After constructing a dependency graph, Gradle performs artifact resolution, mapping the resolved graph to a set of artifacts that will be downloaded during the build.

Artifacts

An artifact is a file that is produced or consumed during the build process. Artifacts are typically files such as compiled libraries, JAR files, AAR files, DLLs, or ZIPs.

Let’s look at the metadata for org.jetbrains.kotlin:kotlin-stdlib:1.8.10 which showcases several variants and artifacts:

kotlin-stdlib-1.8.10.module
{
  "variants": [
    {
      "name": "apiElements",
      "attributes": {
        "org.gradle.category": "library",
        "org.gradle.dependency.bundling": "external",
        "org.gradle.jvm.version": "8",
        "org.gradle.libraryelements": "jar",
        "org.gradle.usage": "java-api"
      },
      "files": [
        {
          "name": "kotlin-stdlib-1.8.10-public.jar"
        },
        {
          "name": "kotlin-stdlib-1.8.10-private.jar"
        }
      ]
    },
    {
      "name": "runtimeElements",
      "attributes": {
        "org.gradle.category": "library",
        "org.gradle.dependency.bundling": "external",
        "org.gradle.jvm.version": "8",
        "org.gradle.libraryelements": "jar",
        "org.gradle.usage": "java-runtime"
      },
      "files": [
        {
          "name": "kotlin-stdlib-1.8.10.jar"
        }
      ]
    },
    {
      "name": "jdk7ApiElements",
      "attributes": {
        "org.gradle.usage": "java-api",
        "org.gradle.jvm.version": "7"
      },
      "files": [
        {
          "name": "kotlin-stdlib-jdk7-1.8.10.jar"
        }
      ]
    },
    {
      "name": "jdk8ApiElements",
      "attributes": {
        "org.gradle.usage": "java-api",
        "org.gradle.jvm.version": "8"
      },
      "files": [
        {
          "name": "kotlin-stdlib-jdk8-1.8.10.jar"
        }
      ]
    }
  ]
}

As we can see there are a number of artifacts available in the metadata:

Variant Artifact(s) Purpose

apiElements

kotlin-stdlib-1.8.10-public.jar, kotlin-stdlib-1.8.10-private.jar

Standard Kotlin runtime (default).

runtimeElements

kotlin-stdlib-jdk7-1.8.10.jar

Provides additional APIs for Java 7 compatibility.

jdk7ApiElements

kotlin-stdlib-jdk8-1.8.10.jar

Provides additional APIs for Java 8 compatibility.

jdk8ApiElements

kotlin-stdlib-common-1.8.10.jar

Shared standard library for Kotlin Multiplatform.

Typically, once a variant is selected, its associated artifacts can be automatically resolved. However, there are specific reasons why artifact selection still happens after variant selection.

For example, what if the metadata for org.jetbrains.kotlin:kotlin-stdlib:1.8.10 looked like this:

kotlin-stdlib-1.8.10.module
{
  "variants": [
    {
      "name": "runtimeElements",
      "attributes": {
        "org.gradle.category": "library",
        "org.gradle.dependency.bundling": "external",
        "org.gradle.jvm.version": "8",
        "org.gradle.libraryelements": "jar",
        "org.gradle.usage": "java-runtime"
      },
      "files": [
        {
          "name": "kotlin-stdlib-1.8.10.jar"
        }
      ]
    },
    {
      "name": "runtimeElements",
      "attributes": {
        "org.gradle.category": "library",
        "org.gradle.dependency.bundling": "external",
        "org.gradle.jvm.version": "8",
        "org.gradle.libraryelements": "classes",
        "org.gradle.usage": "java-runtime"
      },
      "files": [
        {
          "name": "build/classes/java/main/"
        }
      ]
    },
    {
      "name": "runtimeElements",
      "attributes": {
        "org.gradle.category": "library",
        "org.gradle.dependency.bundling": "external",
        "org.gradle.jvm.version": "8",
        "org.gradle.libraryelements": "resources",
        "org.gradle.usage": "java-runtime"
      },
      "files": [
        {
          "name": "build/resources/main/"
        }
      ]
    }
  ]
}

How would Gradle know which file to download?

Artifact Sets

An artifact set is a group of artifacts that belong to a single variant of a module.

A single variant can include multiple artifact sets, each serving a different purpose.

Let’s look at the org.jetbrains.kotlin:kotlin-stdlib:1.8.10 example:

Variant Set Artifacts in the Set

apiElements

1

jar → The packaged library JAR (kotlin-stdlib-1.8.10.jar)

2

tar → The packaged library TAR (kotlin-stdlib-1.8.10.tar)

runtimeElements

1

jar → The packaged library JAR (kotlin-stdlib-1.8.10.jar)

2

classes → The compiled .class files (build/classes/java/main/)

3

resources → The associated resource files (build/resources/main/)

jdk8ApiElements

1

jar → The packaged library JAR (kotlin-stdlib-jdk8-1.8.10.jar)

jdk7ApiElements

1

jar → The packaged library JAR (kotlin-stdlib-jdk7-1.8.10.jar)

The apiElements variant of org.jetbrains.kotlin:kotlin-stdlib:1.8.10 provides two artifact setsjar, and tar—each representing the same distributable in a different form. The runtimeElements variant of org.jetbrains.kotlin:kotlin-stdlib:1.8.10 provides three artifact setsjar, classes, and resources—each representing the same distributable in a different form.

Gradle must now follow a specific process to determine the most appropriate artifact set for the build.

Artifact Resolution Flow

Artifact selection operates on the dependency graph on a node-by-node basis.

Each node (i.e., variant) in the graph may expose multiple sets of artifacts, but only one of those sets may be selected.

dep man adv 4

For each node, or variant, in a graph, Gradle performs attribute matching over each set of artifacts exposed by that node to determine the best artifact set.

If no artifact sets match the requested attributes, Gradle will attempt to construct an artifact transform chain to satisfy the request.

For more details on the attribute matching process, see the previous section.

Available APIs

Gradle APIs can be used to influence the process of artifact selection.

Gradle can then expose the results of artifact selection as an ArtifactCollection. More commonly, the results are exposed as a FileCollection, which is a flat list of files.

Implicit Artifact Selection

By default, the attributes used for artifact selection are the same as those used for variant selection during graph resolution. These attributes are specified by the Configuration.getAttributes() property.

To perform artifact selection (and implicitly, graph resolution) using these default attributes, use the FileCollection and ArtifactCollection APIs.

Files can also be accessed from the configuration’s ResolvedConfiguration, LenientConfiguration, ResolvedArtifact and ResolvedDependency APIs. However, these APIs are in maintenance mode and are discouraged for use in new development. These APIs perform artifact selection using the default attributes.

Resolving files

To resolve files, we first define a task that accepts a ConfigurableFileCollection as input:

build.gradle.kts
abstract class ResolveFiles : DefaultTask() {

    @get:InputFiles
    abstract val files: ConfigurableFileCollection

    @TaskAction
    fun print() {
        files.forEach {
            println(it.name)
        }
    }
}
build.gradle
abstract class ResolveFiles extends DefaultTask {

    @InputFiles
    abstract ConfigurableFileCollection getFiles()

    @TaskAction
    void print() {
        files.each {
            println(it.name)
        }
    }
}

Then, we can wire up a resolvable configuration’s files to the task’s input. The Configuration directly implements FileCollection and can be wired directly:

build.gradle.kts
tasks.register<ResolveFiles>("resolveConfiguration") {
    files.from(configurations.runtimeClasspath)
}
build.gradle
tasks.register("resolveConfiguration", ResolveFiles) {
    files.from(configurations.runtimeClasspath)
}

Running the resolveConfiguration task produces:

> Task :resolveConfiguration
junit-platform-commons-1.11.0.jar
junit-jupiter-api-5.11.0.jar
opentest4j-1.3.0.jar

Resolving artifacts

Instead of consuming the files directly from the implicit artifact selection process, we can consume the artifacts, which contain both the files and the metadata.

This process is slightly more complicated, as in order to maintain Configuration Cache compatibility, we need to split the fields of ResolvedArtifactResult into two task inputs:

build.gradle.kts
data class ArtifactDetails(
    val id: ComponentArtifactIdentifier,
    val variant: ResolvedVariantResult
)

abstract class ResolveArtifacts : DefaultTask() {

    @get:Input
    abstract val details: ListProperty<ArtifactDetails>

    @get:InputFiles
    abstract val files: ListProperty<File>

    fun from(artifacts: Provider<Set<ResolvedArtifactResult>>) {
        details.set(artifacts.map {
            it.map { artifact -> ArtifactDetails(artifact.id, artifact.variant) }
        })
        files.set(artifacts.map {
            it.map { artifact -> artifact.file }
        })
    }

    @TaskAction
    fun print() {
        assert(details.get().size == files.get().size)
        details.get().zip(files.get()).forEach { (details, file) ->
            println("${details.variant.displayName}:${file.name}")
        }
    }
}
build.gradle
class ArtifactDetails {
    ComponentArtifactIdentifier id
    ResolvedVariantResult variant

    ArtifactDetails(ComponentArtifactIdentifier id, ResolvedVariantResult variant) {
        this.id = id
        this.variant = variant
    }
}

abstract class ResolveArtifacts extends DefaultTask {

    @Input
    abstract ListProperty<ArtifactDetails> getDetails()

    @InputFiles
    abstract ListProperty<File> getFiles()

    void from(Provider<Set<ResolvedArtifactResult>> artifacts) {
        details.set(artifacts.map {
            it.collect { artifact -> new ArtifactDetails(artifact.id, artifact.variant) }
        })
        files.set(artifacts.map {
            it.collect { artifact -> artifact.file }
        })
    }

    @TaskAction
    void print() {
        List<ArtifactDetails> allDetails = details.get()
        List<File> allFiles = files.get()

        assert allDetails.size() == allFiles.size()
        for (int i = 0; i < allDetails.size(); i++) {
            def details = allDetails.get(i)
            def file = allFiles.get(i)
            println("${details.variant.displayName}:${file.name}")
        }
    }
}

This task is initialized similarly to the file resolution task:

build.gradle.kts
tasks.register<ResolveArtifacts>("resolveIncomingArtifacts") {
    from(configurations.runtimeClasspath.flatMap { it.incoming.artifacts.resolvedArtifacts })
}
build.gradle
tasks.register("resolveIncomingArtifacts", ResolveArtifacts) {
    from(configurations.runtimeClasspath.incoming.artifacts.resolvedArtifacts)
}

Running this task, we can see that file metadata is included in the output:

org.junit.platform:junit-platform-commons:1.11.0 variant runtimeElements:junit-platform-commons-1.11.0.jar
org.junit.jupiter:junit-jupiter-api:5.11.0 variant runtimeElements:junit-jupiter-api-5.11.0.jar
org.opentest4j:opentest4j:1.3.0 variant runtimeElements:opentest4j-1.3.0.jar

Custom Artifact Selection

The ArtifactView API operates on top of the resolved graph, defined by a ../javadoc/org/gradle/api/artifacts/result/ResolutionResult.html[ResolutionResult].

The API provides flexible ways to access the resolved artifacts:

  • FileCollection - A flat list of files, which is the most commonly way to work with resolved artifacts.

  • ArtifactCollection - Offers access to both the metadata and the files of resolved artifacts, allowing for more advanced artifact handling.

When you call a configuration’s getFiles(), Gradle selects artifacts based on the attributes used during graph resolution. However, the ArtifactView API is more flexible. It allows you to resolve artifacts from the graph with custom attributes.

Artifact views

An ArtifactView allows you to:

  1. Query artifacts with different attributes:

    • Suppose the graph resolved a dependency’s runtime variant. You can use an ArtifactView to extract artifacts from its api variant instead, even if they weren’t originally part of the resolved graph.

  2. Extract specific types of artifacts:

    • You can request only the .jar files or a specific artifact type (e.g., sources, Javadoc) by specifying an attribute like artifactType.

  3. Avoid side effects:

    • Using an ArtifactView allows you to extract artifacts without changing the underlying dependency resolution logic or configuration state.

In the following example, a producer project creates a library with typical Java library variants (runtimeElements, apiElements). We also create a custom variant called apiProductionElements with the artifact production.jar and attribute org.gradle.category:production:

producer/build.gradle.kts
plugins {
    `java-library`
}

repositories {
    mavenCentral()
}

dependencies {
    // Define some dependencies here
}

// Define a task that produces a custom artifact
tasks.register<Jar>("createProductionArtifact") {
    archiveBaseName.set("production")
    from(sourceSets["main"].output)
    destinationDirectory.set(file("build/libs"))
}

configurations {
    // Define a custom configuration and extend from apiElements
    create("apiProductionElements") {
        extendsFrom(configurations.apiElements.get())
        outgoing.artifacts.clear()
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named("production"))
        }
        artifacts {
            add("apiProductionElements", tasks.named("createProductionArtifact"))
        }
    }
}
producer/build.gradle
plugins {
    id 'java-library'
}

repositories {
    mavenCentral()
}

dependencies {
    // Define some dependencies here
}

// Define a task that produces a custom artifact
tasks.register('createProductionArtifact', Jar) {
    archiveBaseName.set('production')
    from(sourceSets.main.output)
    destinationDirectory.set(file('build/libs'))
}

configurations {
    // Define a custom configuration and extend from apiElements
    apiProductionElements {
        extendsFrom(configurations.apiElements)
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category, 'production'))
        }
        artifacts {
            add('apiProductionElements', tasks.named('createProductionArtifact'))
        }
    }
}

We can view the available variants of this library as well as their corresponding artifacts and attributes with a custom task called checkProducerAttributes. Here is an abridged output showing the relevant variants of this library, along with their corresponding artifacts and attributes:

Configuration: apiElements
Attributes:
  - org.gradle.category -> library
  - org.gradle.usage -> java-api
  - org.gradle.libraryelements -> jar
  - org.gradle.dependency.bundling -> external
Artifacts:
  - producer.jar

Configuration: apiProductionElements
Attributes:
  - org.gradle.category -> production
Artifacts:
  - production.jar

Configuration: mainSourceElements
Attributes:
  - org.gradle.dependency.bundling -> external
  - org.gradle.category -> verification
  - org.gradle.verificationtype -> main-sources
Artifacts:
  - /src/main/resources
  - /src/main/java

Configuration: runtimeElements
Attributes:
  - org.gradle.category -> library
  - org.gradle.usage -> java-runtime
  - org.gradle.libraryelements -> jar
  - org.gradle.dependency.bundling -> external
Artifacts:
  - producer.jar

A Java application is a consumer of this Java library:

consumer/build.gradle.kts
plugins {
    application
}

repositories {
    mavenCentral()
}

// Declare the dependency on the producer project
dependencies {
    implementation(project(":producer")) // This references another subproject in the same build
}
consumer/build.gradle
plugins {
    id 'application'
}

repositories {
    mavenCentral()
}

// Declare the dependency on the producer project
dependencies {
    implementation project(':producer')
}

By default, the application, as the consumer, will consume the expected variant when it is run. We can verify this with another custom task called checkResolvedVariant that prints out the following:

RuntimeClasspath Configuration:
- Component: project :producer
    - Variant: runtimeElements
       - org.gradle.category -> library
       - org.gradle.usage -> java-runtime
       - org.gradle.libraryelements -> jar
       - org.gradle.dependency.bundling -> external
       - org.gradle.jvm.version -> 11
- Artifact: producer.jar

As expected, for the runtimeClasspath, the application consumes the runtimeElements variant of the library which is available as the artifact producer.jar. It uses the attributes org.gradle.category:library and org.gradle.usage:java-runtime to select this variant.

Now, let’s create an ArtifactView to select one of the other artifacts provided by the library. We do this by using an ArtifactView with the attribute org.gradle.category:classes so that instead of the jar file, we get the sources:

consumer/build.gradle.kts
tasks.register("artifactWithAttributeAndView") {
    val configuration = configurations.runtimeClasspath
    println("ArtifactView with attribute 'libraryelements = classes' for ${configuration.name}:")
    val artifactView = configuration.get().incoming.artifactView {
        attributes {
            attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named(LibraryElements::class, "classes"))
        }
    }
    println("- Attributes:")
    artifactView.attributes.keySet().forEach { attribute ->
        val value = artifactView.attributes.getAttribute(attribute)
        println("  - ${attribute.name} = ${value}")
    }
    artifactView.artifacts.artifactFiles.files.forEach { file ->
        println("- Artifact: ${file.name}")
    }
}

tasks.register("artifactWithAttributeAndVariantReselectionView") {
    val configuration = configurations.runtimeClasspath
    println("ArtifactView with attribute 'category = production' for ${configuration.name}:")
    val artifactView = configuration.get().incoming.artifactView {
        withVariantReselection()
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category::class, "production"))
        }
    }
    println("- Attributes:")
    artifactView.attributes.keySet().forEach { attribute ->
        val value = artifactView.attributes.getAttribute(attribute)
        println("  - ${attribute.name} = ${value}")
    }
    artifactView.artifacts.artifactFiles.files.forEach { file ->
        println("- Artifact: ${file.name}")
    }
}
consumer/build.gradle
tasks.register('artifactWithAttributeAndView') {
    def configuration = configurations.runtimeClasspath
    println "ArtifactView with attribute 'libraryelements = classes' for ${configuration.name}:"
    def artifactView = configuration.incoming.artifactView {
        attributes {
            attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named(LibraryElements, 'classes'))
        }
    }
    println "- Attributes:"
    artifactView.attributes.keySet().each { attribute ->
        def value = artifactView.attributes.getAttribute(attribute)
        println "  - ${attribute.name} = ${value}"
    }
    artifactView.artifacts.artifactFiles.files.each { file ->
        println "- Artifact: ${file.name}"
    }
}

tasks.register('artifactWithAttributeAndVariantReselectionView') {
    def configuration = configurations.runtimeClasspath
    println "ArtifactView with attribute 'category = production' for ${configuration.name}:"
    def artifactView = configuration.incoming.artifactView {
        withVariantReselection()
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category,'production'))
        }
    }
    println "- Attributes:"
    artifactView.attributes.keySet().each { attribute ->
        def value = artifactView.attributes.getAttribute(attribute)
        println "  - ${attribute.name} = ${value}"
    }
    artifactView.artifacts.artifactFiles.files.each { file ->
        println "- Artifact: ${file.name}"
    }
}

We run the task artifactWithAttributeAndView to see that we get the main artifact instead:

ArtifactView with attribute 'libraryelements = classes' for runtimeClasspath:
- Attributes:
  - org.gradle.libraryelements = classes
  - org.gradle.category = library
  - org.gradle.usage = java-runtime
  - org.gradle.dependency.bundling = external
  - org.gradle.jvm.environment = standard-jvm
- Artifact: main

Now, let’s create an ArtifactView to select our custom variant apiProductionElements by specifying the attribute org.gradle.category:production and forcing Gradle to reselect a new variant:

consumer/build.gradle.kts
tasks.register("artifactWithAttributeAndVariantReselectionView") {
    val configuration = configurations.runtimeClasspath
    println("ArtifactView with attribute 'category = production' for ${configuration.name}:")
    val artifactView = configuration.get().incoming.artifactView {
        withVariantReselection()
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category::class, "production"))
        }
    }
    println("- Attributes:")
    artifactView.attributes.keySet().forEach { attribute ->
        val value = artifactView.attributes.getAttribute(attribute)
        println("  - ${attribute.name} = ${value}")
    }
    artifactView.artifacts.artifactFiles.files.forEach { file ->
        println("- Artifact: ${file.name}")
    }
}
consumer/build.gradle
tasks.register('artifactWithAttributeAndVariantReselectionView') {
    def configuration = configurations.runtimeClasspath
    println "ArtifactView with attribute 'category = production' for ${configuration.name}:"
    def artifactView = configuration.incoming.artifactView {
        withVariantReselection()
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category,'production'))
        }
    }
    println "- Attributes:"
    artifactView.attributes.keySet().each { attribute ->
        def value = artifactView.attributes.getAttribute(attribute)
        println "  - ${attribute.name} = ${value}"
    }
    artifactView.artifacts.artifactFiles.files.each { file ->
        println "- Artifact: ${file.name}"
    }
}

As expected, the apiProductionElements variant is selected along with the production.jar artifact:

ArtifactView with attribute 'libraryelements = classes' for runtimeClasspath:
- Attributes:
  - org.gradle.libraryelements = classes
  - org.gradle.category = library
  - org.gradle.usage = java-runtime
  - org.gradle.dependency.bundling = external
  - org.gradle.jvm.environment = standard-jvm
- Artifact: main