Unraveling the Mystery of Android Resource Parsing: AAPT and AAPT2 Demystified

As an Android developer, you’ve likely encountered the enigmatic `resources.arsc` file, which holds the key to your app’s user interface and behavior. But have you ever wondered how Android parses this file, and why AAPT and AAPT2 sometimes give different results? In this in-depth guide, we’ll delve into the inner workings of Android resource parsing, exploring the roles of AAPT and AAPT2, and providing practical insights to help you master this crucial aspect of Android development.

What is resources.arsc, and why is it important?

The `resources.arsc` file is a binary file containing compiled Android resources, such as layouts, strings, and drawables. It’s generated by the Android asset packaging tool (AAPT) during the build process and is essential for running your app on Android devices. The `resources.arsc` file serves as a centralized repository for your app’s resources, allowing the Android system to efficiently access and utilize them.

The roles of AAPT and AAPT2 in resource parsing

AAPT (Android Asset Packaging Tool) and AAPT2 are two distinct tools used in the Android build process. While they share some similarities, they have different purposes and approaches to resource parsing.

• AAPT (Android Asset Packaging Tool): AAPT is the original asset packaging tool used in older versions of Android. It’s responsible for compiling resources into the `resources.arsc` file, which is then packaged into the APK file. AAPT uses a more traditional approach, relying on the Java-based `android.utils` package to parse resources.
• AAPT2 (Android Asset Packaging Tool 2): Introduced in Android 8.0 (Oreo), AAPT2 is a reimplementation of AAPT, designed to improve performance, security, and flexibility. AAPT2 uses a native, C++-based implementation to parse resources, making it faster and more efficient than AAPT. AAPT2 is the default tool used in modern Android builds.

How Android parses resources.arsc

When an Android app is launched, the system loads the `resources.arsc` file into memory. The parsing process involves the following steps:

1. Resource identification**: Android identifies the resources required by the app, such as layouts, strings, and drawables, based on the app’s configuration and runtime environment.
2. Chunk-based parsing**: The `resources.arsc` file is divided into chunks, each containing a specific type of resource. Android parses these chunks, extracting the necessary resource data.
3. Resource lookup**: Android uses a resource lookup mechanism to locate specific resources within the parsed chunks. This involves hashing the resource identifiers and using the resulting hash values to index into the resource tables.
4. Resource loading**: The required resources are loaded into memory, making them available for the app to use.

android {
    ...
    aaptOptions {
        cruncherEnabled = false
    }
}

android {
    ...
    compileSdkVersion 29
    buildToolsVersion "29.0.3"
}

android {
    ...
    defaultConfig {
        resourceConfigurations += "en"
    }
}