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[[platforms-vs-catalog]]
= Using Catalogs with Platforms

Both platforms and version catalogs help manage dependency versions in a project, but they serve different purposes and have different effects on dependency resolution:

[[sec:version-catalogs]]
== Version Catalogs

- **Purpose:** A version catalog centralizes and standardizes dependency coordinates (group, name, version) and provides type-safe accessors in the build script, making dependencies easier to manage.
- **Effect on Dependency Graph:** Version catalogs **do not** directly affect dependency resolution. The versions defined in the catalog must be explicitly referenced in a `dependencies` block, and once referenced, they behave the same as any locally declared dependency. Additionally, the catalog's contents are **transparent to downstream consumers**, meaning that consumers cannot identify whether a dependency was declared locally or sourced from a catalog.

[source,toml]
.libs.versions.toml
----
[libraries]
mylib = { group = "com.example", name = "mylib", version = "1.0.0" }
----

[[sec:platforms]]
== Platforms

- **Purpose:** A platform is a module in the dependency graph that enforces or aligns versions of dependencies (including transitive dependencies). It influences dependency resolution and ensures version consistency across different modules.
- **Effect on Dependency Graph:** Platforms **apply** or **enforce** versions to dependencies that are declarated locally without versions.  These versions in a platform are propagated through the dependency graph, affecting transitive dependencies and downstream consumers. They are a formal part of the dependency graph and can dictate the version chosen during resolution.

[source,kotlin]
.build.gradle.kts
----
plugins {
    `java-platform`
}

dependencies {
    constraints {
        api("com.example:mylib:2.0.0")
    }
}
----

[[sec:catalog-platform]]
== Using a catalog with a platform

Even if a version catalog defines a version for a dependency, Gradle might pick a different version during resolution if another component (e.g., a platform or a transitive dependency) suggests a different version (unless <<platforms.adoc#sec:enforced-platform,`enforcedPlatform`>> is used).

For example, a version catalog may define `mylib` as version `1.0.0`, but if a platform enforces `2.0.0`, Gradle will select version `2.0.0`.

To ensure consistent version alignment, a good approach is to use a version catalog to define dependency versions alongside a platform to enforce them.

Version Catalog:

====
include::sample[dir="snippets/dependencyManagement/catalogs-platforms/kotlin",files="gradle/libs.versions.toml[]"]
====

Platform:

====
include::sample[dir="snippets/dependencyManagement/catalogs-platforms/kotlin",files="platform/build.gradle.kts[]"]
include::sample[dir="snippets/dependencyManagement/catalogs-platforms/groovy",files="platform/build.gradle[]"]
====

Consumer:

====
include::sample[dir="snippets/dependencyManagement/catalogs-platforms/kotlin",files="consumer/build.gradle.kts[tags=usage]"]
include::sample[dir="snippets/dependencyManagement/catalogs-platforms/groovy",files="consumer/build.gradle[tags=usage]"]
====

Best Practices for using both a catalog and a platform:

1. **Use version catalogs** for defining and sharing dependency coordinates across projects. They make dependency declarations consistent and easier to manage but do not guarantee version alignment.
2. **Use platforms** when you need to influence or enforce version alignment across modules. Platforms ensure that dependencies resolve to the desired version, particularly in large or multi-module projects.
