Jib infers wrong main class if multiple main classes exist

[chanseokoh]

We got a report that Jib uses the string ${start-class) when there are multiple public static void main() defined. I was able to reproduce this on a Spring Boot project. I think the string ${start-class) comes from Spring Boot.

$ cd examples/spring-boot
$ mvn help:effective-pom | grep start-class
                <mainClass>${start-class}</mainClass>
            <mainClass>${start-class}</mainClass>
                <mainClass>${start-class}</mainClass>
                    <mainClass>${start-class}</mainClass>
                <mainClass>${start-class}</mainClass>
            <mainClass>${start-class}</mainClass>
                  <mainClass>${start-class}</mainClass>
              <mainClass>${start-class}</mainClass>

We are supposed to fail and show an error if there are multiple main classes, but seems like this isn't working.

A workaround is to explicitly set the main class with <container><mainClass>.

UPDATE: found a related issue #170

@reechar-goog @saturnism

[chanseokoh]

[WARNING] 'mainClass' is not a valid Java class : ${start-class}
...
[INFO] Container entrypoint set to [java, -cp, /app/resources:/app/classes:/app/libs/*, ${start-class}]

[chanseokoh]

I don't know how exactly Spring Boot determines a main class, but having a single @SpringBootApplication does help. If there is no @SpringBootApplication or multiple @SpringBootApplication annotations, then Spring Boot shows an error:

[ERROR] Failed to execute goal org.springframework.boot:spring-boot-maven-plugin:2.1.6.RELEASE:run (default-cli) on project spring-boot-k8s-example: Execution default-cli of goal org.springframework.boot:spring-boot-maven-plugin:2.1.6.RELEASE:run failed: Unable to find a single main class from the following candidates [hello.Foo, hello.Application] -> [Help 1]

And there seem many other ways Spring Boot determines the main class.

For now, I think we should fix this bug and show an error that multiple main classes are detected.

[chanseokoh]

@reechar-goog @saturnism Jib 2.3.0 is released with the fix.

[saturnism]

Neat. Thank you!