`Implementation-Version` not in Manifest after `spring-boot-maven-plugin` repackage

Question

Given this plugin config in a Maven pom.xml:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <mainClass>acme.Main</mainClass>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Why would MANIFEST.MF be missing Implementation-Version and Implementation-Title, given that all docs that I can get my hands on, either don't mention it, or imply its creation is a default?

The complete generated Manifest is:

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven 3.8.6
Built-By: stewart
Build-Jdk: 17.0.5
Main-Class: org.springframework.boot.loader.JarLauncher
Start-Class: acme.Main
Spring-Boot-Version: 2.7.5
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx
Spring-Boot-Layers-Index: BOOT-INF/layers.idx

Answer 1

Apparently, behind the scenes, spring-boot-maven-plugin still relies on the Manifest created by maven-jar-plugin, because that's the base .jar that is being repackaged.

Having established that, we then find that

Starting with version 2.1, Maven Archiver no longer creates the Implementation and Specification details in the manifest by default. If you want them in your manifest you have to say so explicitly in your configuration.

Thus, what is needed is to add the following to the pom.xml:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <version>3.3.0</version>
    <configuration>
        <archive>
            <manifest>
                <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
            </manifest>
        </archive>
    </configuration>
</plugin>