安卓应用基础-外文文献翻译.doc

Android Application Fundamentals Android applications are written in the Java programming language. The Android SDK pile the code — along with any data and resource files — into an Android package , an archive file with an .apk suffix. All the code ina single .apk file is considered tobeone application and is the file that Android-powered devices use to install the application. Once installed ona device, each Android application lives in its own security sandbox: The Android operating system isa 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'ponents need tobe executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications. In this way, the Android system implements the principle of least privilege . That is, each application, by default, has access only to ponents that it requires todo its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it isnot given permission. However, there are ways for an application to share data with other applications and for an application to access system services: 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