Note: the latest Android Gradle plugin has now built in support for annotation processors and warns and/or blocks android-apt, see this wiki page on how to migrate and this blogpost for more info.

What is this?

The android-apt plugin assists in working with annotation processors in combination with Android Studio. It has two purposes:

• Allow to configure a compile time only annotation processor as a dependency, not including the artifact in the final APK or library
• Set up the source paths so that code that is generated from the annotation processor is correctly picked up by Android Studio.

This plugin requires the android or android-library plugin (version 0.9.x or up) to be configured on your project.

Including and using the plugin in your build script

Add the following to your build script to use the plugin:

#!groovy
buildscript {
    repositories {
      mavenCentral()
    }
    dependencies {
        // replace with the current version of the Android plugin
        classpath 'com.android.tools.build:gradle:1.3.0'
        // the latest version of the android-apt plugin
        classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
    }
}

apply plugin: 'com.android.application'
apply plugin: 'com.neenbedankt.android-apt'

Passing processor arguments

Some annotation processor may require to pass custom arguments, you can use apt.arguments for that purpose. For instance for AndroidAnnotations you can use the following configuration:

#!groovy
apt {
    arguments {
            resourcePackageName android.defaultConfig.applicationId
            androidManifestFile variant.outputs[0]?.processResources?.manifestFile
    }
}

AndroidAnnotations needs to know where your AndroidManifest.xml is. Retrieving it through the variant allows for different AndroidManfest.xml files for each flavor. However, not all variants (e.g. unit tests) might have the properties required, hence the groovy ?. operator to dereference these properties in a safe way. (Yes, this is a bit of a hack).

The arguments are processed for each variant when the compiler is configured. From this closure you can reference android, project and variant for the current variant. Because this is just a Groovy closure things like setting configuration based on the variant name can also be done:

#!groovy

def getManifestByVariant(variant) {
    // return the value based on the variant
    if (variant.name == 'release') {
        return '/my/path/to/the/manifest.xml'
    }
    return variant.outputs[0]?.processResources?.manifestFile
}

apt {
     arguments {
         if (variant.name == 'debug') {
            resourcePackageName "com.myapp.package.name.debug"
            // more options
         }             
         androidManifestFile project.getManifestByVariant(variant)             
     }
}

Configurating a compiler style dependency

Annotation processors generally have a API and a processor that generates code that is used by the API. Depending on the project the processor and the API might be split up in separate dependencies. For example, Dagger uses two artifacts called dagger-compiler and dagger. The compiler artifact is only used during compilation, while the main dagger artifact is required at runtime.

To aid in configuring this dependency, the plugin adds a Gradle configuration named apt that can be used like this:

#!groovy
dependencies {
 apt 'com.squareup.dagger:dagger-compiler:1.1.0'
 compile 'com.squareup.dagger:dagger:1.1.0'
}

If your instrumentation test code requires generated code to be visible in Android Studio, you can use the androidTestApt configuration:

#!groovy
dependencies {
 androidTestApt 'com.github.frankiesardo:android-auto-value-processor:0.1'
 androidTestCompile 'com.github.frankiesardo:android-auto-value:0.1'
}

For unit tests use testApt Note: requires Android Studio 1.4!

#!groovy
dependencies {
 testApt 'com.github.frankiesardo:android-auto-value-processor:0.1'
 testCompile 'com.github.frankiesardo:android-auto-value:0.1'
}

Additional configuration

It's possible to specify the class names of processors that need to be run during compilation and in that case disable the default auto discovery mechanism of javac as well, by specifying the classes in the apt block:

#!groovy
apt {
    processor "my.class.name"
    processor "another.processor.class.name"
    // if you only want to run the processors above
    disableDiscovery true
}

Configuration of other annotation processors

For annotation processors that include the API and processor in one artifact, there's no special setup. You just add the artifact to the compile configuration like usual and everything will work like normal. Additionally, if code that is generated by the processor is to be referenced in your own code, Android Studio will now correctly reference that code and resolve references to it.

FAQ

Q: When do I need this plugin?

A: You need this plugin when you include processors that generate sources that you need to import in your code. For example when using Dagger 2 or Android Annotations. Using this plugin Android Studio will be configured to place the generated sources on the build path, preventing errors in the IDE. When using the apt scope, the annotation processor will not be included in your final APK.

Q: What's the difference in using provided vs using apt for any annotation processor?

A: provided will put the annotation processor classes and it's dependencies on the IDE class path. This means that you could accidentally reference these classes. For example, if the annotation processor uses Guava, you might mistakenly import that in your Android code. At runtime this would cause a crash. When using apt the processor is not added to your class path; it's only used for annotation processing. The plugin will also make sure that Android Studio is configured to put any generated source on the IDE build path so that you can reference those in Android Studio, if needed.

History & Credits

This plugin is based on a script that I've been using for some time which is the result of this post on Google+ and this post on StackOverflow.com. Variations of the SO post and my gists have been floating around for a while on the interwebs. That, and the fact that including scripts is a bit inconvenient pushed me to create this plugin.

License

This plugin is created by Hugo Visser and released in the public domain. Feel free to use and adapt as you like. To get in touch, hit me up on Twitter or Google Plus.