Modularize Your Builds

Modularize your builds by splitting your code into multiple projects.

Explanation

Splitting your build’s source into multiple Gradle projects (modules) is essential for leveraging Gradle’s automatic work avoidance and parallelization features. When a source file changes, Gradle only recompiles the affected projects. If all your sources reside in a single project, Gradle can’t avoid recompilation and won’t be able to run tasks in parallel. Splitting your source into multiple projects can provide additional performance benefits by minimizing each subproject’s compilation classpath and ensuring code generating tools such as annotation and symbol processors run only on the relevant files.

Do this soon. Don’t wait until you hit some arbitrary number of source files or classes to do this, instead structure your build into multiple projects from the start using whatever natural boundaries exist in your codebase.

Exactly how to best split your source varies with every build, as it depends on the particulars of that build. Here are some common patterns we found that can work well and make cohesive projects:

  • API vs. Implementation

  • Front-end vs. Back-end

  • Core business logic vs. UI

  • Vertical slices (e.g., feature modules each containing UI + business logic)

  • Inputs to source generation vs. their consumers

  • Or simply closely related classes.

Ultimately, the specific scheme matters less than ensuring that your build is split logically and consistently.

Expanding a build to hundreds of projects is common, and Gradle is designed to scale to this size and beyond. In the extreme, tiny projects containing only a class or two are probably counterproductive. However, you should typically err on the side of adding more projects rather than fewer.

Example

Don’t Do This

A common way to structure new builds
├── app // This project contains a mix of classes
│    ├── build.gradle.kts
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── CommonsUtil.java
│                        └── GuavaUtil.java
│                        └── Main.java
│                        └── Util.java
├── settings.gradle.kts
A common way to structure new builds
├── app // This project contains a mix of classes
│    ├── build.gradle
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── CommonsUtil.java
│                        └── GuavaUtil.java
│                        └── Main.java
│                        └── Util.java
├── settings.gradle
settings.gradle.kts
include("app") (1)
settings.gradle
include("app") (1)
build.gradle.kts
plugins {
    application (2)
}

dependencies {
    implementation("com.google.guava:guava:31.1-jre") (3)
    implementation("commons-lang:commons-lang:2.6")
}

application {
    mainClass = "org.example.Main"
}
build.gradle
plugins {
    id 'application' (2)
}

dependencies {
    implementation 'com.google.guava:guava:31.1-jre' (3)
    implementation 'commons-lang:commons-lang:2.6'
}

application {
    mainClass = "org.example.Main"
}
1 This build contains only a single project (in addition to the root project) that contains all the source code. If there is any change to any source file, Gradle will have to recompile and rebuild everything. While incremental compilation will help (especially in this simplified example) this is still less efficient then avoidance. Gradle also won’t be able to run any tasks in parallel, since all these tasks are in the same project, so this design won’t scale nicely.
2 As there is only a single project in this build, the application plugin must be applied here. This means that the application plugin will be affect all source files in the build, even those which have no need for it.
3 Likewise, the dependencies here are only needed by each particular implmentation of util. There’s no need for the implementation using Guava to have access to the Commons library, but it does because they are all in the same project. This also means that the classpath for each subproject is much larger than it needs to be, which can lead to longer build times and other confusion.

Do This Instead

A better way to structure this build
├── app
│    ├── build.gradle.kts
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── Main.java
├── settings.gradle.kts
├── util
│    ├── build.gradle.kts
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── Util.java
├── util-commons
│    ├── build.gradle.kts
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── CommonsUtil.java
└── util-guava
    ├── build.gradle.kts
    └── src
        └── main
            └── java
                └── org
                    └── example
                        └── GuavaUtil.java
A better way to structure this build
├── app // App contains only the core application logic
│    ├── build.gradle
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── Main.java
├── settings.gradle
├── util // Util contains only the core utility logic
│    ├── build.gradle
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── Util.java
├── util-commons // One particular implementation of util, using Apache Commons
│    ├── build.gradle
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── CommonsUtil.java
└── util-guava // Another implementation of util, using Guava
    ├── build.gradle
    └── src
        └── main
            └── java
                └── org
                    └── example
                        └── GuavaUtil.java
settings.gradle.kts
include("app") (1)
include("util")
include("util-commons")
include("util-guava")
settings.gradle
include("app") (1)
include("util")
include("util-commons")
include("util-guava")
build.gradle.kts
// This is the build.gradle file for the app module

plugins {
    application (2)
}

dependencies { (3)
    implementation(project(":util-guava"))
    implementation(project(":util-commons"))
}

application {
    mainClass = "org.example.Main"
}
build.gradle
// This is the build.gradle file for the app module

plugins {
    id "application" (2)
}

dependencies { (3)
    implementation project(":util-guava")
    implementation project(":util-commons")
}

application {
    mainClass = "org.example.Main"
}
build.gradle.kts
// This is the build.gradle file for the util-commons module

plugins { (4)
    `java-library`
}

dependencies { (5)
    api(project(":util"))
    implementation("commons-lang:commons-lang:2.6")
}
build.gradle
// This is the build.gradle file for the util-commons module

plugins { (4)
    id "java-library"
}

dependencies { (5)
    api project(":util")
    implementation "commons-lang:commons-lang:2.6"
}
build.gradle.kts
// This is the build.gradle file for the util-guava module

plugins {
    `java-library`
}

dependencies {
    api(project(":util"))
    implementation("com.google.guava:guava:31.1-jre")
}
build.gradle
// This is the build.gradle file for the util-guava module

plugins {
    id "java-library"
}

dependencies {
    api project(":util")
    implementation "com.google.guava:guava:31.1-jre"
}
1 This build logically splits the source into multiple projects. Each project can be built independently, and Gradle can run tasks in parallel. This means that if you change a single source file in one of the projects, Gradle will only need to recompile and rebuild that project, not the entire build.
2 The application plugin is only applied to the app project, which is the only project that needs it.
3 Each project only adds the dependencies it needs. This means that the classpath for each subproject is much smaller, which can lead to faster build times and less confusion.
4 Each project only adds the specific plugins it needs.
5 Each project only adds the dependencies it needs. Projects can effectively use API vs. Implementation separation.

References

Tags

#structuring-builds

Do Not Put Source Files in the Root Project

Do not put source files in your root project; instead, put them in a separate project.

Explanation

The root project is a special Project in Gradle that serves as the entry point for your build.

It is the place to configure some settings and conventions that apply globally to the entire build, that are not configured via Settings. For example, you can declare (but not apply) plugins here to ensure the same plugin version is consistently available across all projects and define other configurations shared by all projects within the build.

Be careful not to apply plugins unnecessarily in the root project - many plugins only affect source code and should only be applied to the projects that contain source code.

The root project should not be used for source files, instead they should be located in a separate Gradle project.

Setting up your build like this from the start will also make it easier to add new projects as your build grows in the future.

Example

Don’t Do This

A common way to structure new builds
├── build.gradle.kts // Applies the `java-library` plugin to the root project
├── settings.gradle.kts
└── src // This directory shouldn't exist
    └── main
        └── java
            └── org
                └── example
                    └── MyClass1.java
A common way to structure new builds
├── build.gradle // Applies the `java-library` plugin to the root project
├── settings.gradle
└── src // This directory shouldn't exist
    └── main
        └── java
            └── org
                └── example
                    └── MyClass1.java
build.gradle.kts
plugins { (1)
    `java-library`
}
build.gradle
plugins {
    id 'java-library' (1)
}
1 The java-library plugin is applied to the root project, as there are Java source files are in the root project.

Do This Instead

A better way to structure new builds
├── core
│    ├── build.gradle.kts // Applies the `java-library` plugin to only the `core` project
│    └── src // Source lives in a "core" (sub)project
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── MyClass1.java
└── settings.gradle.kts
A better way to structure new builds
├── core
│    ├── build.gradle // Applies the `java-library` plugin to only the `core` project
│    └── src // Source lives in a "core" (sub)project
│        └── main
│            └── java
│                └── org
│                    └── example
│                        └── MyClass1.java
└── settings.gradle
settings.gradle.kts
include("core") (1)
settings.gradle
include("core") (1)
build.gradle.kts
// This is the build.gradle.kts file for the core module

plugins { (2)
    `java-library`
}
build.gradle
// This is the build.gradle file for the core module

plugins { (2)
    id 'java-library'
}
1 The root project exists only to configure the build, informing Gradle of a (sub)project named core.
2 The java-library plugin is only applied to the core project, which contains the Java source files.

References

Tags

#structuring-builds

Favor build-logic Composite Builds for Build Logic

You should set up a Composite Build (often called an "included build") to hold your build logic—including any custom plugins, convention plugins, and other build-specific customizations.

Explanation

The preferred location for build logic is an included build (typically named build-logic), not in buildSrc.

The automatically available buildSrc is great for rapid prototyping, but it comes with some subtle disadvantages:

  • There are classloader differences in how these 2 approaches behave that can be surprising; included builds are treated just like external dependencies, which is a simpler mental model. Dependency resolution behaves subtly differently in buildSrc.

  • There can potentially be fewer task invalidations in a build when files in an included build are modified, leading to faster builds. Any change in buildSrc causes the entire build to become out-of-date, whereas changes in a subproject of an included build only cause projects in the build using the products of that particular subproject to be out-of-date.

  • Included builds are complete Gradle builds and can be opened, worked on, and built independently as standalone projects. It is straightforward to publish their products, including plugins, in order to share them with other projects.

  • The buildSrc project automatically applies the java plugin, which may be unnecessary.

One important caveat to this recommendation is when creating Settings plugins. Defining these in a build-logic project requires it to be included in the pluginManagement block of the main build’s settings.gradle(.kts) file, in order to make these plugins available to the build early enough to be applied to the Settings instance. This is possible, but reduces Build Caching capability, potentially impacting performance. A better solution is to use a separate, minimal, included build (e.g. build-logic-settings) to hold only Settings plugins.

Another potential reason to use buildSrc is if you have a very large number of subprojects within your included build-logic. Applying a different set of build-logic plugins to the subprojects in your including build will result in a different classpath being used for each. This may have performance implications and make your build harder to understand. Using different plugin combinations can cause features like Build Services to break in difficult to diagnose ways.

Ideally, there would be no difference between using buildSrc and an included build, as buildSrc is intended to behave like an implicitly available included build. However, due to historical reasons, these subtle differences still exist. As this changes, this recommendation may be revised in the future. For now, these differences can introduce confusion.

Since setting up a composite build requires only minimal additional configuration, we recommend using it over buildSrc in most cases.

Example

Don’t Do This

├── build.gradle.kts
├── buildSrc
│    ├── build.gradle.kts
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        ├── MyPlugin.java
│                        └── MyTask.java
└── settings.gradle.kts
├── build.gradle
├── buildSrc
│    ├── build.gradle
│    └── src
│        └── main
│            └── java
│                └── org
│                    └── example
│                        ├── MyPlugin.java
│                        └── MyTask.java
└── settings.gradle
build.gradle.kts
// This file is located in /buildSrc

plugins {
    `java-gradle-plugin`
}

gradlePlugin {
    plugins {
        create("myPlugin") {
            id = "org.example.myplugin"
            implementationClass = "org.example.MyPlugin"
        }
    }
}
build.gradle
// This file is located in /buildSrc

plugins {
    id "java-gradle-plugin"
}

gradlePlugin {
    plugins {
        create("myPlugin") {
            id = "org.example.myplugin"
            implementationClass = "org.example.MyPlugin"
        }
    }
}

Set up a Plugin Build: This is the same using either method.

settings.gradle.kts
rootProject.name = "favor-composite-builds"
settings.gradle
rootProject.name = "favor-composite-builds"

buildSrc products are automatically usable: There is no additional configuration with this method.

Do This Instead

├── build-logic
│    ├── plugin
│    │    ├── build.gradle.kts
│    │    └── src
│    │        └── main
│    │            └── java
│    │                └── org
│    │                    └── example
│    │                        ├── MyPlugin.java
│    │                        └── MyTask.java
│    └── settings.gradle.kts
├── build.gradle.kts
└── settings.gradle.kts
├── build-logic
│    ├── plugin
│    │    ├── build.gradle
│    │    └── src
│    │        └── main
│    │            └── java
│    │                └── org
│    │                    └── example
│    │                        ├── MyPlugin.java
│    │                        └── MyTask.java
│    └── settings.gradle
├── build.gradle
└── settings.gradle
build.gradle.kts
// This file is located in /build-logic/plugin

plugins {
    `java-gradle-plugin`
}

gradlePlugin {
    plugins {
        create("myPlugin") {
            id = "org.example.myplugin"
            implementationClass = "org.example.MyPlugin"
        }
    }
}
build.gradle
// This file is located in /build-logic/plugin

plugins {
    id "java-gradle-plugin"
}

gradlePlugin {
    plugins {
        create("myPlugin") {
            id = "org.example.myplugin"
            implementationClass = "org.example.MyPlugin"
        }
    }
}

Set up a Plugin Build: This is the same using either method.

settings.gradle.kts
// This file is located in the root project

includeBuild("build-logic") (1)

rootProject.name = "favor-composite-builds"
settings.gradle
// This file is located in the root project

includeBuild("build-logic") (1)

rootProject.name = "favor-composite-builds"
settings.gradle.kts
// This file is located in /build-logic

rootProject.name = "build-logic"

include("plugin") (2)
settings.gradle
// This file is located in /build-logic

rootProject.name = "build-logic"

include("plugin") (2)
1 Composite builds must be explicitly included: Use the includeBuild method to locate and include a build in order to use its products.
2 Structure your included build into subprojects: This allows the main build to only depend on the necessary parts of the included build.

References

Tags

#structuring-builds, #composite-builds

Avoid Unintentionally Creating Empty Projects

When using a hierarchical directory structure to organize your Gradle projects, make sure to avoid unintentionally creating empty projects in your build.

Explanation

When you use the Settings.include() method to include a project in your Grade settings file, you typically include projects by supplying the directory name like include("featureA"). This usage assumes that featureA is located at the root of your build.

You can also include projects located in subdirectories using hierarchical paths with the : charactor as a separator. This allows you to organize large builds containing many projects to improve comprehensibility, versus having all your projects live at the root of the build. For instance, if project search was located in a subdirectory named features, itself located in a subdirectory named subs, you could call include(":subs:features:search") to include it.

However, without further configuration, Gradle will create empty projects for every element of the path. In the example above, Gradle will create a project named :subs, a project named :subs:features, and a project named :subs:features:search. This probably isn’t intended, as you likely only want to include the search project. Additional projects - even if empty - will slow down your build, clutter reporting, and make your build harder to understand.

To avoid this, be sure to explicitly set the Project.projectDir property of any projects included in nested directories: project(':my-web-module').projectDir = file("subs/web/my-web-module").

As ab added benefit, this will allow you to reference the project by the shorter name :my-web-module instead of the full logical path :subs:web:my-web-module.

Example

Don’t Do This

├── settings.gradle
├── app/ (1)
│   ├── build.gradle
│   └── src/
└── subs/ (2)
    └── web/ (3)
        ├── my-web-module/ (4)
            ├── src/
            └── build.gradle.kts
├── settings.gradle
├── app/ (1)
│   ├── build.gradle
│   └── src/
└── subs/ (2)
    └── web/ (3)
        ├── my-web-module/ (4)
            ├── src/
            └── build.gradle.kts
1 A project named app located at the root of the build
2 A directory named subs that is not intended to represent a Gradle project, but is used to organize the build
3 Another organizational directory not intended to represent a Gradle project
4 A Gradle project named my-web-module that should be included in the build
settings.gradle.kts
include(":app") (1)
include(":subs:web:my-web-module") (2)
settings.gradle
include(":app") (1)
include(":subs:web:my-web-module") (2)
1 Including the app project located at the root of the build requires no additional configuration
2 Including a project named :subs:my-web-module located in a nested subdirectory causes Gradle to create empty projects for each element of the path
avoidEmptyProjects-avoid.out
> Task :projects

Projects:

------------------------------------------------------------
Root project 'avoidEmptyProjects-avoid'
------------------------------------------------------------

Root project 'avoidEmptyProjects-avoid'
+--- Project ':app'
\--- Project ':subs'
     \--- Project ':subs:web'
          \--- Project ':subs:web:my-web-module'

To see a list of the tasks of a project, run gradle <project-path>:tasks
For example, try running gradle :app:tasks

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

The output of running the projects report on the above build shows that Gradle created empty projects for :subs and :subs:web.

Do This Instead

settings.gradle.kts
include(":app")

include(":my-web-module")
project(":my-web-module").projectDir = file("subs/web/my-web-module") (1)
settings.gradle
include(":app")

include(":my-web-module")
project(":my-web-module").projectDir = file("subs/web/my-web-module") (1)
1 After including the :subs:web:my-web-module project, its projectDir property is set to the physical location of the project
avoidEmptyProjects-do.out
> Task :projects

Projects:

------------------------------------------------------------
Root project 'avoidEmptyProjects-do'
------------------------------------------------------------

Root project 'avoidEmptyProjects-do'
+--- Project ':app'
\--- Project ':my-web-module'

To see a list of the tasks of a project, run gradle <project-path>:tasks
For example, try running gradle :app:tasks

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

The output of running the projects report on the above build shows that now Gradle only creates the intended projects for this build.

You can also now invoke tasks on the my-web-module project using the shorter name :my-web-module like gradle :my-web-module:build, instead of gradle :subs:web:my-web-module:build.

References

Tags

#structuring-builds