长春理工大学光电信息学院毕业设计

长春理工大学光电信息学院毕业设计

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App lication Fundamentals

An droid app licati ons are writte n in the Java p rogram ming Ian guage. The comp iled Java code— along with any data and resource files required by the app licatio— is bun dled by the aapt tool i nto an An droid p ackage an archive file marked by an.apk suffix. This file is the vehicle for distribut ing the app licati on and in stalli ng it on mobile devices; it's the file users dow nl oad to their devices. All the code in a sin glea pk file is con sidered to be oneapp licati on.

In many ways, each An droid app licati on lives in its own world:

By default, every app licati on runs in its own Linux p rocess. An droid starts the

p rocess whe n any of the app licatio n's code n eeds to be executed, and shuts dow n the p rocess whe n it's no Ion ger n eeded and system resources are required by other app licati ons.

Each p rocess has its own virtual machi ne (VM), so app licati on code runs in isolati on from the code of all other app licati ons.

By default, each app licati on is assig ned a unique Linux user ID. P ermissi ons are set so that the app licati on's files are visible on ly to that user and only to the app licati on itself —although there are ways to export them to other app licati ons as well.

It's p ossible to arrange for two app licati ons to share the same user ID, i n which case they will be able to see each other's files. To con serve system resources, app licati ons with the same ID can also arrange to run in the same Linux p rocess, shari ng the same VM.

App lication Comp onents

A cen tral feature of An droid is that one app licatio n can make use of eleme nts of other app licatio ns (p rovided those app licati ons p ermit it). For exa mp le, if your app licati on n eeds to dis play a scrolli ng list of images and ano ther app licatio n has devel oped a suitable scroller and made it available to others, you can call upon that scroller to do the work, rather tha n devel op y