# Dependency Management

version 9.0.0

Software projects typically depend on other libraries to function. These libraries can either be sourced from other projects in the same build or from external repositories.

Gradle’s dependency management infrastructure provides APIs to declare, resolve, and expose binaries required by and provided by a project.

Understanding dependency management in Gradle is important for structuring projects into components. It is also important when you want to reuse existing libraries, or you need to upgrade those libraries while managing their versions.

Let’s look at a Java project where the code relies on Guava which is a suite of Google Core libraries for Java. The build file of the project includes the following:

build.gradle.kts

    dependencies {
        implementation("com.google.guava:guava:32.1.2-jre") (2)
        api("org.apache.juneau:juneau-marshall:8.2.0")      (3)
    }

build.gradle

    dependencies {
        implementation("com.google.guava:guava:32.1.2-jre") (2)
        api("org.apache.juneau:juneau-marshall:8.2.0")      (3)
    }

Within the `dependencies` block, there are three things to notice when a dependency is declared:

1. The configuration: `implementation` also known as the scope the dependency is applied to

2. The module ID: `com.google.guava:guava` is made up of a `group` and an artifact `name` which are uniquely identifiable

3. The version: `32.1.2-jre` which is not always required

Dependencies can be local or external. To let Gradle know where to find external dependencies, use the `repositories{}` block in the Build File.

Let’s expand our example:

build.gradle.kts

    repositories {
        google()
        mavenCentral()
    }

    dependencies {
        implementation("com.google.guava:guava:32.1.2-jre") (2)
        api("org.apache.juneau:juneau-marshall:8.2.0")      (3)
    }

build.gradle

    repositories {
        google()
        mavenCentral()
    }

    dependencies {
        implementation("com.google.guava:guava:32.1.2-jre") (2)
        api("org.apache.juneau:juneau-marshall:8.2.0")      (3)
    }

In this example, Gradle fetches the `guava` and `juneau-marshall` dependencies from the Maven Central and Google repositories.

## Learning the Basics

If you want to understand the basics of dependency management and you are new to Gradle, start here.

### 1. Declaring Dependencies

You can add external libraries to your Java project, such as `Guava`. These libraries are dependencies of your project. They are added using the `dependencies{}` block in your build file.

### 2. Dependency Configurations

Every dependency declared for a Gradle project applies to a specific scope, known as a configurations. These configurations are typically created by applying plugins or must be created directly using APIs.

### 3. Declaring Repositories

You can declare repositories to tell Gradle where to fetch external dependencies. During a build, Gradle locates and downloads the dependencies, a process called dependency resolution.

### 4. Centralizing Dependencies

To keep dependencies and their versions declared in a single, manageable location (i.e., centralized), you can use platforms and version catalogs.

A platform is a set of modules intended to be used together. A version catalog is a centralized list of dependency coordinates that can be referenced in multiple projects.

### 5. Managing Constraints and Conflicts

Conflicts can arise when the same library is declared multiple times or when different libraries provide the same functionality. This usually leads to failing builds.

You can manage conflicts using resolution rules or dependency locking.

## Advanced Concepts

In order to influence how Gradle resolves dependencies, it’s important to understand how it works.

### 1. Dependency Resolution

Gradle’s dependency resolution process determines which modules and versions are required to fulfill a build’s declared dependencies. It is done in two steps, graph resolution and artifact resolution.

### 2. Graph Resolution

During graph resolution, Gradle constructs a dependency graph, resolving version conflicts and ensuring the intended dependencies are selected.

### 3. Variant Selection

Variant selection traverses the resolved dependency graph by choosing the most compatible variant of each module based on attributes and capabilities.

### 4. Artifact Resolution

Finally, Artifact resolution is how Gradle determines which files or artifacts published by the selected variants to download and use during the build.