毕业论文外文文献翻译Android-ApplicationFundamentals安卓应用基础Word下载.doc

毕业论文外文文献翻译Android-ApplicationFundamentals安卓应用基础Word下载.doc

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2017.02.14

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英语原文

AndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecode—alongwithanydataandresourcefiles—intoan 

Androidpackage,anarchivefilewithan 

.apk 

suffix.Allthecodeinasingle 

fileisconsideredtobeoneapplicationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.

Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:

lTheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.

lBydefault,thesystemassignseachapplicationauniqueLinuxuserID（theIDisusedonlybythesystemandisunknowntotheapplication）.ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.

lEachprocesshasitsownvirtualmachine（VM）,soanapplication'

scoderunsinisolationfromotherapplications.

lBydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'

scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'

snolongerneededorwhenthesystemmustrecovermemoryforotherapplications.

Inthisway,theAndroidsystemimplementsthe 

principleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenvironmentinwhichanapplicationcannotaccesspartsofthesystemforwhichitisnotgivenpermission.

However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:

lIt'

spossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachother'

sfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocessandsharethesameVM（theapplicationsmustalsobesignedwiththesamecertificate）.

lAnapplicationcanrequestpermissiontoaccessdevicedatasuchastheuser'

scontacts,SMSmessages,themountablestorage（SDcard）,camera,Bluetooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.

ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Therestofthisdocumentintroducesyouto:

lThecoreframeworkcomponentsthatdefineyourapplication.

lThemanifestfileinwhichyoudeclarecomponentsandrequireddevicefeaturesforyourapplication.

lResourcesthatareseparatefromtheapplicationcodeandallowyourapplicationtogracefullyoptimizeitsbehaviorforavarietyofdeviceconfigurations.

ApplicationComponents

ApplicationcomponentsaretheessentialbuildingblocksofanAndroidapplication.Eachcomponentisadifferentpointthroughwhichthesystemcanenteryourapplication.Notallcomponentsareactualentrypointsfortheuserandsomedependoneachother,buteachoneexistsasitsownentityandplaysaspecificrole—eachoneisauniquebuildingblockthathelpsdefineyourapplication'

soverallbehavior.

Therearefourdifferenttypesofapplicationcomponents.Eachtypeservesadistinctpurposeandhasadistinctlifecyclethatdefineshowthecomponentiscreatedanddestroyed.

Herearethefourtypesofapplicationcomponents:

Activities

An 

activity 

representsasinglescreenwithauserinterface.Forexample,anemailapplicationmighthaveoneactivitythatshowsalistofnewemails,anotheractivitytocomposeanemail,andanotheractivityforreadingemails.Althoughtheactivitiesworktogethertoformacohesiveuserexperienceintheemailapplication,eachoneisindependentoftheothers.Assuch,adifferentapplicationcanstartanyoneoftheseactivities（iftheemailapplicationallowsit）.Forexample,acameraapplicationcanstarttheactivityintheemailapplicationthatcomposesnewmail,inorderfortheusertoshareapicture.

Anactivityisimplementedasasubclassof 

Activity 

andyoucanlearnmoreaboutitinthe 

Activities 

developerguide.

Services

A 

service 

isacomponentthatrunsinthebackgroundtoperformlong-runningoperationsortoperformworkforremoteprocesses.Aservicedoesnotprovideauserinterface.Forexample,aservicemightplaymusicinthebackgroundwhiletheuserisinadifferentapplication,oritmightfetchdataoverthenetworkwithoutblockinguserinteractionwithanactivity.Anothercomponent,suchasanactivity,canstarttheserviceandletitrunorbindtoitinordertointeractwithit.

Aserviceisimplementedasasubc