Using JLink to create a custom and secure JVM

It is recommended not to use a plain JDK (or much better a JRE) at production stage. You get all the stuff you did not need and also some critical vulnerabilities. As there are often CVEs in the desktop/client parts of the JVM you can simple remove them.

This is a example how to create a custom JVM with jlink for a application build with maven. In this sample we are using spring boot, but this does not matter and works also with Quarkus or plain Java programs.

Extend Maven

Extend Maven with a plugin to get a list or all maven java dependencies. Use the maven-dependency-plugin in the build section:

<plugin>
    <artifactId>maven-dependency-plugin</artifactId>
    <version>3.6.1</version>
    <executions>
        <execution>
            <id>copy-dependencies</id>
            <phase>package</phase>
            <goals>
                <goal>copy-dependencies</goal>
            </goals>
            <configuration>
                <outputDirectory>${project.build.directory}/dependency</outputDirectory>
            </configuration>
        </execution>
    </executions>
</plugin>

This will copy all JARs in the given folder.

Determine Java modules and create JVM

With the tool jdeps it is possible to create a list of Java modules which are needed from the application:

jdeps --ignore-missing-deps -q --recursive --multi-release 17 --print-module-deps --class-path 'target/dependency/*' target/*.jar > target/deps.info

After getting a complete list of the used Java modules the tool jlink can create a custom JVM containing only the needed Java modules:

jlink  --add-modules "$(cat target/deps.info)" --strip-debug --compress 2 --no-header-files --no-man-pages --output target/jvm

After this the JVM is placed in target/jvm and can be used.

Important

The JVM is created for the system/architecture it is running for!

Additional Hints

Optimized build

The script buildJvm.sh contains all steps to create the custom JVM. It also uses the command strip to shrink the size of the created libraries to address a bug in some jlink implementations which still exists.

Docker multistage build

In Dockerfile is shown how to create a custom JVM in a multistage docker build (this should worked in all environments) and use a very small distroless image at runtime.