有关android技术英文文献翻译Word下载.docx

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●Bydefault,thesystemassignseachapplicationauniqueLinuxuserID（theIDisusedonlybythesystemandisunknowntotheapplication）.ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.

●Eachprocesshasitsownvirtualmachine（VM）,soanapplication'

scoderunsinisolationfromotherapplications.

●Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'

scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'

snolongerneededorwhenthesystemmustrecovermemoryforotherapplications.

Inthisway,theAndroidsystemimplementsthe 

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

However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:

●It'

spossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachother'

sfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocessandsharethesameVM（theapplicationsmustalsobesignedwiththesamecertificate）.

●Anapplicationcanrequestpermissiontoaccessdevicedatasuchastheuser'

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

ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Therestofthisdocumentintroducesyouto:

●Thecoreframeworkcomponentsthatdefineyourapplication.

●Themanifestwhichyoudeclarecomponentsandrequireddevicefeaturesforyourapplication.

●Resourcesthatareseparatefromtheapplicationcodeandallowyourapplicationtogracefullyoptimizeitsbehaviorforavarietyofdeviceconfigurations.

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.

Aserviceisimplementedasasubclassof 

Service 

Services 

Contentproviders

contentprovider 

managesasharedsetofapplicationdata.Youcanstorethedatainthe,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata（ifthecontentproviderallowsit）.Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuser'

scontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider（suchas 

ContactsContract.Data）toreadandwriteinformationaboutaparticularperson.

Contentprovidersarealsousefulforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,the 

NotePad 

sampleapplicationusesacontentprovidertosavenotes.

Acontentproviderisimplementedasasubclassof 

ContentProvider 

andmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,seethe 

ContentProviders 

Broadcastreceivers

broadcastreceiver 

isacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystem—forexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcasts—forexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdon'

tdisplayauserinterface,theymay 

createastatusbarnotification 

toalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcastreceiverisjusta"

gateway"

toothercomponentsandisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.

Abroadcastreceiverisimplementedasasubclassof 

BroadcastReceiver 

andeachbroadcastisdeliveredasan 

Intent 

object.Formoreinformation,seetheBroadcastReceiver 

class.

AuniqueaspectoftheAndroidsystemdesignisthatanyapplicationcanstartanotherapplication’scomponent.Forexample,ifyouwanttheusertocaptureaphotowiththedevicecamera,there'

sprobablyanotherapplicationthatdoesthatandyourapplicationcanuseit,insteadofdevelopinganactivitytocaptureaphotoyourself.Youdon'

tneedtoincorporateorevenlinktothecodefromthecameraapplication.Instead,youcansimplystarttheactivityinthecameraapplicationthatcapturesaphoto.Whencomplete,thephotoisevenreturnedtoyourapplicationsoyoucanuseit.Totheuser,itseemsasifthecameraisactuallyapartofyourapplication.

Whenthesystemstartsacomponent,itstartstheprocessforthatapplication（ifit'

snotalreadyrunning）andinstantiatestheclassesneededforthecomponent.Forexample,ifyourapplicationstartstheactivityinthecameraapplicationthatcapturesaphoto,thatactivityrunsintheprocessthatbelongstothecameraapplication,notinyourapplication'

sprocess.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdon'

thaveasingleentrypoint（there'

sno 

main（） 

function,forexample）.

Becausethesystemrunseachapplicationinaseparateprocesswiththatrestrictaccesstootherapplications,yourapplicationcannotdirectlyactivateacomponentfromanotherapplication.TheAndroidsystem,however,can.So,toactivateacomponentinanotherapplication,youmustdeliveramessagetothesystemthatspecifiesyour 

intent 

tostartaparticularcomponent.Thesystemthenactivatesthecomponentforyou.

ActivatingComponents

Threeofthefourcomponenttypes—activities,services,andbroadcastreceivers—areactivatedbyanasynchronousmessagecalledan 

intent.Intentsbindindividualcomponentstoeachotheratruntime（youcanthinkofthemasthemessengersthatrequestanactionfromothercomponents）,whetherthecomponentbelongstoyourapplicationoranother.

Anintentiscreatedwithan 

object,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecific 

type 

ofcomponent—anintentcanbeeitherexplicitorimplicit,respectively.

Foractivitiesandservices,anintentdefinestheactiontoperform（forexample,to"

view"

or"

send"

something）andmayspecifytheURIofthedatatoacton（amongotherthingsthatthecomponentbeingstartedmightneedtoknow）.Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivitytoreceivearesult,inwhichcase,theactivityalsoreturnstheresultinan 

（forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyou—thereturnintentincludesaURIpointingtothechosencontact）.

Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast（forexample,abroadcasttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicates"

batteryislow"

）.

Theothercomponenttype,contentprovider,isnotactivatedbyintents.Rather,it