may be around 18 MB, with the BOOT-INF/lib directory consuming almost the entire size.

Solution

Step 1: Build the normal JAR and extract the lib folder

Use the standard Maven build and then unzip the resulting JAR to obtain the BOOT-INF/lib directory.

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <mainClass>com.johnnian.App</mainClass>
                <layout>ZIP</layout>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Run mvn clean install, then copy the BOOT-INF/lib folder to the target directory.

Step 2: Modify pom.xml to produce a JAR without the embedded libraries

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <mainClass>com.johnnian.App</mainClass>
                <layout>ZIP</layout>
                <includes>
                    <include>
                        <groupId>nothing</groupId>
                        <artifactId>nothing</artifactId>
                    </include>
                </includes>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>repackage</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

After updating the configuration, run mvn clean install again. The new JAR is much smaller because external JARs are no longer packaged.

Step 3: Run the slim JAR with external libraries

Place the previously extracted lib folder and the newly built JAR in the same directory, then start the application with:

java -Dloader.path=/path/to/lib -jar /path/to/springboot-jsp-0.0.1-SNAPSHOT.jar

Alternatively, you can copy the required dependencies using Maven:

mvn dependency:copy-dependencies -DoutputDirectory=F:\ideaWorkPlace\AnalysisEngine\lib -DincludeScope=runtime

Replace /path/to with the actual path of your lib directory. The final directory layout looks like:

├── lib   # external libraries
└── springboot-jsp-0.0.1-SNAPSHOT.jar

Conclusion

Once the project’s architecture is fixed, the set of external JARs rarely changes; most future changes are in business logic. By rebuilding only the lightweight JAR, deployment becomes faster and more efficient.