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Multi-Project Build Setup in Gradle: Focusing on Composite Builds

Purpose:

A multi-project build in Gradle allows developers to organize and manage large codebases by breaking them into smaller, logically separated modules or projects. This setup enhances modularity, enabling better collaboration, faster builds, and more manageable maintenance of large systems.

A specific aspect of multi-project builds is Composite Builds, which enable combining multiple independent builds into a unified one, while retaining the independence of the individual projects.

Key Concepts of Multi-Project Builds:

In Gradle, a multi-project build setup can be categorized into two types:

Traditional Multi-Project Build: A single Gradle project hierarchy where multiple subprojects share the same root settings.gradle.
Composite Builds: Allows integrating separate, independent Gradle builds together to work as a single build, without merging them into a single repository or file structure.

Composite Builds Approach:

1. What Are Composite Builds?

A Composite Build in Gradle refers to a setup where you combine multiple independent Gradle builds into one. Unlike a traditional multi-project build, composite builds do not require the projects to reside in the same repository or share a single root. This flexibility makes them ideal for situations where you need to combine different projects temporarily or retain autonomy over separate codebases.

2. Why Use Composite Builds?

Separation of Concerns: Each project can exist independently, but when required, multiple projects can work together.
Development Efficiency: You can work on multiple related projects without switching repositories.
Dependency Substitution: Composite builds allow for dependency substitution, meaning you can substitute a binary dependency with a source dependency, reducing the need for artifact publication during development.

3. How Composite Builds Work

In composite builds, Gradle allows you to combine multiple independent builds by including them in a single root project’s settings.gradle file. This is achieved by linking them with the includeBuild directive, which adds these separate builds into the root project's build logic.

// Root Project settings.gradle

includeBuild('../some-other-project')

This simple inclusion means that tasks and dependencies across builds can now interact as if they were part of the same build hierarchy.

4. Composite Builds vs Traditional Multi-Project Builds

In traditional multi-project builds, projects are physically located under the same root directory, and they share common configuration files (settings.gradle and build.gradle). Conversely, composite builds allow projects to stay in their original repositories or directories, preserving their independence. Composite builds avoid the complexity of merging repositories, and the projects can be developed and maintained separately.

5. Advantages of Composite Builds

Independent Version Control: Projects in a composite build can remain in separate repositories with their own version control.
Collaboration Between Projects: Composite builds allow developers to work across multiple independent projects without needing to merge them into one repository.
Avoiding Binary Dependencies: During development, instead of relying on published binaries, you can substitute them with local source dependencies using composite builds. This reduces the overhead of publishing artifacts and enables faster iteration.

6. Configuring Dependency Substitution

Composite builds enable dependency substitution, which means you can replace a binary dependency (usually a published artifact) with a source dependency during development. This feature is highly beneficial when working on related projects across repositories.

For example, suppose projectA depends on projectB. You can substitute the published dependency of projectB with its source by specifying:

// Root project build.gradle

dependencies {
    implementation('com.example:projectB:1.0') // The binary dependency

    // In development, substitute with source
    components {
        withModule('com.example:projectB') {
            all { componentMetadata ->
                componentMetadata.status = 'integration'
            }
        }
    }
}

With composite builds, you can avoid publishing the binary and instead link projectB as a local project dependency.

7. Composite Builds in Practice: Example

Assume we have two independent projects, library-project and app-project, in separate repositories. You are working on app-project, which depends on a library that library-project provides.

Without composite builds, you would need to publish library-project as a binary dependency, and app-project would need to fetch it from a repository.
With composite builds, you can work on library-project in tandem with app-project without publishing the library every time.

To achieve this:

In the app-project, modify the settings.gradle file:

// app-project/settings.gradle
includeBuild('../library-project') // Including the library as a composite build

Now, Gradle will recognize that instead of fetching the published library-project dependency, it can use the source from the included build.

This approach significantly simplifies development when you're working on multiple related projects simultaneously, eliminating the need for constant publishing.

8. Running Composite Builds

To run tasks across composite builds, Gradle allows you to execute them just like any other task. For instance, you can run a build for both the app-project and library-project together:

gradle build

Gradle will detect the included build and execute all necessary tasks across the projects.

Conclusion:

Composite builds in Gradle offer a powerful mechanism for working across multiple independent projects, without sacrificing autonomy or requiring extensive repository merges. By allowing dependency substitution and enabling collaboration between independent builds, composite builds are essential for managing large-scale, interconnected systems in a modular and efficient way.

Backlinks

Composite Build