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1、英语原文AndroidApplicationFundamentalsAndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecode—alongwithanydataandresourcefiles—intoan Androidpackage,anarchivefilewithan .apk suffix.Allthecodeinasingle .apk fileisconsideredtobeoneapp
2、licationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:lTheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.lBydefault,thesystemassigns
3、eachapplicationauniqueLinuxuserID(theIDisusedonlybythesystemandisunknowntotheapplication).ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.lEachprocesshasitsownvirtualmachine(VM),soanapplication'scoder
4、unsinisolationfromotherapplications.lBydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'snolongerneededorwhenthesystemmustrecovermemoryforotherapplications.In
5、thisway,theAndroidsystemimplementsthe principleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenvironmentinwhichanapplicationcannotaccesspartsofthesystemforwhichitisnotgivenpe
6、rmission.However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:lIt'spossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachother'sfiles.Toconservesystemresources,appli
7、cationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocess共21页第20页andsharethesameVM(theapplicationsmustalsobesignedwiththesamecertificate).lAnapplicationcanrequestpermissiontoaccessdevicedatasuchastheuser'scontacts,SMSmessages,themountablestorage(SDcard),ca
8、mera,Bluetooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Th
