Jib should handle the absence of source files gracefully

The behaviour of Jib and java plugins differs if a project does not have any source files. Cosnider the following difference.

Java plugin handles the absence of source files gracefully by skipping the compileJava task:

> Task :root-project:sub-project:compileJava NO-SOURCE
file or directory '<project_path>/src/main/java', not found
Skipping task ':root-project:sub-project:compileJava' as it has no source files and no previous output files.

Jib, on the other hand, terminates gradle execution:

warning: Could not find build output <project_path>/build/classes/java/main'
warning: No classes files were found - did you compile your project?

> Task :jibDockerBuild FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':jibDockerBuild'.
> Main class was not found, perhaps you should add a `mainClass` configuration to jib

Caused by: com.google.cloud.tools.jib.frontend.MainClassInferenceException: Main class was not found, perhaps you should add a `mainClass` configuration to jib
        at com.google.cloud.tools.jib.frontend.MainClassFinder.resolveMainClass(MainClassFinder.java:96)

It would be convenient to be able to acheive the same no-source handling behaviour in both plugins for a simpler configuration.

I would suggest to either hardcode the improvement or add a configuration option such as lenient=true/false, where in case of true the plugin would skip the task with no source files and in case of false Jib would terminate as it does now.

[chanseokoh]

PS. Jib-created images have timestamps of something like CREATED: 48 years ago or 292 years ago.

This is intended. Please see #578 and #413.

[loosebazooka]

Yeah, since maven is assuming we’re calling the goal on all modules, to avoid this you can use one of two options:

1. -pl which will only execute on a specific module.

$ mvn clean compile && mvn jib:build -pl <my-module-name>

1. Attaching goals to lifecycle phases in a profile [EDIT: this can be done without profiles]. Assuming we’re using both <modules> and the parent pom patterns. And lets assume we want the jib goal to execute in the package phase our lifecycle. pom.xml (in our jib module)

     <profiles>
      <profile>
        <id>jib</id>
        <build>
          <plugins>
            <plugin>
              <groupId>com.google.cloud.tools</groupId>
              <artifactId>jib-maven-plugin</artifactId>
              <executions>
                <execution>
                  <phase>package</phase>
                  <goals>
                    <goal>build</goal>
                  </goals>
                </execution>
              </executions>
            </plugin>
          </plugins>
        </build>
      </profile>
    </profiles>

to execute this with almost the same results as mvn clean compile jib:build

$ mvn clean package -DskipTests -Pjib

We’re skipping tests, because previously compile jib:build didn’t even activate the test phase.

I must also say that the pattern of the plugin enabling/disabling itself appears in the maven ecosystem. And while it may be strange, I can’t deny that people seem to want to use it. Let me know how this works for you, and if it doesn’t we can work on another solution that fits within the standard maven model.