Spring Boot repackages the jar file for this project using a custom layout factory defined in the additional jar file, provided as a dependency to the build plugin:

<project>
  ...
  <build>
    ...
    <plugins>
      ...
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
        <executions>
          <execution>
            <id>repackage</id>
            <goals>
              <goal>repackage</goal>
            </goals>
            <configuration>
              <layoutFactory implementation="com.example.CustomLayoutFactory">
                <customProperty>value</customProperty>
              </layoutFactory>
            </configuration>
          </execution>
        </executions>
        <dependencies>
          <dependency>
            <groupId>com.example</groupId>
            <artifactId>custom-layout</artifactId>
            <version>0.0.1.BUILD-SNAPSHOT</version>
          </dependency>
        </dependencies>
        ...
      </plugin>
      ...
    </plugins>
    ...
  </build>
  ...
</project>

The layout factory is provided as an implementation of LayoutFactory (from spring-boot-loader-tools) explicitly specified in the pom. If there is only one custom LayoutFactory on the plugin classpath and it is listed in META-INF/spring.factories then it is unnecessary to explicitly set it in the plugin configuration.

Layout factories are always ignored if an explicit layout is set.