If you do not want to move files around, it is possible to keep the Eclipse folder structure in your project. To do that, you need to change the source set configuration. We mentioned source sets earlier when talking about the project structure. Gradle and the Android plugin have their defaults, as usual, but it is possible to override those.

The first thing you need to do is to create a build.gradle file in the project directory. This file should apply the Android plugin and define the required properties for Gradle and the Android plugin. In its simplest form, it looks like this:

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.2.3'
    }
}

apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"
}

Then you can start by changing the source set. Usually, overriding the main source set to comply with the Eclipse structure looks like this:

android {
  sourceSets {
    main {
      manifest.srcFile 'AndroidManifest.xml'
      java.srcDirs = ['src']
      resources.srcDirs = ['src']
      aidl.srcDirs = ['src']
      renderscript.srcDirs = ['src']
      res.srcDirs = ['res']
      assets.srcDirs = ['assets']
    }

  androidTest.setRoot('tests')
  }
}

In the Eclipse folder structure, all source files will reside in the same folder, so you need to tell Gradle that all these components can be found in the src folder. You only need to include the components that are in your project, but adding them all does no harm.

If you have any dependencies on JAR files, you need to tell Gradle where the dependencies are located. Assuming the JAR files are in a folder called libs, the configuration looks like this:

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
}

This one-liner includes every file with the extension .jar inside the libs directory as a dependency.

If you decide to convert to the new project structure manually, you need to create a few folders and move some files. This table shows an overview of the most important files and folders, and where you need to move them to convert to the new project structure:

If you have any unit tests, you need to move the source code for those to app/src/test/java/ to have Gradle recognize them automatically. Functional tests belong in the app/src/androidTest/java/ folder.

The next step is to create a settings.gradle file in the root of the project. This file needs to contain only one line, and its purpose is to tell Gradle to include the app module in the build:

include: ':app'

When that is ready, you need two build.gradle files for a successful Gradle build. The first one belongs in the root of the project (on the same level as settings.gradle) and is used to define project-wide settings:

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.2.3'
    }
}

This sets up a few properties for all modules in the project. The second build.gradle goes in the app folder and contains module-specific settings:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"
}

These are the absolute basics. If you have a simple Android app that does not depend on third-party code, this will suffice. If you have any dependencies, you need to migrate those to Gradle as well.

If you have any libraries in your project that contain Android-specific code, those also need to use Gradle in order for them to play nice with the app module. The same basics apply, but you need to use the Android library plugin instead of the Android application plugin. The details of this process are discussed in Chapter 5, Managing Multimodule Builds.