[[variant_attributes]]
= Working with Variant Attributes

As explained in the section on <<variant_model.adoc#sec:variant-aware-matching,variant aware matching>>, attributes give semantics to variants and are used to perform the selection between them.

As a user of Gradle, attributes are often hidden as implementation details.
But it might be useful to understand the _standard attributes_ defined by Gradle and its core plugins.

As a plugin author, these attributes, and the way they are defined, can serve as a basis for <<#sec:declaring_attributes,building your own set of attributes>> in your eco system plugin.

[[sec:standard_attributes]]
== Standard attributes defined by Gradle

Gradle defines a list of standard attributes used by Gradle's core plugins.

=== Ecosystem-independent standard attributes

.Ecosystem-independent standard variant attributes
[%header%autowidth,compact]
|===
| Attribute name | Description | Values | compatibility and disambiguation rules

| link:{javadocPath}/org/gradle/api/attributes/Usage.html#USAGE_ATTRIBUTE[`org.gradle.usage`]
| Indicates main purpose of variant
| `Usage` values built from constants defined in link:{javadocPath}/org/gradle/api/attributes/Usage.html[Usage]
| Following ecosystem semantics (e.g. `java-runtime` can be used in place of `java-api` but not the opposite)

| link:{javadocPath}/org/gradle/api/attributes/Category.html#CATEGORY_ATTRIBUTE[`org.gradle.category`]
| Indicates the category of this software component
| `Category` values built from constants defined in link:{javadocPath}/org/gradle/api/attributes/Category.html[Category]
| Following ecosystem semantics (e.g. `library` is default on the JVM, no compatibility otherwise)

| link:{javadocPath}/org/gradle/api/attributes/LibraryElements.html#LIBRARY_ELEMENTS_ATTRIBUTE[`org.gradle.libraryelements`]
| Indicates the contents of a `org.gradle.category=library` variant
| `LibraryElements` values built from constants defined in link:{javadocPath}/org/gradle/api/attributes/LibraryElements.html[LibraryElements]
| Following ecosystem semantics(e.g. in the JVM world, `jar` is the default and is compatible with `classes`)

| link:{javadocPath}/org/gradle/api/attributes/DocsType.html#DOCS_TYPE_ATTRIBUTE[`org.gradle.docstype`]
| Indicates the contents of a `org.gradle.category=documentation` variant
| `DocsType` values built from constants defined in link:{javadocPath}/org/gradle/api/attributes/DocsType.html[DocsType]
| No default, no compatibility

| link:{javadocPath}/org/gradle/api/attributes/Bundling.html#BUNDLING_ATTRIBUTE[`org.gradle.dependency.bundling`]
| Indicates how dependencies of a variant are accessed.
| `Bundling` values built from constants defined in link:{javadocPath}/org/gradle/api/attributes/Bundling.html[Bundling]
| Following ecosystem semantics (e.g. in the JVM world, `embedded` is compatible with `external`)
|===

.Ecosystem-independent standard component attributes
[%header%autowidth,compact]
|===
| Attribute name | Description | Values | compatibility and disambiguation rules

| `org.gradle.status`
| Component level attribute, derived
| Based on <<component_metadata_rules.adoc#sec:custom_status_scheme,a status scheme>>, with a default one existing based on the source repository.
| Based on the scheme in use
|===

[[sub:jvm_default_attributes]]
=== JVM ecosystem specific attributes

In addition to the ecosystem independent attributes defined above, the JVM ecosystem adds the following attribute:

.JVM ecosystem standard component attributes
[%header%autowidth,compact]
|===
| Attribute name | Description | Values | compatibility and disambiguation rules

| link:{javadocPath}/org/gradle/api/attributes/java/TargetJvmVersion.html#TARGET_JVM_VERSION_ATTRIBUTE[`org.gradle.jvm.version`]
| Indicates the JVM version compatibility.
| Integer using the version after the `1.` for Java 1.4 and before, the major version for Java 5 and beyond.
| Defaults to the JVM version used by Gradle, lower is compatible with higher, prefers highest compatible.
|===

The JVM ecosystem also contains a number of compatibility and disambiguation rules over the different attributes.
The reader willing to know more can take a look at the code for `org.gradle.api.internal.artifacts.JavaEcosystemSupport`.

=== Native ecosystem specific attributes

In addition to the ecosystem independent attributes defined above, the native ecosystem adds the following attributes:

.Native ecosystem standard component attributes
[%header%autowidth,compact]
|===
| Attribute name | Description | Values | compatibility and disambiguation rules

| link:{javadocPath}/org/gradle/language/cpp/CppBinary.html#DEBUGGABLE_ATTRIBUTE[`org.gradle.native.debuggable`]
| Indicates if the binary was built with debugging symbols
| Boolean
| N/A

| link:{javadocPath}/org/gradle/language/cpp/CppBinary.html#OPTIMIZED_ATTRIBUTE[`org.gradle.native.optimized`]
| Indicates if the binary was built with optimization flags
| Boolean
| N/A

| link:{javadocPath}/org/gradle/nativeplatform/MachineArchitecture.html#ARCHITECTURE_ATTRIBUTE[`org.gradle.native.architecture`]
| Indicates the target architecture of the binary
| `MachineArchitecture` values built from constants defined in link:{javadocPath}/org/gradle/nativeplatform/MachineArchitecture.html[MachineArchitecture]
| None

| link:{javadocPath}/org/gradle/nativeplatform/OperatingSystemFamily.html#OPERATING_SYSTEM_ATTRIBUTE[`org.gradle.native.operatingSystem`]
| Indicates the target operating system of the binary
| `OperatingSystemFamily` values built from constants defined in link:{javadocPath}/org/gradle/nativeplatform/OperatingSystemFamily.html[OperatingSystemFamily]
| None
|===

[[sec:declaring_attributes]]
== Declaring custom attributes

If you are extending Gradle, e.g. by writing a plugin for another ecosystem, declaring custom attributes could be an option if you want to support variant-aware dependency management features in your plugin.
However, you should be cautious if you also attempt to publish libraries.
Semantics of new attributes are usually defined through a plugin, which can carry <<#sec:abm_compatibility_rules,compatibility>> and <<sec:abm_disambiguation_rules,disambiguation>> rules.
Consequently, builds that consume libraries published for a certain ecosystem, also need to apply the corresponding plugin to interpret attributes correctly.
If your plugin is intended for a larger audience, i.e. if it is openly available and libraries are published to public repositories, defining new attributes effectively extends the semantics of Gradle Module Metadata and comes with responsibilities.
E.g., support for attributes that are already published should not be removed again, or should be handled in some kind of compatibility layer in future versions of the plugin.

== Creating attributes in a build script or plugin

Attributes are _typed_. An attribute can be created via the `Attribute<T>.of` method:

.Define attributes
====
include::sample[dir="snippets/dependencyManagement/attributeMatching/groovy",files="build.gradle[tags=define_attribute]"]
include::sample[dir="snippets/dependencyManagement/attributeMatching/kotlin",files="build.gradle.kts[tags=define_attribute]"]
====

Currently, only attribute types of `String`, or anything extending `Named` is supported.
Attributes must be declared in the _attribute schema_ found on the `dependencies` handler:

.Registering attributes on the attributes schema
====
include::sample[dir="snippets/dependencyManagement/attributeMatching/groovy",files="build.gradle[tags=register-attributes]"]
include::sample[dir="snippets/dependencyManagement/attributeMatching/kotlin",files="build.gradle.kts[tags=register-attributes]"]
====

Then configurations can be configured to set values for attributes:

.Setting attributes on configurations
====
include::sample[dir="snippets/dependencyManagement/attributeMatching/groovy",files="build.gradle[tags=attributes-on-configurations]"]
include::sample[dir="snippets/dependencyManagement/attributeMatching/kotlin",files="build.gradle.kts[tags=attributes-on-configurations]"]
====

For attributes which type extends `Named`, the value of the attribute **must** be created via the _object factory_:

.Named attributes
====
include::sample[dir="snippets/dependencyManagement/attributeMatching/groovy",files="build.gradle[tags=named-attributes]"]
include::sample[dir="snippets/dependencyManagement/attributeMatching/kotlin",files="build.gradle.kts[tags=named-attributes]"]
====

[[sec:abm_compatibility_rules]]
=== Attribute compatibility rules

Attributes let the engine select _compatible variants_.
However, there are cases where a provider may not have _exactly_ what the consumer wants, but still something that it can use.
For example, if the consumer is asking for the API of a library, there's a possibility that the producer doesn't have such a variant, but only a _runtime_ variant.
This is typical of libraries published on external repositories.
In this case, we know that even if we don't have an exact match (API), we can still compile against the runtime variant (it contains _more_ than what we need to compile but it's still ok to use).
To deal with this, Gradle provides link:{javadocPath}/org/gradle/api/attributes/AttributeCompatibilityRule.html[attribute compatibility rules].
The role of a compatibility rule is to explain what variants are _compatible_ with what the consumer asked for.

Attribute compatibility rules have to be registered via the link:{javadocPath}/org/gradle/api/attributes/AttributeMatchingStrategy.html[attribute matching strategy] that you can obtain from the link:{javadocPath}/org/gradle/api/attributes/AttributesSchema.html[attributes schema].

[[sec:abm_disambiguation_rules]]
=== Attribute disambiguation rules

Because multiple values for an attribute can be _compatible_ with the requested attribute, Gradle needs to choose between the candidates.
This is done by implementing an link:{javadocPath}/org/gradle/api/attributes/AttributeDisambiguationRule.html[attribute disambiguation rule].

Attribute disambiguation rules have to be registered via the link:{javadocPath}/org/gradle/api/attributes/AttributeMatchingStrategy.html[attribute matching strategy] that you can obtain from the link:{javadocPath}/org/gradle/api/attributes/AttributesSchema.html[attributes schema].
