Gradle is an advanced build system. Android Studio adopted gradle due to its flexibility, good tooling API, dependency management through Maven etc. The language used by Gradle is Groovy. A gradle project has a build.gradle file in the root of the project directory. This gradle file describes the build configuration for the project.

This article is about using gradle Extra Properties Extension to manage dependency versions and configurations, conveniently across the entire project.

Problem

Whenever there’s a version update to a library or build tools version, most developers are daunted by the repetitive task of manually inputting the new version for every library dependency. For e.g. if android support libraries version changes then you will have to update the versions everywhere they are used.

A basic build.gradle file within the app module would look similar to this:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.1"

    defaultConfig {
        minSdkVersion 21
        targetSdkVersion 24
        ...
    }

dependencies {
    ...
    compile 'com.android.support:appcompat-v7:24.1.1'
    compile 'com.android.support:appcompat-v4:24.1.1'
    compile 'com.android.support:recyclerview-v7:24.1.1'
    compile 'com.android.support:design:24.1.1'
    compile 'com.google.android.gms:play-services:9.4.0'
}

From the snippet above, we have four repeated versions 24.1.1 from com.android.support library dependencies alone. It gets worse and compounds easily if you have multiple modules within your project. However, there is a solution.

Solution

Externalize hardcoded values in the root project module build.gradle file. (Not in the app module or any sub module). This allows you to access values from any module within your project.

// Top-level build file where you can add configuration options common to all sub-projects/modules.

ext {
    compileSdkVersion = 24
    buildToolsVersion = '24.0.1'
    minSdkVersion = 21
    targetSdkVersion = 24
    supportLibraryVersion = '24.2.0'
    playServicesVersion = '9.4.0'
}

After you’ve externalized the hardcoded values in the root project directory, other sub-modules within the project can now access the values, hence making version updates a breeze. Below is a sample snippet of the app module’s build.gradle file that shows how to conveniently access the hardcoded values.

apply plugin: 'com.android.application'

android {
    compileSdkVersion rootProject.compileSdkVersion
    buildToolsVersion rootProject.buildToolsVersion

    defaultConfig {
        minSdkVersion rootProject.minSdkVersion
        targetSdkVersion rootProject.targetSdkVersion
        ...
    }

dependencies {
    ...
    // support libraries
    compile "com.android.support:appcompat-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:support-v4:$rootProject.supportLibraryVersion"
    compile "com.android.support:recyclerview-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:design:$rootProject.supportLibraryVersion"

    // play services
    compile "com.google.android.gms:play-services:$rootProject.playServicesVersion"
}

Important

Note the use of double quotation within dependencies (instead of single quotation).

We changed from single quoted strings e.g.

compile 'com.android.support:appcompat-v7:24.1.1'

to double quoted strings e.g.

compile "com.android.support:appcompat-v7:$rootProject.supportLibraryVersion"

Also note the use of $, which is simply the symbol for string interpolation in Groovy.

We cannot use single quotation here because the official documentation clearly states that any Groovy expression can be interpolated in all string literals, apart from single and triple single quoted strings.

For further reading, please refer to the references below.

References

Gradle Plugin User Guide
http://tools.android.com/tech-docs/new-build-system/user-guide

Groovy Language Documentation
http://docs.groovy-lang.org/latest/html/documentation/index.html