Android 启动过程介绍

一般开机过程大致可以分为三个大阶段：

1. OS级别，由bootloader载入linux kernel后，kernel开始初始化, 并载入built-in的驱动程序。Kernel完成开机后，载入init process，切换至user-space后，结束kernel 的循序过程（sequence），进入排程模式（process scheduling）。

2. Android-level，由init process 开始，读取init.rc，Native服务启动，并启动重要的外部程序，例如：servicemanager、Zygote以及System Server。

3. Zygote-Mode，Zygote启动完System Server后，进入Zygote Mode，在Socket 等候命令。随后，使用者将看到一个桌面环境（Home Screen）。桌面环境由一个名为Launcher的应用程序负责提供。

注：Zygote主要负责启动System server和执行android程序（APK）。成功启动System server后会使用socket方式监听（monitorandroid apps/prcesses）

查看init.rc文件显示，系统首先启动servicemanager，然后启动zygote：

service ueventd /sbin/ueventd
    class core
    critical

service console /system/bin/sh
    class core
    console
    disabled
    user shell
    group log
service adbd /sbin/adbd
    class core
    disabled
service servicemanager /system/bin/servicemanager
    class core
    user system
    group system
    critical
    onrestart restart zygote
    onrestart restart media
    onrestart restart surfaceflinger
    onrestart restart drm

service vold /system/bin/vold
    class core
    socket vold stream 0660 root mount
    ioprio be 2

service netd /system/bin/netd
    class main
    socket netd stream 0660 root system
    socket dnsproxyd stream 0660 root inet
    socket mdns stream 0660 root system

service debuggerd /system/bin/debuggerd
    class main
service surfaceflinger /system/bin/surfaceflinger
    class main
    user system
    group graphics
    onrestart restart zygote

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart media
    onrestart restart netd

service drm /system/bin/drmserver
    class main
    user drm
    group drm system inet drmrpc sdcard_r

service media /system/bin/mediaserver
    class main
    user media
    group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc system
    ioprio rt 4

service bootanim /system/bin/bootanimation
    class main
    user graphics
    group graphics
    disabled
    oneshot

service dbus /system/bin/dbus-daemon --system --nofork
    class main
    socket dbus stream 660 bluetooth bluetooth
    user bluetooth
    group bluetooth net_bt_admin

service bluetoothd /system/bin/bluetoothd -n
    class main
    socket bluetooth stream 660 bluetooth bluetooth
    socket dbus_bluetooth stream 660 bluetooth bluetooth
    # init.rc does not yet support applying capabilities, so run as root and
    # let bluetoothd drop uid to bluetooth with the right linux capabilities
    group bluetooth net_bt_admin misc
    disabled

service installd /system/bin/installd
    class main
    socket installd stream 600 system system

service flash_recovery /system/etc/install-recovery.sh
    class main
    oneshot
service racoon /system/bin/racoon
    class main
    socket racoon stream 600 system system
    # IKE uses UDP port 500. Racoon will setuid to  after binding the port.
    group  net_admin inet
    disabled
    oneshot

service mtpd /system/bin/mtpd
    class main
    socket mtpd stream 600 system system
    user 
    group  net_admin inet net_raw
    disabled
    oneshot

service keystore /system/bin/keystore /data/misc/keystore
    class main
    user keystore
    group keystore drmrpc
    socket keystore stream 666

service dumpstate /system/bin/dumpstate -s
    class main
    socket dumpstate stream 0660 shell log
    disabled
    oneshot

service sshd /system/bin/start-ssh
    class main
    disabled

service mdnsd /system/bin/mdnsd
    class main
    user mdnsr
    group inet net_raw
    socket mdnsd stream 0660 mdnsr inet
    disabled
    oneshot

service vtserver /system/bin/vtserver
    user root
    disabled
    oneshot