How to build sources JAR with Gradle?

Solution as of Gradle 6.0

Assuming that you use the java/java-library plugin with Gradle 6.0 or later, you can get a sourcesJar task using the following configuration:

java {
    withSourcesJar()
    // and/or analogously use “withJavadocJar()” to get a “javadocJar” task
}

If you additionally use the maven-publish/ivy-publish plugin (recommended nowadays), then this will also publish a *-sources.jar artifact along with your main Java publication.

See also the Gradle docs.

When using:

• Gradle: 5+
• java and maven-publish plugins

task sourceJar(type: Jar) {
    from sourceSets.main.allJava
    archiveClassifier = "sources"
}

publishing {
    publications {
        maven(MavenPublication) {
            from components.java
            artifact sourceJar
        }
    }
}

https://docs.gradle.org/current/dsl/org.gradle.api.publish.maven.MavenPublication.html

This is how I included Dokka (view it online) and sources JARs for my Android Kotlin library using Kotlin DSL (build.gradle.kts):

plugins {
    // ...
    id("org.jetbrains.dokka") version "1.4.32"
    id("maven-publish")
}

lateinit var sourcesArtifact: PublishArtifact
lateinit var javadocArtifact: PublishArtifact

tasks {
    val sourcesJar by creating(Jar::class) {
        archiveClassifier.set("sources")
        from(android.sourceSets["main"].java.srcDirs)
    }
    val dokkaHtml by getting(org.jetbrains.dokka.gradle.DokkaTask::class)
    val javadocJar by creating(Jar::class) {
        dependsOn(dokkaHtml)
        archiveClassifier.set("javadoc")
        from(dokkaHtml.outputDirectory)
    }
    artifacts {
        sourcesArtifact = archives(sourcesJar)
        javadocArtifact = archives(javadocJar)
    }
}

publishing {
    // ...
    publications {
        create<MavenPublication>("MyPublication") {
            from(components["release"])
            artifact(sourcesArtifact)
            artifact(javadocArtifact)
            // ...
        }
    }
}

Android:

  task androidSourcesJar(type: Jar) {
        getArchiveClassifier().set('sources')
        from android.sourceSets.main.java.srcDirs//full sources
  }

Java:

 task sourcesJar(type: Jar, dependsOn: classes) {
        getArchiveClassifier().set('sources')
        from sourceSets.main.allSource
    }