Android Initialization Process

【网学网提醒】：网学会员为您提供Android Initialization Process参考，解决您在Android Initialization Process学习中工作中的难题，参考学习。

    AndroidInitializationProcess---Sinkin(jinzhcheng@tom)
    init是内核启动后创建的第一个进程，由相关代码组成：system/core/init/builtins.csystem/core/init/init.csystem/core/init/devices.csystem/core/init/property_service.csystem/core/init/util.csystem/core/init/parser.csystem/core/init/logo.cItdoesthefollowingtasksstepbystep:（以下摘自Steve’sSpace）1.Initializelogsystem.2.Parse/init.rcand/init.%hardware%.rc.3.Executeearly-initactioninthetwofilesparsedinstep2.4.Devicespecificinitialize.Forexample,makealldevicenodein/devanddownloadfirmwares.5.Initializepropertysystem.Actuallythepropertysystemisworkingasasharememory.LogicallyitlookslikearegistryunderWindowssystem.6.Executeinitactioninthetwofilesparsedinstep2.7.Startpropertyservice.8.Executeearly-bootandbootactionsinthetwofilesparsedinstep2.9.Executepropertyactioninthetwofilesparsedinstep2.10.Enterintoanindefinitelooptowaitfordevice/propertyset/childprocessexitevents.Forexample,ifanSDcardisplugined,initwillreceiveadeviceaddevent,soitcanmakenodeforthedevice.Mostoftheimportantprocessisforkedininit,soifanyofthemcrashed,initwillreceiveaSIGCHLDthentranslateitintoachildprocessexitevent,sointheloopinitcanhandletheprocessexiteventandexecutethecommandsdefinedin*.rc(itwillruncommandonrestart).均通过Socket来完成监控。其中用于监控device的Socket是用来监控内核kobject子系统发送的NETLINK_KOBJECT_UEVENT事件The.rcfileisascriptfiledefinedbyAndroid.Thedefaultisdevice/system/rootdir/init.rc.Wecantakealootatthefileformat(device/system/init/readme.txtisagoodoverallintroductionofthescript).Basicallythescriptfilecontainsactionsandservices.Actions------Actionsarenamedsequencesofcommands.Actionshaveatriggerwhichisusedtodeterminewhentheactionshouldoccur.Whenaneventoccurswhichmatchesan
     action'strigger,thatactionisaddedtothetailofato-be-executedqueue(unlessitisalreadyonthequeue).Eachactioninthequeueisdequeuedinsequenceandeachcommandinthatactionisexecutedinsequence.Inithandlesotheractivities(devicecreation/destruction,propertysetting,processrestarting)"between"theexecutionofthecommandsinactivities.Actionstaketheformof:on...Services-------Servicesareprogramswhichinitlaunchesand(optionally)restartswhentheyexit.Servicestaketheformof:service[]*...Options------Optionsaremodifierstoservices.Theyaffecthowandwheninitrunstheservice.Triggers-------Triggersarestringswhichcanbeusedtomatchcertainkindsofeventsandusedtocauseanactiontooccur.Thebuiltinsupportedcommandsaredefinedindevice/sys
    tem/init/keywords.h.Commandsareimplementdindevice/system/init/bultins.c.Theinitprogramonlyexecutesfivekindsoftriggers:“early-init”,“init”,“early-boot”,“boot”,“property:*”.Takealookatthefollowinglineindefaultinit.rc.class_startdefaultThislineisacommandfortheactioncorrespondingto“boot”trigger.Itwillstartallserviceswhoseclassnameequalsto“default”.Bydefault,ifnoclassoptionisdefinedforaservice,theservice’sclassnameis“default”.Sothislinewillstartalltheservicesintheorderofpositioninthefilebydefault.(BTW,youcanstartanyserviceusingstartcommands,ifyoulike.)Anyserviceisrunasaforkedprocessofinit,takealookatthesourcecodeofservice_startindevice/system/init.c.Soaccordingtothedefaultinit.rc,thefollowingserviceswillbeexecutedstepbystep:
     console:starashell.Thesourceisindevice/system/bin/ash.adbd:startadbdaemon.Thesourceisindevice/tools/adbd.Bydefaultisdisabled.servicemanager:startbindersystem.Thesourceisinframeworks/base/cmds/servicemanager/service_manager.c.mountd:mountallfsdefinedin/system/etc/mountd.confifstarted,receivecommandsthroughlocalsockettomountanyfs.Thesourceisindevice/system/bin/mountd.debuggerd:startdebugsystem.Thesourceisindevice/system/bin/debuggerd.rild:startradiointerfacelayerdaemon.Thesourceisindevice/commands/rind.zygote:startAndroidJavaRuntimeandstartsystemserver.It’sthemostimportantservice.Thesourceisindevice/servers/app.media:startAudioFlinger,MediaPlayerServiceandCameraService.Thesourceisindevice/commands/mediaserver.bootsound:playthedefaultbootsound/system/media/audio/ui/boot.mp3.Thesourceisindevice/commands/playmp3.dbus:startdbusdaemon,it’sonlyusedbyBlueZ.Thesourceisindevice/system/Bluetooth/dbus-daemon.hcid:redirecthcid’sstdoutandstderrtotheAndroidloggingsystem.Thesourceisindevice/system/bin/logwrapper.Bydefaultisdisabled.hfag:startBluetoothhandsfreeaudiogateway,it’sonlyusedbyBlueZ.Thesourceisindevice/system/Bluetooth/bluez-utils.Bydefaultisdisabled.hsag:startBluetoothheadsetaudiogateway,it’sonlyusedbyBlueZ.Thesourceisindevice/system/Bluetooth/bluez-utils.Bydefaultisdisabled.installd:startinstallpackagedaemon.Thesourceisindevice/servers/installd.flash_recovery:load/system/recovery.img.Thesourceisindevice/commands/recovery/mtdutils.在init.rc中针对Zygote的service描述为：
    servicezygote/system/bin/app_process-Xzygote/system/bin--zygote--start-system-serversocketzygotestream666//init在启动zygoteservice时，同时创建socketonrestartwrite/sys/android_power/request_statewakeonrestartwrite/sys/power/stateon启动service的代码可参考system/core/init.c/service_start()/system/bin/app_process代码可参考fra
    meworks/base/cmds/app_process/app_main.cpp在app_process的main函数里，调用类AppRuntime(AndroidRuntime的继承类)的start成员函数：runtime.start("com.android.internal.os.ZygoteInit",startSystemServer);在该成员函数里，创建VM，并调用com.android.internal.os.ZygoteInit的main函数。
    Zygoteservicedoesthefollowingtasksstepbystep:1.CreateJAVAVM.2.RegisterandroidnativefunctionforJAVAVM.3.CallthemainfunctionintheJAVAclassnamedcom.android.internal.os.ZygoteInitwhosesourceisdevice/java/android/com/android/internal/os/ZygoteInit.java.a)LoadZygoteInitclass
     b)
    Registerzygotesocket
    首先获得ANDROID_SOCKET_zygote的环境变量值。注意init在启动zygoteservice时，同时根据service描述创建了socket，并根据socketfd更新了ANDROID_SOCKET_zygote环境变量值。调用createFileDescriptor这个Nativefunction。AndroidRuntime::startReg()里向VM注在册了很多NativeMethods。创建LocalServerSocket类对象，监听socket。
    c)d)e)
    Loadpreloadclasses(thedefaultfileisdevice/java/android/preloaded-classes)Loadpreloadresources如果启动zygote时带有参数--start-system-server，则会CallZygote::forkSystemServer(implementedindevice/dalvik/vm/InternalNative.c)toforkanewprocess.Inthenewprocess,callthemainfunctionintheJAVAclassnamedcom.android.server.SystemServer,whosesourceisinbase/java/services/com/android/server/SystemServer.java.在子进程里，会执行RuntimeInit.zygoteInit()，并调用类com.android.server.SystemServer的main函数，在该函数里完成：i.Loadlibandroid_servers.soii.CallJNInativeinit1functionimplementedinbase/core/jni/server/android_servers/com_android_server_SystemServers.Itonlycallssystem_initimplementedinbase/cmds/system_server/library/system_init.cpp.Ifrunningonsimulator,instantiateAudioFlinger,MediaPlayerServiceandCameraServicehere.Callinit2functioninJAVAclassnamedcom.android.server.SystemServer,whosesourceisindevice/java/services/com/android/server.ThisfunctionisverycriticalforAndroidbecauseitstartallofAndroidJAVAservices.(SystemServer.java/init2())Ifnotrunningonsimulator,callIPCThreadState::self()->joinThreadPool()toenterintoservicedispatcher.
    SystemServer::init2willstartanewthreadtostartallJAVAservicesasfollows:CoreServices:1.StartingPowerManager2.CreatingActivityManager3.StartingTelephonyRegistry4.StartingPackageManager5.SetActivityManagerServiceasSystemProcess6.StartingContextManager7.StartingSystemContextProviders8.StartingBatteryService
     9.StartingAlarmManager10.StartingSensorService11.StartingWindowManager12.StartingBluetoothService13.StartingMountServiceOtherservices1.StartingStatusBarService2.StartingHardwareService3.StartingNetStatService4.StartingConnectivityServic
    e5.StartingNotificationManager6.StartingDeviceStorageMonitorService7.StartingLocationManager8.StartingSearchService9.StartingClipboardService10.StartingCheckinService11.StartingWallpaperService12.StartingAudioService13.StartingHeadsetObserver14.StartingAdbSettingsObserverFinallySystemServer::init2willcallActivityManagerService.systemReadytolaunchthefirstactivitybysentingIntent.CATEGORY_HOMEintent.Thereisanotherwaytostartsystemserver,whichisthroughaprogramnamedsystem_serverwhosesourceisdevice/servers/system/system_main.cpp.Italsocallssystem_inittostartsystemservices.Sothereisaquestion:whydoesAndroidhavetwomethodstostartsystemservices?Myguessisthatdirectlystartsystem_servermayhavesynchronousproblemwithzygotebecausesystem_serverwillcallJNItostartSystemServer::init2,whileatthattimezygotemaynotstartJAVAVMyet.SoAndroidusesanothermethod.Afterzynoteisinitialized,forkanewprocesstostartsystemservices.