Android App Basics

Overview

Android apps are built as a combination of components that can be invoked individually.
For example, an activity is a type of app component that provides a user interface.

The "main" activity is what starts when the user taps your app icon
Other components such as broadcast receivers and services also allow your app to perform background tasks without a user interface.

Android apps can be written using Kotlin, Java, and C++ languages. The Android SDK tools compile your code along with any data and resource files into an APK, an Android package, which is an archive file with an .apk suffix. One APK file contains all the contents of an Android app and is the file that Android-powered devices use to install the app.

Each Android app lives in its own security sandbox, protected by the following Android security features:

• The Android operating system is a multi-user Linux system in which each app is a different user.
• By default, the system assigns each app a unique Linux user ID (the ID is used only by the system and is unknown to the app). The system sets permissions for all the files in an app so that only the user ID assigned to that app can access them.
• Each process has its own virtual machine (VM), so an app's code runs in isolation from other apps.
• By default, every app runs in its own Linux process. The Android system starts the process when any of the app's components need to be executed, and then shuts down the process when it's no longer needed or when the system must recover memory for other apps.

The Android system implements the principle of least privilege. That is, each app, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an app cannot access parts of the system for which it is not given permission.

App components

App resources

For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your app code or from other resources defined in XML. For example, if your app contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo. This ID maps to an app-specific integer, which you can use to reference the image and insert it in your user interface.

App Manifest

Every app project must have an AndroidManifest.xml file (with precisely that name) at the root of the project source set. The manifest file describes essential information about your app to the Android build tools, the Android operating system, and Google Play.

The manifest file is required to declare the following:

• The app's package name
• The components of the app
• The permissions that the app needs in order to access protected parts of the system or other apps.
• The hardware and software features the app requires