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init is the first process after kernel started. The corresponding source code lies in: device/system/init. It does the following tasks step by step: 1.Initialize log system. 2.Parse /init.rc and /init.hardware.rc. 3. Execute early-init action in the two files parsed in step 2. 4. Device specific initialize. For example make all device node in /dev and download firmwares. 5. Initialize property system. Actually the property system is working as a share memory. Logically it looks like a registry under Windows system. 6. Execute init action in the two files parsed in step 2. 7. Start property service. 8. Execute early-boot and boot actions in the two files parsed in step 2. 9. Execute property action in the two files parsed in step 2. 10. Enter into an indefinite loop to wait for device/property set/child process exit events. For example if an SD card is plugined init will receive a device add event so it can make node for the device. Most of the important process is forked in init so if any of them crashed init will receive a SIGCHLD then translate it into a child process exit event so in the loop init can handle the process exit event and execute the commands defined in .rcit will run command onrestart. The .rc file is a script file defined by Android. The default is device/system/rootdir/init.rc. We can take a loot at the file formatdevice/system/init/readme.txt is a good overall introduction of the script. Basically the script file contains actions and services. Actions Actions are named sequences of commands. Actions have a trigger which is used to determine when the action should occur. When an event occurs which matches an actions trigger that action is added to the tail of a to-be-executed queue unless it is already on the queue. Each action in the queue is dequeued in sequence and each command in that action is executed in sequence. Init handles other activities device creation/destruction property setting process restarting between the execution of the commands in activities. Actions take the form of: on ... Services Services are programs which init launches and optionally restarts when they exit. Services take the form of: service ... Options Options are modifiers to services. They affect how and when init runs the service. Triggers Triggers are strings which can be used to match certain kinds of events and used to cause an action to occur. The builtin supported commands are defined in device/system/init/keywords.h. Commands are implementd in device/system/init/bultins.c. The init program only executes five kinds of triggers: “early-init” “init” “early-boot” “boot” “property:”. Take a look at the following line in default init.rc. class_start default This line is a command for the action corresponding to “boot” trigger. It will start all services whose class name equals to “default”. By default if no class option is defined for a service the service’s class name is “default”. So this line will start all the services in the order of position in the file by default. BTW you can start any service using start commands if you like. Any service is run as a forked process of init take a look at the source code of service_start in device/system/init.c. So according to the default init.rc the following services will be executed step by step: console: star a shell. The source is in device/system/bin/ash. adbd: start adb daemon. The source is in device/tools/adbd. By default is disabled. servicemanager: start binder system. The source is in device/commands/binder. mountd: mount all fs defined in /system/etc/mountd.conf if started receive commands through local socket to mount any fs. The source is in device/system/bin/mountd. debuggerd: start debug system. The source is in device/system/bin/debuggerd. rild: start radio interface layer daemon. The source is in device/commands/rind. zygote: start Android Java Runtime and start system server. It’s the most important service. The source is in device/servers/app. media: start AudioFlinger MediaPlayerService and CameraService. The source is in device/commands/mediaserver. bootsound: play the default boot sound /system/media/audio/ui/boot.mp3. The source is in device/commands/playmp3. dbus: start dbus daemon it’s only used by BlueZ. The source is in device/system/Bluetooth/dbus-daemon. hcid: redirect hcid’s stdout and stderr to the Android logging system. The source is in device/system/bin/logwrapper. By default is disabled. hfag: start Bluetooth handsfree audio gateway it’s only used by BlueZ. The source is in device/system/Bluetooth/bluez-utils. By default is disabled. hsag: start Bluetooth headset audio gateway it’s only used by BlueZ. The source is in device/system/Bluetooth/bluez-utils. By default is disabled. installd: start install package daemon. The source is in device/servers/installd. flash_recovery: load /system/recovery.img. The source is in device/commands/recovery/mtdutils. Zygote service does the following tasks step by step: 1. Create JAVA VM. 2. Register android native function for JAVA VM. 3. Call the main function in the JAVA class named com.android.internal.os.ZygoteInit whose source is device/java/android/com/android/internal/os/ZygoteInit.java. a Load ZygoteInit class b Register zygote socket c Load preload classesthe default file is device/java/android/preloaded-classes d Load preload resources e Call Zygote::forkSystemServer implemented in device/dalvik/vm/InternalNative.c to fork a new process. In the new process call the main function in the JAVA class named com.android.server.SystemServer whose source is in device/java/services/com/android/server. i. Load libandroid_servers.so ii. Call JNI native init1 function implemented in device/libs/android_servers/com_android_server_SystemServers. It only calls system_init implemented in device/servers/system/library/system_init.cpp. If running on simulator instantiate AudioFlinger MediaPlayerService and CameraService here. Call init2 function in JAVA class named com.android.server.SystemServer whose source is in device/java/services/com/android/server. This function is very critical for Android because it start all of Android JAVA services. If not running on simulator call IPCThreadState::self-joinThreadPool to enter into service dispatcher. SystemServer::init2 will start a new thread to start all JAVA services as follows: Core Services: 1. Starting Power Manager // 2. Creating Activity Manager 3. Starting Telephony Registry // 4. Starting Package Manager // 5. Set Activity Manager Service as System Process 6. Starting Context Manager 7. Starting System Context Providers 8. Starting Battery Service // 9. Starting Alarm Manager // 10. Starting Sensor Service 11. Starting Window Manager 12. Starting Bluetooth Service // 13. Starting Mount Service // Other services // 1. Starting Status Bar Service 2. Starting Hardware Service 3. Starting NetStat Service 4. Starting Connectivity Service 5. Starting Notification Manager 6. Starting DeviceStorageMonitor Service 7. Starting Location Manager 8. Starting Search Service 9. Starting Clipboard Service 10. Starting Checkin Service 11. Starting Wallpaper Service 12. Starting Audio Service 13. Starting HeadsetObserver 14. Starting AdbSettingsObserver Finally SystemServer::init2 will call ActivityManagerService.systemReady to launch the first activity by senting Intent.CATEGORY_HOME intent. There is another way to start system server which is through a program named system_server whose source is device/servers/system/system_main.cpp. It also calls system_init to start system services. So there is a question: why does Android have two methods to start system services My guess is that directly start system_server may have synchronous problem with zygote because system_server will call JNI to start SystemServer::init2 while at that time zygote may not start JAVA VM yet. So Android uses another method. After zynote is initialized fork a new process to start system services. Android 启动过程 1. Boot系统初始化 具体过程参见systemcoreinitInit.c中的main函数这时候手机或者模拟器出现的画面是一个console显示“ANDROID”msg。