Adding support for Kotlin sources to existing Java multi-project Gradle build

Question

I have a Spring Boot Java project that builds using Gradle (v6.2.2).

build.gradle.kts

plugins {
    base
    java
    id("org.springframework.boot") apply false
}

val gradleVersionProperty: String by project
val javaVersion: String by project
val springBootVersion: String by project
val springCloudStarterParentBomVersion: String by project

if (JavaVersion.current() != JavaVersion.VERSION_11) {
    throw GradleException("This build must be run with JDK 11")
} else {
    println("Building with JDK " + JavaVersion.current())
}

tasks.withType<Wrapper> {
    gradleVersion = gradleVersionProperty
    distributionType = Wrapper.DistributionType.ALL
}

allprojects {
    group = "com.meanwhile.in.hell"
    version = "$version"

    // Repos used in dependencyManagement section of settings.gradle
    repositories {
        mavenLocal()
        mavenCentral()
        maven("https://repo.spring.io/snapshot")
        maven("https://repo.spring.io/milestone")
    }
}

subprojects {

    if (!project.name.startsWith("platform")) {
        apply {
            plugin("java-library")
        }

        java.sourceCompatibility = JavaVersion.VERSION_11
        java.targetCompatibility = JavaVersion.VERSION_11

        // Change the default test logging settings
        tasks.withType<Test>() {
            useJUnitPlatform()
            testLogging {
                events = setOf(
                    org.gradle.api.tasks.testing.logging.TestLogEvent.FAILED,
                    org.gradle.api.tasks.testing.logging.TestLogEvent.PASSED,
                    org.gradle.api.tasks.testing.logging.TestLogEvent.SKIPPED
                )
                exceptionFormat = org.gradle.api.tasks.testing.logging.TestExceptionFormat.FULL
            }
            enableAssertions = false
            ignoreFailures = gradle.startParameter.isContinueOnFailure

            maxParallelForks =
                (Runtime.getRuntime().availableProcessors() / 2).takeIf { it > 0 } ?: 1
        }

        dependencies {
            "api"(enforcedPlatform("org.springframework.boot:spring-boot-dependencies:$springBootVersion"))
            "api"(enforcedPlatform("org.springframework.cloud:spring-cloud-dependencies:$springCloudStarterParentBomVersion"))

            "implementation"(enforcedPlatform(project(":platform-dependencies")))

            "testCompile"("org.springframework.boot:spring-boot-starter-test") 
        }
    }
}

However, I would like to add support for Spring Boot Kotlin sub-projects within it. I have used a very simple sample project from a Kotlin-only project I have that builds fine within it. Without any changes to my root build.gradle.kts file, my current build error is:

* What went wrong:
Execution failed for task ':kotlin-sample-project:bootJar'.
> Main class name has not been configured and it could not be resolved

I have not configured the main class for any of the Java sub-projects and neither have I in my Kotlin-only other project.

My build.gradle.kts in kotlin-sample-project is very simple:

plugins {
    id("org.springframework.boot")
}

dependencies {
    implementation("org.springframework.cloud:spring-cloud-starter-gateway")
}

And my main class looks like:

src/main/kotlin/sample/KotlinSampleApplication.kts

package com.meanwhile.in.hell.kotlinsample

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
open class KotlinSampleApplication

fun main(args: Array<String>) {
    runApplication<KotlinSampleApplication>(*args)
}

I have tried to add the kotlin plugin, but the build fails instantly not knowing what it is.

plugins {
    base
    java
    kotlin
}

Error:

Line 9:   kotlin
          ^ Unresolved reference. None of the following candidates is applicable because of receiver type mismatch:
              public val <T : Any> Class<TypeVariable(T)>.kotlin: KClass<TypeVariable(T)> defined in kotlin.jvm

madhead · Accepted Answer · 2020-05-26 19:03:46Z

2

I have tried to add the kotlin plugin, but the build fails instantly not knowing what it is.

It should be:

build.gradle.kts:

plugins {
    kotlin("jvm").version("1.3.72")
}

dependencies {
    implementation(kotlin("stdlib-jdk8"))
}

Bot Kotlin JVM plugin should be applied and Kotlin stdlib be present on compile classpath.

answered May 26, 2020 at 19:03

madhead

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2 Comments

MeanwhileInHell Over a year ago

Great, thanks a lot @madhead. I was wondering, is there a reason to use stdlib-jdk8 while using JDK 11? I see there is no stdlib-jdk9/10/11.

madhead Over a year ago

And that's the answer. stdlib-jdk8 is the highest version on Kotlin's standard library currently. They don't have anything "newer" yet. So, if you run Java 8+ – use it.

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