Sample of how to configure Spring Boot's layered jar support for multi-module build that includes your own libs

spring-boot-snippet.gradle

subprojects { subproject ->
    // ...
    
    plugins.withId('org.springframework.boot') {
        springBoot {
            bootJar {
                layered {
                    // application follows Boot's defaults
                    application {
                        intoLayer("spring-boot-loader") {
                            include "org/springframework/boot/loader/**"
                        }
                        intoLayer("application")
                    }
                    // for dependencies we ensure that our own libraries are put into a separate layer
                    dependencies {
                        intoLayer("libs-dependencies") {
                            include "nl.trifork:*:*"
                        }
                        intoLayer("dependencies")
                    }
                    layerOrder = ["dependencies", "spring-boot-loader", "libs-dependencies", "application"]
                }
            }
            buildInfo {
                properties {
                    // ensure builds result in the same artifact when nothing's changed:
                    time = null
                }
            }
        }
    }
}

Do you have any idea if this is possible with maven?

The Maven plugin provides the same features, but using an external XML config file instead of internal DSL: https://docs.spring.io/spring-boot/docs/2.3.1.RELEASE/maven-plugin/reference/html/#repackage-layers