Cannot use plugin tasks due to "org.gradle.api.tasks.bundling.Jar.getArchivePath()" because "jar" is null

I’ve a minimal Spring Boot application using Java 17 and I’m trying to get AppEngine Gradle plugin to work…

As documented I have set it up like:

settings.gradle:

pluginManagement {
    repositories {
        gradlePluginPortal()
        mavenCentral()
    }
    resolutionStrategy {
        eachPlugin {
            if (requested.id.id.startsWith("com.google.cloud.tools.appengine")) {
                useModule("com.google.cloud.tools:appengine-gradle-plugin:${requested.version}")
            }
        }
    }
}

build.gradle:

plugins {
	id "java"
	id "com.google.cloud.tools.appengine-appyaml" version "2.4.4"
	id "io.spring.dependency-management" version "1.0.12.RELEASE"
	id "org.springframework.boot" version "2.6.10"
}

group = "com.github.marceloverdijk"
version = "0.0.1-SNAPSHOT"
sourceCompatibility = "17"

ext {
	set("springCloudVersion", "2021.0.3")
	set("springCloudGcpVersion", "3.3.0")
}

configurations {
	compileOnly {
		extendsFrom annotationProcessor
	}
}

repositories {
	mavenCentral()
}

dependencyManagement {
	imports {
		mavenBom "com.google.cloud:spring-cloud-gcp-dependencies:${springCloudGcpVersion}"
		mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
	}
}

dependencies {
	// implementation "com.google.cloud:spring-cloud-gcp-starter"
	implementation "org.apache.commons:commons-lang3"
	implementation "org.springframework.boot:spring-boot-starter-actuator"
	implementation "org.springframework.boot:spring-boot-starter-jetty"
	implementation("org.springframework.boot:spring-boot-starter-web") {
		exclude module: "spring-boot-starter-tomcat"
	}
	annotationProcessor "org.springframework.boot:spring-boot-configuration-processor"
	developmentOnly "org.springframework.boot:spring-boot-devtools"
	testImplementation "org.springframework.boot:spring-boot-starter-test"
}

appengine {
	tools {
                // https://github.com/GoogleCloudPlatform/app-gradle-plugin/issues/431
		cloudSdkHome = "/usr/local/Caskroom/google-cloud-sdk/latest/google-cloud-sdk"
	}
	deploy {
		projectId = "GCLOUD_CONFIG"
		version = "GCLOUD_CONFIG"
		promote = true
		stopPreviousVersion = true
	}
}

tasks.named("test") {
	useJUnitPlatform()
}

but when running any GAE tasks I get:

❯ ./gradlew appengineShowConfiguration

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'aircraft-notifier'.
> Cannot invoke "org.gradle.api.tasks.bundling.Jar.getArchivePath()" because "jar" is null

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 587ms

Even the bootRun tasks now fails:

❯ ./gradlew bootRun

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'aircraft-notifier'.
> Cannot invoke "org.gradle.api.tasks.bundling.Jar.getArchivePath()" because "jar" is null

If I remove just the appengine { .. } configuration block from build.gradle (so GAE plugin itself still configured) the app starts up again with bootRun…

[elefeint]

Ah! Use this appengine configuration: https://github.com/GoogleCloudPlatform/app-gradle-plugin/issues/410#issuecomment-1207646519

[TWiStErRob]

If anyone is interested the reason why jar {} worked, is because it forced Gradle to eagerly create and configure (with nothing) the jar Task. This task is used by: https://github.com/GoogleCloudPlatform/app-gradle-plugin/blob/f50be8dcdde14ec6ef94947f1e21f5ea855be792/src/main/java/com/google/cloud/tools/gradle/appengine/appyaml/AppEngineAppYamlPlugin.java#L95-L96

Notice the getProperties().get("jar"), that triggers some magical Groovy path to resolve the property, but since Gradle is lazy, the task is not materialized yet as a property. If this plugin used tasks.named or tasks.getByName directly, then that would look in the right (Task) container and the plugin would not get null.