Spring Boot Layered jar: Extract layers.xml to separate dependency

I'm starting to catch up on the capabilities of the repackage goal in spring-boot-maven-plugin.It looks promising, but I need to fine-tune it a little.

I can easily do that by creating a layers.xml file somewhere in my project, but the problem is that I don't only have 1 project, but rather half a dozen. All of the projects need the same kind of layering, but I don't really want to copy the same configuration for every project I want to use it on.

A nice-looking solution would be to extract that configuration file into a separate jar for example and have the plugin take the config file from there, but I see no way of doing it. Is there any other solution that doesn't involve me copying the configuration file to every project I have?

Unfortunately, even though the projects I have use the same parent, but are not in the same multi-module project.

I managed to work out a solution.

Before asking the question my spring-boot-mave-plugin config looked something like this:

            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <layers>
                        <enabled>true</enabled>
                    </layers>
                </configuration>
                <executions>
                    <execution>
                        <id>repackage</id>
                        <goals>
                            <goal>repackage</goal>
                        </goals>
                        <phase>package</phase>
                        <configuration>
                            <layers>
                                <enabled>true</enabled>
                                <configuration><!-- something like classpath:layers.xml --></configuration>
                            </layers>
                            <classifier>exec</classifier>
                        </configuration>
                    </execution>
                </executions>
                <dependencies>
                    <dependency>
                        <groupId>artifact</groupId>
                        <artifactId>with-layers.xml</artifactId>
                        <version>...</version>
                    </dependency>
                </dependencies>
            </plugin>

The solution becomes one step more convoluted, by bringing in the maven-dependency-plugin, which downloads the before mentioned dependency and unpacks it in the build folder with this configuration:

            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <version>3.1.1</version>
                <executions>
                    <execution>
                        <id>copy-shc-build-tools</id>
                        <goals>
                            <goal>unpack</goal>
                        </goals>
                        <phase>package</phase>
                        <configuration>
                            <artifactItems>
                                <artifactItem>
                                    <groupId>artifact</groupId>
                                    <artifactId>with-layers.xml</artifactId>
                                    <version>...</version>
                                    <type>jar</type>
                                    <outputDirectory>${project.build.directory}</outputDirectory>
                                    <includes>**/layers.xml</includes>
                                </artifactItem>
                            </artifactItems>
                        </configuration>
                    </execution>
                </executions>
                <configuration>
                    <overWriteReleases>false</overWriteReleases>
                    <overWriteSnapshots>true</overWriteSnapshots>
                </configuration>
            </plugin>

In turn, the line <configuration><!-- something like classpath:layers.xml --></configuration> becomes <configuration>${project.build.directory}/layers/layers.xml</configuration>.

The documentation for Spring Boot Maven Plugin states you can set the path to the layers.xml manually, so why not have all pom.xml point to the same location?

<project>
	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
				<version>2.3.2.RELEASE</version>
				<configuration>
					<layers>
						<enabled>true</enabled>
						<configuration>${project.basedir}/../layers.xml</configuration>
					</layers>
				</configuration>
			</plugin>
		</plugins>
	</build>
</project>

The /../ means one level higher from the project directory. So say you have a bunch of projects in one directory, put the layers.xml there and it'll work.

Another approach could be to reuse the Maven Plugin declaration by moving it to a so-called parent POM. This is a technique where the common/shared parts of the POM files of a series of projects is moved to a single POM file (the parent POM). Here's an example