springboot Building docker image for multi-module project fails

E.g. try with start.spring.io:

$ mvn spring-boot:build-image
...
[INFO] Reactor Summary for start.spring.io 0.0.1-SNAPSHOT:
[INFO] 
[INFO] start.spring.io .................................... SUCCESS [  0.354 s]
[INFO] start.spring.io client ............................. FAILURE [01:05 min]
[INFO] start.spring.io website ............................ SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:06 min
[INFO] Finished at: 2020-05-29T16:57:49+01:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.3.0.RELEASE:build-image (default-cli) on project start-client: Execution default-cli of goal org.springframework.boot:spring-boot-maven-plugin:2.3.0.RELEASE:build-image failed: Unable to find main class -> [Help 1]
...

It's trying to build an image for the start-client project which isn't an app. Maybe the plugin should only try to build an image for a project that has the Spring Boot Maven Plugin configured by default, or something?

Comment From: snicoll

There is nothing we can do about that. You are asking maven to run a specific goal on the reactor and it does exactly that.

Maybe the plugin should only try to build an image for a project that has the Spring Boot Maven Plugin configured by default, or something?

That's what executions are for. You can enable the execution of such goal and use a regular phase (i.e. package).

Comment From: philwebb

@snicoll Can't we have some kind of intelligent 'enabled' property that does as Dave suggests? It feels a bit nasty that users can't just run spring-boot:build-image.

Comment From: philwebb

We just double checked things again and I was getting a little confused. I thought that the build-image goal required a repackaged jar but it actually doesn't so we shouldn't make the skipping too intelligent.

So for the original example, if you run mvn spring-boot:build-image from the root of the project you're asking the build-image goal to run on all of the children. It then fails for start-client because it's impossible to build an image for that module.

If you really want to be able to run the build-image goal from the root you'd have to update start-client with something like following:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <executions>
                <execution>
                    <id>skip-build-image</id>
                    <goals>
                        <goal>build-mage</goal>
                    </goals>
                    <configuration>
                        <skip>true</skip>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

You might also be able to use a spring-boot.build-image.skip property (but I'm not totally sure about that).

For the start.spring.io project, it's probably best to execute the goal only on the single project:

mvn spring-boot:build-image -pl start-site

Comment From: philwebb

Another option might be to configure the start-site module to bind the build-image goal to a maven lifecycle element such as package.