How to configure bootBuildImage task to be up-to-date when no changes to source code

Question

I'm trying to use spring boot 2.3's new support for creating docker images, but the bootBuildImage gradle task is never up-to-date. This unfortunately causes a new docker image to be generated even if no source code was changed.

My goal is to have a static build command that doesn't result in new images being produced unnecessarily. So something like one of the two scenarios below:

./gradlew bootBuildImage (but does nothing if no source code has changed)

OR

./gradlew someOtherTask (if this task is not up-to-date, it triggers bootBuildImage)

My latest effort was to configure bootBuildImage to only run if the bootJar task is not up to date:

tasks {
    val bootJarTask: TaskProvider<BootJar> = this.bootJar
    bootBuildImage {
        outputs.upToDateWhen {
            bootJarTask.get().state.upToDate
        }
    }
}

But this fails with this error (for some reason this particular task hates jars as inputs)

> Unable to store input properties for task ':bootBuildImage'. Property 'jar' with value '/demo/build/libs/demo-0.0.1-SNAPSHOT.jar' cannot be serialized.

Surely I'm missing something obvious here! The reason I need bootBuildImage to only produce an image when necessary is because I've got a multi-project build. I don't want subprojects to generate and push a new image even when nothing in them changed.

Using Spring Boot 2.3.4, Gradle 6.6.1, Java 11.

Answer 1

This seems to work:

    val bootJarTask: TaskProvider<BootJar> = this.bootJar
    
    bootBuildImage {
        onlyIf {
            !bootJarTask.get().state.skipped
        }
    }