Migrate Android Project to Gradle Build System

Gradle is a new build system that Android is currently promoting. It can be used to build Android project by adding a Gradle Android plugin. It is also possible that Android would move to Gradle-only build system, ditching the old ant-way to build things. The Android Studio only supports Gradle projects.

Quickstart

To join the new family, we need to write a new build.gradle file in project root. An general example of build.gradle would look like the snippet below. Please note that the system is still in active development, version numbers could change very soon. I’m using Gradle 1.7, plugin 0.5.6, Android Build Tools 17 and my target SDK version is 15.

buildscript {
  repositories {
    mavenCentral()
  }

  dependencies {
    classpath 'com.android.tools.build:gradle:0.5.6'
  }
}

apply plugin: 'android'

android {
  compileSdkVersion 15
  buildToolsVersion "17"

  sourceSets {
    main {
      manifest.srcFile 'AndroidManifest.xml'
        java.srcDirs = ['src']
        res.srcDirs = ['res']
    }
  }
}

This simple gradle file should be able to build most of standard Android projects. But what if I have special need? I’m going to talk about how to add support for AndroidAnnotation and token replacement below.

Android Annotation

AndroidAnnotation is a set of very useful annotations that you could use in your code to make your code shorter and more easier to read. The framework utilize a annotation processor to scan through your source code file and generate helper classes. To integrate it, we need to do 2 things:

  • Add processor flags to Java compiler
  • Add androidannotation-api dependency

Manipulating Compile Task

To add extra flags to compiler, we need to use the interface Android Gradle plugin exposed to access relevant tasks. The build plugin organize tasks into different “variants”, some basic variants includes debug and release. To iterate through all variants, we could use android.applicationVariants.all.

// Code based on https://github.com/excilys/androidannotations/issues/676
def getSourceSetName(variant) {
  return new File(variant.dirName).getName();
}

android.applicationVariants.all { variant ->

  def aptOutputDir = project.file("build/source/apt")
  def aptOutput = new File(aptOutputDir, variant.dirName)

  android.sourceSets[getSourceSetName(variant)].java.srcDirs += aptOutput.getPath()

  variant.javaCompile.options.compilerArgs += [
    '-processorpath', configurations.apt.getAsPath(),
    '-s', aptOutput
  ]

  variant.javaCompile.source = variant.javaCompile.source.filter { p ->
    return !p.getPath().startsWith(aptOutputDir.getPath())
  }

  variant.javaCompile.doFirst {
    aptOutput.mkdirs()
  }

}

The configurations.apt is missing at the moment. We used the variable to store path to Android Annotation processor and we use Gradle’s dependencies managing feature to resolve the path for us.

configurations {
  apt
}

dependencies {
  apt files('compile-libs/androidannotations-2.7.1.jar')
}

Add dependencies

For managing dependencies, Gradle uses a dependencies block. The following code snippets shows different ways to include dependencies. fileTree can be used to iterate files in directory, string will be resolved using repositories(default includes Maven Central) and project is used to reference dependent local projects. We will talk about multi-project build support later.

dependencies {
  compile fileTree(dir: 'libs', include: '*.jar', exclude: 'android-support-v4.jar')
  compile 'com.android.support:support-v4:18.0.+'
  compile project(':extra:actionbarsherlock')
  compile project(':extra:ViewPagerIndicator')
}

Say if you put the Android Annotation API jar into libs directory, the build system should picked it up by now.

Token Replacement

Sometimes we would like to have a way to reference build version, git revisions from Java code. To accomplish this, we need to collect those informations and put it in a Java source file which will be picked out later during compilation.

Let’s say if you have a AppBuild.java looks like below and we put it in compile-libs directory.

package idv.Zero.example;

public class AppBuild {
  public static final String GIT_REV = "@git-rev@";
}

Note the @git-rev@ is the token we will replace with current git revision number each time we build the project. Now we have the template, we need to rig it into the build system. The way I’m doing this is by adding a generation task per variant and setup the dependency for compiling task to be depend on this generation task.

import org.apache.tools.ant.filters.ReplaceTokens

android.applicationVariants.all { variant ->
  def taskName = "generateAppBuild${variant.dirName.capitalize()}"

  task (taskName, type: Copy) {
    def todir = "build/source/AppBuild/${variant.dirName}/cc/hypo/pieceroids"
    new File(todir).mkdirs()

    from project.file('compile-libs/AppBuild.java')
    into todir
    outputs.upToDateWhen { false }

    def proc = "git rev-parse --verify HEAD".execute()
    proc.waitFor()

    filter(ReplaceTokens, tokens:['git-rev': proc.in.text.trim()])
  }
  tasks["compile${variant.dirName.capitalize()}"].dependsOn(taskName)
}

What this code snippets do is to initiate a copy task by the name generateAppBuild${variant}. Note the outputs.upToDateWhen line would cause the task to always run, otherwise it’ll be skipped if the source file is not changed, which is not what we want. You’ll also need to import ReplaceTokens filter from our good old friend, ant.

Multi-Projects

It is very common that you might have some sub-projects that needs to be build together. Previously, I showed that you could have compile project(':path:to:project') in dependencies block to have it being included during compile time. However, you need some extra setup to make it work. You need an extra file called settings.gradle with those lines.

include ":extra:actionbarsherlock"
include ":extra:ViewPagerIndicator"

This indicates you have two sub-projects, they’re located at ${project.root}/extra/actionbarsherlock and ${project.root}/extra/ViewPagerIndicator. Gradle will look for build.gradle inside those directories and set up project dependencies automatically.

Conclusion

In this article, I showed how to migrate your android project to utilize Gradle as the build system. Additionally, I also showed a way to integrate Android Annotations and build information generation. I hope these would be enough to get your started with Gradle. Here is the final build.gradle you should have after all these extra codes.

import org.apache.tools.ant.filters.ReplaceTokens

buildscript {
  repositories {
    mavenCentral()
  }

  dependencies {
    classpath 'com.android.tools.build:gradle:0.5.6'
  }
}

apply plugin: 'android'

configurations {
  apt
}

dependencies {
  compile fileTree(dir: 'libs', include: '*.jar', exclude: 'android-support-v4.jar')
  compile 'com.android.support:support-v4:18.0.+'
  compile project(':extra:actionbarsherlock')
  compile project(':extra:ViewPagerIndicator')

  apt files('compile-libs/androidannotations-2.7.1.jar')
}

def getSourceSetName(variant) {
    return new File(variant.dirName).getName();
}

android.applicationVariants.all { variant ->
  def aptOutputDir = project.file("build/source/apt")
  def aptOutput = new File(aptOutputDir, variant.dirName)

  android.sourceSets[getSourceSetName(variant)].java.srcDirs += aptOutput.getPath()

  variant.javaCompile.options.compilerArgs += [
    '-processorpath', configurations.apt.getAsPath(),
    '-s', aptOutput
  ]

  variant.javaCompile.source = variant.javaCompile.source.filter { p ->
    return !p.getPath().startsWith(aptOutputDir.getPath())
  }

  variant.javaCompile.doFirst {
    aptOutput.mkdirs()
  }

  android.sourceSets[getSourceSetName(variant)].java.srcDirs += "build/source/AppBuild/${variant.dirName}"

  def taskName = "generateAppBuild${variant.dirName.capitalize()}"
  task (taskName, type: Copy) {
    def todir = "build/source/AppBuild/${variant.dirName}/cc/hypo/pieceroids"
    new File(todir).mkdirs()

    from project.file('compile-libs/AppBuild.java')
    into todir
    outputs.upToDateWhen { false }

    def proc = "git rev-parse --verify HEAD".execute()
    proc.waitFor()

    filter(ReplaceTokens, tokens:['git-rev': proc.in.text.trim()])
  }
  tasks["compile${variant.dirName.capitalize()}"].dependsOn(taskName)
}

android {
  compileSdkVersion 15
  buildToolsVersion "17"

  sourceSets {
    main {
      manifest.srcFile 'AndroidManifest.xml'
        java.srcDirs = ['src']
        res.srcDirs = ['res']
    }
  }
}