Organizing Gradle Projects

Source code and build logic of every software project should be organized in a meaningful way. This page lays out the best practices that lead to readable, maintainable projects. The following sections also touch on common problems and how to avoid them.

Separate language-specific source files

Gradle’s language plugins establish conventions for discovering and compiling source code. For example, a project applying the Java plugin will automatically compile the code in the directory src/main/java. Other language plugins follow the same pattern. The last portion of the directory path usually indicates the expected language of the source files.

Some compilers are capable of cross-compiling multiple languages in the same source directory. The Groovy compiler can handle the scenario of mixing Java and Groovy source files located in src/main/groovy. Gradle recommends that you place sources in directories according to their language, because builds are more performant and both the user and build can make stronger assumptions.

The following source tree contains Java and Kotlin source files. Java source files live in src/main/java, whereas Kotlin source files live in src/main/kotlin.

.
├── build.gradle.kts
└── src
    └── main
        ├── java
        │   └── HelloWorld.java
        └── kotlin
            └── Utils.kt

.
├── build.gradle
└── src
    └── main
        ├── java
        │   └── HelloWorld.java
        └── kotlin
            └── Utils.kt

Separate source files per test type

It’s very common that a project defines and executes different types of tests e.g. unit tests, integration tests, functional tests or smoke tests. Optimally, the test source code for each test type should be stored in dedicated source directories. Separated test source code has a positive impact on maintainability and separation of concerns as you can run test types independent from each other.

Have a look at the sample that demonstrates how a separate integration tests configuration can be added to a Java-based project.

Use standard conventions as much as possible

All Gradle core plugins follow the software engineering paradigm convention over configuration. The plugin logic provides users with sensible defaults and standards, the conventions, in a certain context. Let’s take the Java plugin as an example.

It defines the directory src/main/java as the default source directory for compilation.
The output directory for compiled source code and other artifacts (like the JAR file) is build.

By sticking to the default conventions, new developers to the project immediately know how to find their way around. While those conventions can be reconfigured, it makes it harder to build script users and authors to manage the build logic and its outcome. Try to stick to the default conventions as much as possible except if you need to adapt to the layout of a legacy project. Refer to the reference page of the relevant plugin to learn about its default conventions.

Always define a settings file

Gradle tries to locate a settings.gradle (Groovy DSL) or a settings.gradle.kts (Kotlin DSL) file with every invocation of the build. For that purpose, the runtime walks the hierarchy of the directory tree up to the root directory. The algorithm stops searching as soon as it finds the settings file.

Always add a settings.gradle to the root directory of your build to avoid the initial performance impact. The file can either be empty or define the desired name of the project.

A multi-project build must have a settings.gradle(.kts) file in the root project of the multi-project hierarchy. It is required because the settings file defines which projects are taking part in a multi-project build. Besides defining included projects, you might need it to add libraries to your build script classpath.

The following example shows a standard Gradle project layout:

.
├── settings.gradle.kts
├── subproject-one
│   └── build.gradle.kts
└── subproject-two
    └── build.gradle.kts

.
├── settings.gradle
├── subproject-one
│   └── build.gradle
└── subproject-two
    └── build.gradle

Use `buildSrc` to abstract imperative logic

Complex build logic is usually a good candidate for being encapsulated either as custom task or binary plugin. Custom task and plugin implementations should not live in the build script. It is very convenient to use buildSrc for that purpose as long as the code does not need to be shared among multiple, independent projects.

The directory buildSrc is treated as an included build. Upon discovery of the directory, Gradle automatically compiles this code and puts it in the classpath of your build script. For multi-project builds there can be only one buildSrc directory, which has to sit in the root project directory. buildSrc should be preferred over script plugins as it is easier to maintain, refactor and test the code.

buildSrc uses the same source code conventions applicable to Java and Groovy projects. It also provides direct access to the Gradle API. Additional dependencies can be declared in a dedicated build.gradle under buildSrc.

Example 1. Custom buildSrc build script

buildSrc/build.gradle.kts

repositories {
    mavenCentral()
}

dependencies {
    testImplementation("junit:junit:4.13")
}

buildSrc/build.gradle

repositories {
    mavenCentral()
}

dependencies {
    testImplementation 'junit:junit:4.13'
}

A typical project including buildSrc has the following layout. Any code under buildSrc should use a package similar to application code. Optionally, the buildSrc directory can host a build script if additional configuration is needed (e.g. to apply plugins or to declare dependencies).

.
├── buildSrc
│   ├── build.gradle.kts
│   └── src
│       ├── main
│       │   └── java
│       │       └── com
│       │           └── enterprise
│       │               ├── Deploy.java
│       │               └── DeploymentPlugin.java
│       └── test
│           └── java
│               └── com
│                   └── enterprise
│                       └── DeploymentPluginTest.java
├── settings.gradle.kts
├── subproject-one
│   └── build.gradle.kts
└── subproject-two
    └── build.gradle.kts

.
├── buildSrc
│   ├── build.gradle
│   └── src
│       ├── main
│       │   └── java
│       │       └── com
│       │           └── enterprise
│       │               ├── Deploy.java
│       │               └── DeploymentPlugin.java
│       └── test
│           └── java
│               └── com
│                   └── enterprise
│                       └── DeploymentPluginTest.java
├── settings.gradle
├── subproject-one
│   └── build.gradle
└── subproject-two
    └── build.gradle

A change in buildSrc causes the whole project to become out-of-date.

Thus, when making small incremental changes, the --no-rebuild command-line option is often helpful to get faster feedback. Remember to run a full build regularly.

Declare properties in `gradle.properties` file

In Gradle, properties can be defined in the build script, in a gradle.properties file or as parameters on the command line.

It’s common to declare properties on the command line for ad-hoc scenarios. For example you may want to pass in a specific property value to control runtime behavior just for this one invocation of the build. Properties in a build script can easily become a maintenance headache and convolute the build script logic. The gradle.properties helps with keeping properties separate from the build script and should be explored as viable option. It’s a good location for placing properties that control the build environment.

A typical project setup places the gradle.properties file in the root directory of the build. Alternatively, the file can also live in the GRADLE_USER_HOME directory if you want it to apply to all builds on your machine.

.
├── gradle.properties
└── settings.gradle.kts
├── subproject-a
│   └── build.gradle.kts
└── subproject-b
    └── build.gradle.kts

.
├── gradle.properties
└── settings.gradle
├── subproject-a
│   └── build.gradle
└── subproject-b
    └── build.gradle

Avoid overlapping task outputs

Tasks should define inputs and outputs to get the performance benefits of incremental build functionality. When declaring the outputs of a task, make sure that the directory for writing outputs is unique among all the tasks in your project.

Intermingling or overwriting output files produced by different tasks compromises up-to-date checking causing slower builds. In turn, these filesystem changes may prevent Gradle’s build cache from properly identifying and caching what would otherwise be cacheable tasks.

Standardizing builds with a custom Gradle distribution

Often enterprises want to standardize the build platform for all projects in the organization by defining common conventions or rules. You can achieve that with the help of initialization scripts. Initialization scripts make it extremely easy to apply build logic across all projects on a single machine. For example, to declare a in-house repository and its credentials.

There are some drawbacks to the approach. First of all, you will have to communicate the setup process across all developers in the company. Furthermore, updating the initialization script logic uniformly can prove challenging.

Custom Gradle distributions are a practical solution to this very problem. A custom Gradle distribution is comprised of the standard Gradle distribution plus one or many custom initialization scripts. The initialization scripts come bundled with the distribution and are applied every time the build is run. Developers only need to point their checked-in Wrapper files to the URL of the custom Gradle distribution.

Custom Gradle distributions may also contain a gradle.properties file in the root of the distribution, which provide an organization-wide set of properties that control the build environment.

The following steps are typical for creating a custom Gradle distribution:

Implement logic for downloading and repackaging a Gradle distribution.
Define one or many initialization scripts with the desired logic.
Bundle the initialization scripts with the Gradle distribution.
Upload the Gradle distribution archive to a HTTP server.
Change the Wrapper files of all projects to point to the URL of the custom Gradle distribution.

Example 2. Building a custom Gradle distribution

build.gradle

plugins {
    id 'base'
}

// This is defined in buildSrc
import org.gradle.distribution.DownloadGradle

version = '0.1'

tasks.register('downloadGradle', DownloadGradle) {
    description = 'Downloads the Gradle distribution with a given version.'
    gradleVersion = '4.6'
}

tasks.register('createCustomGradleDistribution', Zip) {
    description = 'Builds custom Gradle distribution and bundles initialization scripts.'

    dependsOn downloadGradle

    def projectVersion = project.version
    archiveFileName = downloadGradle.gradleVersion.map { gradleVersion ->
        "mycompany-gradle-${gradleVersion}-${projectVersion}-bin.zip"
    }

    from zipTree(downloadGradle.destinationFile)

    from('src/init.d') {
        into "${downloadGradle.distributionNameBase.get()}/init.d"
    }
}

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