Android Fundamentals

Before directly going to the application development, I thought its better to read something on the Android OS. It was quite interesting to me when I understand that each applications in this OS is treated as different user with different permissions to the assoicated files.More over this OS runs with Linux Kernal as a multi-user Linux system in which each application is a different user.

•By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that application can access them.

•Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.

•By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications

•It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).

•An application can request permission to access device data such as the user's contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.

Refer Application Fundamentals