APK building

Android application project

Note: The directory names are conventional so they could be any name. Additionally, the contents of each directory could be spread across multiple folders.

Mandatory:

• 🗎 AndroidManifest.xml declares mainly the app’s package name, components, access permissions, as well as hardware and software features the app requires and the API libraries the app needs to be linked against.
• 🗎 Signing key to sign the .apk file. This is required to install or update the app on an Android device.
• 🗎 android.jar includes a set of Android platform API classes. Usually this file is already integrated into the toolchain and the programmer doesn’t need to take care himself.

Optionals:

• Resources
  • 📁 res: primarily includes elements appearing in or describing the UI.
  • 📁 assets: stores further files and will be integrated as-is into an APK to access them with ordinary file I/O.
• Sources
  • 📁 java: contains code targeting the Java Virtual Machine (JVM), so written in Java and/or Kotlin.
  • 📁 cpp: holds native code written in C/C++.
• Libraries
  • 📁 libs: comprises Java .jar and/or Android .aar archive files mostly for the compilation phase.
  • 📁 lib: contains native shared .so and/or static .a library files.

Android package (apk)

An APK is essentially a ZIP archive with an .apk extension.

While only AndroidManifest.xml and META-INF are mandatory, an APK almost always includes the following files and directories:

• 🗎 AndroidManifest.xml: the application’s manifest file, stored in Android’s binary XML format.
• 🗎 classes.dex (or classesN.dex): the compiled application code in the Dalvik Executable format. Multi-dex applications may contain multiple files (e.g., classes2.dex).
• 🗎 resources.arsc: the resource table file in binary format, optimizing the access to the UI resources.
• 📁 META-INF: contains the CERT.SF and CERT.RSA signature files, as well as the MANIFEST.MF manifest file.
• 📁 res: includes all UI resources except those from the res/values
• 📁 assets: contains further resources packed as-is into the .apk file.
• 📁 lib: contains native shared libraries of the package.

Note: An APK may contain further files and folders.

Building process

Awesome resource written by Michael Zent.

1. 📁 res (without res/values) and 🗎 AndroidManifest.xml are compiled (aapt [compile] tool)

   • -> 🗎 R.java, 🗎 resources.asrc, 📁 res (compiled), 🗎 AndroidManifest.xml (compiled)

2. 📁 java, 📁 libs, 🗎 R.java, 🗎 android.jar are compiled (java compiler [e.g. javac] and/or kotlin compiler [e.g. kotlinc])

   • -> Java bytecode (.class) files. These are then compiled/converted (d8 tool)

     • -> Dalvik bytecode (.dex) file/s

3. Native code is embedded as dynamic shared libraries called Shared Object (.so)

4. All the output are packaged (aapt [link] tool)

   • -> .apk (unsigned)

5. zipalign and apksigner to sign the APK and make it installable and updatable on an Android device.