How to add new sourceset with gradle kotlin-dsl

The answer of @s1m0nw1 is correct to add a new sourceset. But to just add a new source-folder in an existing sourceset, this can be used:

java.sourceSets["main"].java {
    srcDir("src/gen/java")
}

You should try the following:

java.sourceSets.create("src/gen/java")

Hope it's what you need!

Worked for me on Gradle 4.10.2:

sourceSets.create("integrationTest") {
    java.srcDir("src/integrationTest/java")
    java.srcDir("build/generated/source/apt/integrationTest")
    resources.srcDir("src/integrationTest/resources")
}

Worked for me on Gradle 4.10.2:

sourceSets.getByName("main") {
    java.srcDir("src/main/java")
    java.srcDir("src/main/kotlin")
}
sourceSets.getByName("test") {
    java.srcDir("src/test/java")
    java.srcDir("src/test/kotlin")
}

The codes above can also be used in subprojects block.

I wanted to add a source set with the name "test-integration" and the source directory src/test-integration/kotlin. I was able to accomplish that by combining the two pre-existing answers:

java.sourceSets.create("test-integration").java {
    srcDir("src/test-integration/kotlin")
}

kotlin-dsl

sourceSets {
        this.getByName("androidTest"){
            //Adds the given source directory to this set.
            this.java.srcDir("src/mock/java")
        }
        this.getByName("test"){
            this.java.srcDir("src/mock/java")
        }
    }

As of Gradle 7.5:

sourceSets {
    main {
        java.sourceSets {
            create("gen"){
                java.srcDir("src/gen/java")
            }
        }
    }
}

This is what I had before:

main.kotlin.srcDirs = main.java.srcDirs = ['src']
test.kotlin.srcDirs = test.java.srcDirs = ['test']
main.resources.srcDirs = ['resources']
test.resources.srcDirs = ['testresources']

Above now translates to:

sourceSets {
main {
    java {
        srcDirs("src")
    }
    resources {
        srcDirs("resources")
    }
}
test {
    java {
        srcDirs("test")
    }
    resources {
        srcDirs("testresources")
    }
}}