和大多數(shù)Linux發(fā)行版一樣，在執(zhí)行開機(jī)后，Android也是通過引導(dǎo)加載器（bootloader）首先將Linux內(nèi)核裝載到內(nèi)存中，然后啟動(dòng)初始化進(jìn)程。初始化進(jìn)程在完成一系列初始化工作后，會(huì)啟動(dòng)相關(guān)的守護(hù)進(jìn)程、Zygote虛擬機(jī)和相關(guān)的實(shí)時(shí)進(jìn)程，進(jìn)而完成整個(gè)啟動(dòng)過程。圖1-1顯示了Android在啟動(dòng)過程中各進(jìn)程的啟動(dòng)順序。
圖1-1 Android的啟動(dòng)過程
在啟動(dòng)system\core\rootdir\init.rc腳本后，會(huì)首先創(chuàng)建系統(tǒng)運(yùn)行所需的各種環(huán)境變量，接著創(chuàng)建各種文件系統(tǒng)（如系統(tǒng)文件系統(tǒng)、用戶文件系統(tǒng)、SD Card、緩存（Cache）等）的掛載點(diǎn)，并將這些文件系統(tǒng)掛載到MTD（Memory Technology Device）上。
然后會(huì)為接下來的守護(hù)進(jìn)程和實(shí)時(shí)進(jìn)程的創(chuàng)建做些準(zhǔn)備，如設(shè)置進(jìn)程的內(nèi)存門檻、系統(tǒng)服務(wù)和守護(hù)進(jìn)程的權(quán)限等，并為各種網(wǎng)絡(luò)（如WIFI、UMTS、EDGE、GPRS、CDMA（Android 1.6后支持）等）設(shè)置所需的TCP緩存，然后即創(chuàng)建各種守護(hù)進(jìn)程，具體初始化過程如下：
初始化過程
on init
sysclktz 0 //設(shè)置系統(tǒng)時(shí)區(qū)
loglevel 3 //日志級(jí)別
# 設(shè)置全局環(huán)境變量
export PATH /sbin:/system/sbin:/system/bin:/system/xbin //設(shè)置環(huán)境變量
export LD_LIBRARY_PATH /system/lib //共享庫路徑
export ANDROID_BOOTLOGO 1
export ANDROID_ROOT /system //根路徑
export ANDROID_ASSETS /system/app
export ANDROID_DATA /data
export EXTERNAL_STORAGE /sdcard //SD卡
export BOOTCLASSPATH
/system/framework/core.jar:/system/framework/ext.jar:/system/framework/
framework.jar:/system/framework/android.policy.jar:/system/framework/services.jar //核心庫
# 后向兼容
symlink /system/etc /etc
#創(chuàng)建掛載點(diǎn)并將tmpfs掛載到sqlite_stmt_journals上
mkdir /sdcard 0000 system system
mkdir /system
mkdir /data 0771 system system
mkdir /cache 0770 system cache
mkdir /sqlite_stmt_journals 01777 root root
mount tmpfs tmpfs /sqlite_stmt_journals size=4m
mount rootfs rootfs / ro remount
write /proc/sys/kernel/panic_on_oops 1
write /proc/sys/kernel/hung_task_timeout_secs 0
write /proc/cpu/alignment 4
write /proc/sys/kernel/sched_latency_ns 10000000
write /proc/sys/kernel/sched_wakeup_granularity_ns 2000000
# 掛載MTD分區(qū)
# 首先掛載System分區(qū)以保存文件系統(tǒng)檢查點(diǎn)
mount yaffs2 mtd@system /system //掛載yaffs2文件系統(tǒng)
mount yaffs2 mtd@system /system ro remount
# 掛載Data分區(qū)，設(shè)置權(quán)限
mount yaffs2 mtd@userdata /data nosuid nodev
chown system system /data
chmod 0771 /data
# 掛載Cache分區(qū)，并設(shè)置權(quán)限
mount yaffs2 mtd@cache /cache nosuid nodev
chown system cache /cache
chmod 0770 /cache
# 設(shè)置修復(fù)路徑權(quán)限
chown system system /cache/recovery
chmod 0770 /cache/recovery
# 創(chuàng)建基本的文件系統(tǒng)結(jié)構(gòu)
mkdir /data/misc 01771 system misc
mkdir /data/misc/hcid 0770 bluetooth bluetooth
mkdir /data/local 0771 shell shell
mkdir /data/local/tmp 0771 shell shell
mkdir /data/data 0771 system system
mkdir /data/app-private 0771 system system
mkdir /data/app 0771 system system
mkdir /data/property 0700 root root
# 配置dalvik-cache權(quán)限
mkdir /data/dalvik-cache 0771 system system
chown system system /data/dalvik-cache
chmod 0771 /data/dalvik-cache
# 創(chuàng)建lost+found目錄
mkdir /data/lost+found 0770
mkdir /cache/lost+found 0770
# 配置lost+found權(quán)限
chown root root /data/lost+found
chmod 0770 /data/lost+found
chown root root /cache/lost+found
chmod 0770 /cache/lost+found
on boot
# 基本網(wǎng)絡(luò)初始化
ifup lo
hostname localhost
domainname localdomain
# 設(shè)置RLIMIT_NICE，使優(yōu)先級(jí)范圍為19～-20
setrlimit 13 40 40
# 設(shè)置rmnet超時(shí)值
write /sys/devices/virtual/net/rmnet0/timeout_suspend 5000000
//定義ActivityManagerService 用到的系統(tǒng)屬性
setprop ro.FOREGROUND_APP_ADJ 0
setprop ro.VISIBLE_APP_ADJ 1
setprop ro.SECONDARY_SERVER_ADJ 2
setprop ro.HOME_APP_ADJ 4
setprop ro.HIDDEN_APP_MIN_ADJ 7
setprop ro.CONTENT_PROVIDER_ADJ 14
setprop ro.EMPTY_APP_ADJ 15
//定義垃圾回收用到的內(nèi)存門檻
setprop ro.FOREGROUND_APP_MEM 1536
setprop ro.VISIBLE_APP_MEM 2048
setprop ro.SECONDARY_SERVER_MEM 4096
setprop ro.HOME_APP_MEM 4096
setprop ro.HIDDEN_APP_MEM 5120
setprop ro.CONTENT_PROVIDER_MEM 5632
setprop ro.EMPTY_APP_MEM 6144
write /sys/module/lowmemorykiller/parameters/adj 0,1,2,7,14,15
write /proc/sys/vm/overcommit_memory 1
write /proc/sys/vm/min_free_order_shift 4
write /sys/module/lowmemorykiller/parameters/minfree 1536,2048,4096,5120,5632,6144
#設(shè)置init進(jìn)程能夠創(chuàng)建的子進(jìn)程的數(shù)量
write /proc/1/oom_adj -16
#設(shè)置系統(tǒng)服務(wù)和守護(hù)進(jìn)程的權(quán)限
chown radio system /sys/android_power/state
chown radio system /sys/android_power/request_state
chown radio system /sys/android_power/acquire_full_wake_lock
chown radio system /sys/android_power/acquire_partial_wake_lock
chown radio system /sys/android_power/release_wake_lock
chown radio system /sys/power/state
chown radio system /sys/power/wake_lock
chown radio system /sys/power/wake_unlock
chmod 0660 /sys/power/state
chmod 0660 /sys/power/wake_lock
chmod 0660 /sys/power/wake_unlock
chown system system /sys/class/timed_output/vibrator/enable
chown system system /sys/class/leds/keyboard-backlight/brightness
chown system system /sys/class/leds/lcd-backlight/brightness
chown system system /sys/class/leds/button-backlight/brightness
chown system system /sys/class/leds/jogball-backlight/brightness
chown system system /sys/class/leds/red/brightness
chown system system /sys/class/leds/green/brightness
chown system system /sys/class/leds/blue/brightness
chown system system /sys/class/leds/red/device/grpfreq
chown system system /sys/class/leds/red/device/grppwm
chown system system /sys/class/leds/red/device/blink
chown system system /sys/class/leds/red/brightness
chown system system /sys/class/leds/green/brightness
chown system system /sys/class/leds/blue/brightness
chown system system /sys/class/leds/red/device/grpfreq
chown system system /sys/class/leds/red/device/grppwm
chown system system /sys/class/leds/red/device/blink
chown system system /sys/class/timed_output/vibrator/enable
chown system system /sys/module/sco/parameters/disable_esco
chown system system /sys/kernel/ipv4/tcp_wmem_min
chown system system /sys/kernel/ipv4/tcp_wmem_def
chown system system /sys/kernel/ipv4/tcp_wmem_max
chown system system /sys/kernel/ipv4/tcp_rmem_min
chown system system /sys/kernel/ipv4/tcp_rmem_def
chown system system /sys/kernel/ipv4/tcp_rmem_max
chown root radio /proc/cmdline
#定義各種網(wǎng)絡(luò)情況下的TCP緩沖
# ReadMin, ReadInitial, ReadMax, WriteMin, WriteInitial, WriteMax,
setprop net.tcp.buffersize.default 4096,87380,110208,4096,16384,110208
setprop net.tcp.buffersize.wifi 4095,87380,110208,4096,16384,110208
setprop net.tcp.buffersize.umts 4094,87380,110208,4096,16384,110208
setprop net.tcp.buffersize.edge 4093,26280,35040,4096,16384,35040
setprop net.tcp.buffersize.gprs 4092,8760,11680,4096,8760,11680
class_start default
# 初始化守護(hù)進(jìn)程
service console /system/bin/sh //啟動(dòng)Shell
console
# adbd由persist.service.adb.enable系統(tǒng)屬性控制
service adbd /sbin/adbd //啟動(dòng)adb守護(hù)進(jìn)程
disabled
on property:ro.kernel.qemu=1
start adbd
on property:persist.service.adb.enable=1
start adbd
on property:persist.service.adb.enable=0
stop adbd
service servicemanager /system/bin/servicemanager //啟動(dòng)IBinder通信
user system
critical
onrestart restart zygote
onrestart restart media
service vold /system/bin/vold
socket vold stream 0660 root mount
service debuggerd /system/bin/debuggerd //啟動(dòng)調(diào)試系統(tǒng)
service ril-daemon /system/bin/rild //啟動(dòng)RIL（Radio Interface Layer）守護(hù)進(jìn)程
socket rild stream 660 root radio
socket rild-debug stream 660 radio system
user root //root用戶權(quán)限
group radio cache inet misc
service zygote /system/bin/app_process -Xzygote/
system/bin --zygote --start-system-server
socket zygote stream 666
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
service media /system/bin/mediaserver //啟動(dòng)音頻管理器等媒體服務(wù)
user media
group system audio camera graphics inet net_bt net_bt_admin
service bootsound /system/bin/playmp3 //啟動(dòng)開機(jī)音樂
user media
group audio
oneshot
service dbus /system/bin/dbus-daemon --system –nofork //啟動(dòng)Dbus守護(hù)進(jìn)程
socket dbus stream 660 bluetooth bluetooth
user bluetooth
group bluetooth net_bt_admin
service hcid /system/bin/hcid -s -n -f /etc/bluez/hcid.conf //重定向hcid
socket bluetooth stream 660 bluetooth bluetooth
socket dbus_bluetooth stream 660 bluetooth bluetooth
group bluetooth net_bt_admin misc
disabled
service hfag /system/bin/sdptool add --channel=10 HFAG //啟動(dòng)藍(lán)牙免提音頻網(wǎng)關(guān)
user bluetooth
group bluetooth net_bt_admin
disabled
oneshot
service hsag /system/bin/sdptool add --channel=11 HSAG //啟動(dòng)藍(lán)牙耳機(jī)音頻網(wǎng)關(guān)
user bluetooth
group bluetooth net_bt_admin
disabled
oneshot
service installd /system/bin/installd //啟動(dòng)安裝APK包守護(hù)進(jìn)程
socket installd stream 600 system system
service flash_recovery /system/bin/flash_image recovery /system/recovery.img
oneshot