第一步：啓動linux
1.Bootloader
2.Kernel
第二步android系統啓動：入口爲init.rc(system\core\rootdir)
1./system/bin/service manager: Binder 守護進程；
2.Runtime;
3.Zygote :app-process/app-main;
4.Start VM;
5.Start server
6.Start android service:Register to service Manager
7.Start Launcher
第三步：應用程序啓動：運行package Manager
1.Init進程
Android系統在啓動時首先會啓動Linux系統，引導加載Linux Kernel並啓動init進程。Init進程是一個由內核啓動的用戶級進程，是Android系統的第一個進程。該進程的相關代碼在platform\system\core\init\init.c。在main函數中，有如下代碼：
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open_devnull_stdio();
log_init();
INFO("reading config file\n");
init_parse_config_file("/init.rc");
/* pull the kernel commandline and ramdisk properties file in */
import_kernel_cmdline(0);
get_hardware_name(hardware, &revision);
snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);
init_parse_config_file(tmp);
這裏會加載解析init.rc和init.hardware.rc兩個初始化腳本。*.rc文件定義了在init進程中需要啓動哪些進程服務和執行哪些動作。其詳細說明參見platform\system\core\init\reademe.txt。init.rc見如下定義：
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service servicemanager /system/bin/servicemanager
user system
critical
onrestart restart zygote
onrestart restart media
service vold /system/bin/vold
socket vold stream 0660 root mount
ioprio be 2
service netd /system/bin/netd
socket netd stream 0660 root system
socket dnsproxyd stream 0660 root inet
service debuggerd /system/bin/debuggerd
service ril-daemon /system/bin/rild
socket rild stream 660 root radio
socket rild-debug stream 660 radio system
user root
group radio cache inet misc audio sdcard_rw
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
onrestart restart media
onrestart restart netd
service drm /system/bin/drmserver
user drm
group system root inet
具體解析過程見platform\system\core\init\Init_parser.c。解析所得服務添加到service_list中，動作添加到action_list中。
接下來在main函數中執行動作和啓動進程服務：
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execute_one_command();
restart_processes();
通常init過程需要創建一些系統文件夾並啓動USB守護進程、Android Debug Bridge守護進程、Debug守護進程、ServiceManager進程、Zygote進程等。
2. ServiceManager進程
ServiceManager進程是所有服務的管理器。由init.rc對ServiceManager的描述service servicemanager/system/bin/servicemanager可知servicemanager進程從platform\frameworks\base\cmd\servicemanager\Service_manager.cpp啓動。在main函數中有如下代碼：
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int main(int argc, char **argv)
{
struct binder_state *bs;
void *svcmgr = BINDER_SERVICE_MANAGER;
bs = binder_open(128*1024);
if (binder_become_context_manager(bs)) {
LOGE("cannot become context manager (%s)\n", strerror(errno));
return -1;
}
svcmgr_handle = svcmgr;
binder_loop(bs, svcmgr_handler);
return 0;
}
首先調用binder_open()打開Binder設備(/dev/binder)，調用binder_become_context_manager()把當前進程設置爲ServiceManager。ServiceManager本身就是一個服務。
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int binder_become_context_manager(struct binder_state *bs)
{
return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
}
最後binder_loop()進入循環狀態，並設置svcmgr_handler回調函數等待添加、查詢、獲取服務等請求。
3. Zygote進程
Zygote進程用於產生其他進程。由init.rc對zygote的描述service zygot /system/bin/app_process可知zygote進程從platfrom\frameworks\base\cmds\app_process\App_main.cpp啓動。在main函數中有如下代碼：
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if (0 == strcmp("--zygote", arg)) {
bool startSystemServer = (i < argc) ?
strcmp(argv[i], "--start-system-server") == 0 : false;
setArgv0(argv0, "zygote");
set_process_name("zygote");
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer);
} else {
set_process_name(argv0);
runtime.mClassName = arg;
// Remainder of args get passed to startup class main()
runtime.mArgC = argc-i;
runtime.mArgV = argv+i;
LOGV("App process is starting with pid=%d, class=%s.\n",
getpid(), runtime.getClassName());
runtime.start();
}
首先創建AppRuntime，即AndroidRuntime，建立了一個Dalvik虛擬機。通過這個runtime傳遞com.android.internal.os.ZygoteInit參數，從而由Dalvik虛擬機運行ZygoteInit.java的main()，開始創建Zygote進程。在其main()中，如下所示：
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registerZygoteSocket();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preloadClasses();
//cacheRegisterMaps();
preloadResources();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gc();
// If requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("true")) {
startSystemServer();
} else if (!argv[1].equals("false")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
首先通過registerZygoteSocket()登記端口，接着preloadClasses()裝載相關類。這裏大概要裝載1000多個類，具體裝載類見platform\frameworks\base\preloaded-classes。這個文件有WritePreloadedClassFile類自動生成。分析該類的main函數，有如下一段篩選類的代碼：
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// Preload classes that were loaded by at least 2 processes. Hopefully,
// the memory associated with these classes will be shared.
for (LoadedClass loadedClass : root.loadedClasses.values()) {
Set<String> names = loadedClass.processNames();
if (!Policy.isPreloadable(loadedClass)) {
continue;
}
if (names.size() >= MIN_PROCESSES ||
(loadedClass.medianTimeMicros() > MIN_LOAD_TIME_MICROS && names.size() > 1)) {
toPreload.add(loadedClass);
}
}
int initialSize = toPreload.size();
System.out.println(initialSize
+ " classses were loaded by more than one app.");
// Preload eligable classes from applications (not long-running
// services).
for (Proc proc : root.processes.values()) {
if (proc.fromZygote() && !Policy.isService(proc.name)) {
for (Operation operation : proc.operations) {
LoadedClass loadedClass = operation.loadedClass;
if (shouldPreload(loadedClass)) {
toPreload.add(loadedClass);
}
}
}
}
其中MIN_LOAD_TIME_MICROS等於1250，當類的裝載時間大於1.25ms，則需要預裝載。
Policy.isPreloadable()定於如下：
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/**Reports if the given class should be preloaded. */
public static boolean isPreloadable(LoadedClass clazz) {
return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);
}
其中EXCLUDED_CLASSES如下定義：
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/**
* Classes which we shouldn't load from the Zygote.
*/
private static final Set<String> EXCLUDED_CLASSES
= new HashSet<String>(Arrays.asList(
// Binders
"android.app.AlarmManager",
"android.app.SearchManager",
"android.os.FileObserver",
"com.android.server.PackageManagerService$AppDirObserver",
// Threads
"android.os.AsyncTask",
"android.pim.ContactsAsyncHelper",
"java.lang.ProcessManager"
));
這幾個Binders和Thread是不會被預加載的。
另外還有一些application需要裝載，要求滿足條件proc.fromZygote()且不是屬於常駐內存的服務。SERVICES定義如下：
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/**
* Long running services. These are restricted in their contribution to the
* preloader because their launch time is less critical.
*/
// TODO: Generate this automatically from package manager.
private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(
"system_server",
"com.google.process.content",
"android.process.media",
"com.android.bluetooth",
"com.android.calendar",
"com.android.inputmethod.latin",
"com.android.phone",
"com.google.android.apps.maps.FriendService", // pre froyo
"com.google.android.apps.maps:FriendService", // froyo
"com.google.android.apps.maps.LocationFriendService",
"com.google.android.deskclock",
"com.google.process.gapps",
"android.tts"
));
preloaded-classes是在下載源碼的時候生成，WritePreloadedClassFile類並沒有被用到，但可以通過這個類瞭解Android系統對預加載類的默認要求，參考修改preloaded-classes文件，減少開機初始化時要預加載的類，提高開機速度。
最後來通過startSystemServer()啓動SystemServer進程。見如下代碼：
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/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
/*
* Enable debugging of the system process if *either* the command line flags
* indicate it should be debuggable or the ro.debuggable system property
* is set to "1"
*/
int debugFlags = parsedArgs.debugFlags;
if ("1".equals(SystemProperties.get("ro.debuggable")))
debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids, debugFlags, null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities)
Zygote包裝了Linux的fork。forkSystemServer()調用forkAndSpecialize()，最終穿過虛擬機調用platform\dalvik\vm\native\dalvik_system_Zygote.c中Dalvik_dalvik_system_Zygote_forkAndSpecialize()。由dalvik完成fork新的進程。
main()最後會調用runSelectLoopMode()，進入while循環，由peers創建新的進程。
4. SystemService進程
SystemService用於創建init.rc定義的服務之外的所有服務。在main()的最後有如下代碼：
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// The system server has to run all of the time, so it needs to be
// as efficient as possible with its memory usage.
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
System.loadLibrary("android_servers");
init1(args);
Init1()是在native空間實現的，用於啓動native空間的服務，其實現在com_android_server_SystemServer.cpp中的android_server_SystemServer_init1()：
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static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
system_init();
}
而system_init()服務初始化創建native層的各個服務：
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// Start the sensor service
SensorService::instantiate();
// On the simulator, audioflinger et al don't get started the
// same way as on the device, and we need to start them here
if (!proc->supportsProcesses()) {
// Start the AudioFlinger
AudioFlinger::instantiate();
// Start the media playback service
MediaPlayerService::instantiate();
// Start the camera service
CameraService::instantiate();
// Start the audio policy service
AudioPolicyService::instantiate();
}
最後通過如下代碼:
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LOGI("System server: starting Android services.\n");
runtime->callStatic("com/android/server/SystemServer", "init2");
回到SystemServer.java，調用init2()：
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public static final void init2() {
Slog.i(TAG, "Entered the Android system server!");
Thread thr = new ServerThread();
thr.setName("android.server.ServerThread");
thr.start();
}
Init2啓動一個線程，專門用來啓動java空間的所有服務。如下代碼所示啓動部分服務：
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// Critical services...
try {
Slog.i(TAG, "Entropy Service");
ServiceManager.addService("entropy", new EntropyService());
Slog.i(TAG, "Power Manager");
power = new PowerManagerService();
ServiceManager.addService(Context.POWER_SERVICE, power);
Slog.i(TAG, "Activity Manager");
context = ActivityManagerService.main(factoryTest);
Slog.i(TAG, "Telephony Registry");
ServiceManager.addService("telephony.registry", new TelephonyRegistry(context));
AttributeCache.init(context);
Slog.i(TAG, "Package Manager");
// Only run "core" apps if we're encrypting the device.
String cryptState = SystemProperties.get("vold.decrypt");
boolean onlyCore = false;
if (ENCRYPTING_STATE.equals(cryptState)) {
Slog.w(TAG, "Detected encryption in progress - only parsing core apps");
onlyCore = true;
} else if (ENCRYPTED_STATE.equals(cryptState)) {
Slog.w(TAG, "Device encrypted - only parsing core apps");
onlyCore = true;
}
pm = PackageManagerService.main(context,
factoryTest != SystemServer.FACTORY_TEST_OFF,
onlyCore);
boolean firstBoot = false;
try {
firstBoot = pm.isFirstBoot();
} catch (RemoteException e) {
}
ActivityManagerService.setSystemProcess();
mContentResolver = context.getContentResolver();
// The AccountManager must come before the ContentService
try {
Slog.i(TAG, "Account Manager");
ServiceManager.addService(Context.ACCOUNT_SERVICE,
new AccountManagerService(context));
} catch (Throwable e) {
Slog.e(TAG, "Failure starting Account Manager", e);
}
Slog.i(TAG, "Content Manager");
ContentService.main(context,
factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);
Slog.i(TAG, "System Content Providers");
ActivityManagerService.installSystemProviders();
Slog.i(TAG, "Lights Service");
lights = new LightsService(context);
Slog.i(TAG, "Battery Service");
battery = new BatteryService(context, lights);
ServiceManager.addService("battery", battery);
Slog.i(TAG, "Vibrator Service");
ServiceManager.addService("vibrator", new VibratorService(context));
// only initialize the power service after we have started the
// lights service, content providers and the battery service.
power.init(context, lights, ActivityManagerService.self(), battery);
Slog.i(TAG, "Alarm Manager");
alarm = new AlarmManagerService(context);
ServiceManager.addService(Context.ALARM_SERVICE, alarm);
Slog.i(TAG, "Init Watchdog");
Watchdog.getInstance().init(context, battery, power, alarm,
ActivityManagerService.self());
Slog.i(TAG, "Window Manager");
wm = WindowManagerService.main(context, power,
factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL,
!firstBoot);
ServiceManager.addService(Context.WINDOW_SERVICE, wm);
ActivityManagerService.self().setWindowManager(wm);
// Skip Bluetooth if we have an emulator kernel
// TODO: Use a more reliable check to see if this product should
// support Bluetooth - see bug 988521
if (SystemProperties.get("ro.kernel.qemu").equals("1")) {
Slog.i(TAG, "No Bluetooh Service (emulator)");
} else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {
Slog.i(TAG, "No Bluetooth Service (factory test)");
} else {
Slog.i(TAG, "Bluetooth Service");
bluetooth = new BluetoothService(context);
ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);
bluetooth.initAfterRegistration();
bluetoothA2dp = new BluetoothA2dpService(context, bluetooth);
ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,
bluetoothA2dp);
bluetooth.initAfterA2dpRegistration();
int airplaneModeOn = Settings.System.getInt(mContentResolver,
Settings.System.AIRPLANE_MODE_ON, 0);
int bluetoothOn = Settings.Secure.getInt(mContentResolver,
Settings.Secure.BLUETOOTH_ON, 0);
if (airplaneModeOn == 0 && bluetoothOn != 0) {
bluetooth.enable();
}
}
} catch (RuntimeException e) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting core service", e);
}
並且把這些服務添加到ServiceManager中，以便管理和進程間通訊。
在該線程後半部分，ActivityManagerService會等待AppWidget、WallPaper、IMM等systemReady後調用自身的systemReady()。
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Slog.i(TAG, "Content Manager");
ContentService.main(context,
factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);
((ActivityManagerService)ServiceManager.getService("activity"))
.setWindowManager(wm);
// Skip Bluetooth if we have an emulator kernel
// TODO: Use a more reliable check to see if this product should
// support Bluetooth - see bug 988521
if (SystemProperties.get("ro.kernel.qemu").equals("1")) {
Slog.i(TAG, "Registering null Bluetooth Service (emulator)");
ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);
} else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {
Slog.i(TAG, "Registering null Bluetooth Service (factory test)");
ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);
} else {
Slog.i(TAG, "Bluetooth Service");
bluetooth = new BluetoothService(context);
ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);
bluetooth.initAfterRegistration();
bluetoothA2dp = new BluetoothA2dpService(context, bluetooth);
ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,
bluetoothA2dp);
int bluetoothOn = Settings.Secure.getInt(mContentResolver,
Settings.Secure.BLUETOOTH_ON, 0);
if (bluetoothOn > 0) {
bluetooth.enable();
}
}
而在ActivityManagerService的systemReady()最後會執行如下代碼：
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mMainStack.resumeTopActivityLocked(null);
由於Activity管理棧爲空，因此啓動Launcher。
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// Find the first activity that is not finishing.
ActivityRecord next = topRunningActivityLocked(null);
// Remember how we'll process this pause/resume situation, and ensure
// that the state is reset however we wind up proceeding.
final boolean userLeaving = mUserLeaving;
mUserLeaving = false;
if (next == null) {
// There are no more activities! Let's just start up the
// Launcher...
if (mMainStack) {
return mService.startHomeActivityLocked();
}
}
在startHomeActivityLocked()中創建一個帶Category爲CATEGORY_HOME的Intent，由此去啓動相應Activity，即Launcher。
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Intent intent = new Intent(
mTopAction,
mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
這樣，Android系統便啓動起來進入到待機界面。
摘自 亨利摩根的專欄
Android開機啓動流程初探 .
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