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1、英语原文AndroidApplicationFundamentalsAndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecode—alongwithanydataandresourcefiles—intoan Androidpackage,anarchivefilewithan .apk suffix.Allthecodeinasingle .apk fileisconsideredt
2、obeoneapplicationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:lTheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.lBydefaul
3、t,thesystemassignseachapplicationauniqueLinuxuserID(theIDisusedonlybythesystemandisunknowntotheapplication).ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.lEachprocesshasitsownvirtualmachine
4、(VM),soanapplication'scoderunsinisolationfromotherapplications.lBydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'snolongerneededorwhenthesystemmust
5、recovermemoryforotherapplications.Inthisway,theAndroidsystemimplementsthe principleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenvironmentinwhichanapplicationcanno
6、taccesspartsofthesystemforwhichitisnotgivenpermission.However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:lIt'spossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareableto
7、accesseachother'sfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocess共21页第20页andsharethesameVM(theapplicationsmustalsobesignedwiththesamecertificate).lAnapplicationcanrequestpermissiontoaccessdevicedatasuc
8、hastheuser'scontacts,SMSmessages,themountablestorage(SDcard),camera,Bluetooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Th
