HB Blog 51: Android Build System - How Android Apk Is Built.

The Android build system is the toolkit you use to build, test, run and package your apps. The build system can run as an integrated tool from the Android Studio menu and independently from the command line.The build process involves many tools and processes that generate intermediate files on the way to producing an .apk. The following diagram depicts the different tools and processes that are involved in a build:

The general process for a typical build is outlined below. The build system merges all the resources from the configured product flavors, build types, and dependencies. If different folders contain resources with the same name or setting, the following override priority order is: dependencies override build types, which override product flavors, which override the main source directory.

• The Android Asset Packaging Tool (aapt) takes your application resource files, such as the AndroidManifest.xml file and the XML files for your Activities, and compiles them. An R.java is also produced so you can reference your resources from your Java code.
• The aidl tool converts any .aidl interfaces that you have into Java interfaces.
• All of your Java code, including the R.java and .aidl files, are compiled by the Java compiler and .class files are output.
• The dex tool converts the .class files to Dalvik byte code. Any 3rd party libraries and .class files that you have included in your module build are also converted into .dex files so that they can be packaged into the final .apk file.
• All non-compiled resources (such as images), compiled resources, and the .dex files are sent to the apkbuilder tool to be packaged into an .apk file.
• Once the .apk is built, it must be signed with either a debug or release key before it can be installed to a device.
• Finally, if the application is being signed in release mode, you must align the .apk with the zipalign tool. Aligning the final .apk decreases memory usage when the application is -running on a device.