第一步：启动linux 可以参考上文
1.Bootloader
2.Kernel
第二步android系统启动：入口为init.rc(system\core\rootdir)
1./system/bin/service manager: Binder 守护进程；
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：应用程序启动：运行package Manager
1.Init进程
Android系统在启动时首先会启动Linux系统，引导加载Linux Kernel并启动init进程。Init进程是一个由内核启动的用户级进程，是Android系统的第一个进程。该进程的相关代码在system\core\init\init.c。在main函数中，有如下代码：
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1. open_devnull_stdio();
2. log_init();
3. INFO("reading config file\n");
4. init_parse_config_file("/init.rc");
5.
6. /* pull the kernel commandline and ramdisk properties file in */
7. import_kernel_cmdline(0);
8. get_hardware_name(hardware, &revision);
9. snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);
10. init_parse_config_file(tmp);
这里会加载解析init.rc和init.hardware.rc两个初始化脚本。*.rc文件定义了在init进程中需要启动哪些进程服务和执行哪些动作。其详细说明参见system\core\init\reademe.txt。init.rc见如下定义：
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1. service servicemanager /system/bin/servicemanager
2. user system
3. critical
4. onrestart restart zygote
5. onrestart restart media
6.
7. service vold /system/bin/vold
8. socket vold stream 0660 root mount
9. ioprio be 2
10.
11. service netd /system/bin/netd
12. socket netd stream 0660 root system
13. socket dnsproxyd stream 0660 root inet
14.
15. service debuggerd /system/bin/debuggerd
16.
17. service ril-daemon /system/bin/rild
18. socket rild stream 660 root radio
19. socket rild-debug stream 660 radio system
20. user root
21. group radio cache inet misc audio sdcard_rw
22.
23. service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
24. socket zygote stream 666
25. onrestart write /sys/android_power/request_state wake
26. onrestart write /sys/power/state on
27. onrestart restart media
28. onrestart restart netd
29.
30. service drm /system/bin/drmserver
31. user drm
32. group system root inet
具体解析过程见system\core\init\Init_parser.c。解析所得服务添加到service_list中，动作添加到action_list中。
接下来在main函数中执行动作和启动进程服务：
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1. execute_one_command();
2. restart_processes();
通常init过程需要创建一些系统文件夹并启动USB守护进程、Android Debug Bridge守护进程、Debug守护进程、ServiceManager进程、Zygote进程等。
2. ServiceManager进程
ServiceManager进程是所有服务的管理器。由init.rc对ServiceManager的描述service servicemanager /system/bin/servicemanager可知servicemanager进程从frameworks/base/cmds/servicemanager/service_manager.c启动。在main函数中有如下代码：
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1. int main(int argc, char **argv)
2. {
3. struct binder_state *bs;
4. void *svcmgr = BINDER_SERVICE_MANAGER;
5.
6. bs = binder_open(128*1024);
7.
8. if (binder_become_context_manager(bs)) {
9. "cannot become context manager (%s)\n", strerror(errno));
10. return -1;
11. }
12.
13. svcmgr_handle = svcmgr;
14. binder_loop(bs, svcmgr_handler);
15. return 0;
16. }
首先调用binder_open()打开Binder设备(/dev/binder)，调用binder_become_context_manager()把当前进程设置为ServiceManager。ServiceManager本身就是一个服务。
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1. int binder_become_context_manager(struct binder_state *bs)
2. {
3. return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);
4. }
最后binder_loop()进入循环状态，并设置svcmgr_handler回调函数等待添加、查询、获取服务等请求。
3. Zygote进程
Zygote进程用于产生其他进程。由init.rc对zygote的描述service zygot /system/bin/app_process可知zygote进程从platfrom\frameworks\base\cmds\app_process\App_main.cpp启动。在main函数中有如下代码：
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1. if (0 == strcmp("--zygote", arg)) {
2. bool startSystemServer = (i < argc) ?
3. "--start-system-server") == 0 : false;
4. "zygote");
5. "zygote");
6. "com.android.internal.os.ZygoteInit",
7. startSystemServer);
8. else {
9. set_process_name(argv0);
10. runtime.mClassName = arg;
11.
12. // Remainder of args get passed to startup class main()
13. runtime.mArgC = argc-i;
14. runtime.mArgV = argv+i;
15.
16. "App process is starting with pid=%d, class=%s.\n",
17. getpid(), runtime.getClassName());
18. runtime.start();
19. }
首先创建AppRuntime，即AndroidRuntime，建立了一个Dalvik虚拟机。通过这个runtime传递com.android.internal.os.ZygoteInit参数，从而由Dalvik虚拟机运行ZygoteInit.java的main()，开始创建Zygote进程。在其main()中，如下所示：
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1. registerZygoteSocket();
2. EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
3. SystemClock.uptimeMillis());
4. preloadClasses();
5. //cacheRegisterMaps();
6. preloadResources();
7. EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
8. SystemClock.uptimeMillis());
9.
10. // Finish profiling the zygote initialization.
11. SamplingProfilerIntegration.writeZygoteSnapshot();
12.
13. // Do an initial gc to clean up after startup
14. gc();
15.
16. // If requested, start system server directly from Zygote
17. if (argv.length != 2) {
18. throw new RuntimeException(argv[0] + USAGE_STRING);
19. }
20.
21. if (argv[1].equals("true")) {
22. startSystemServer();
23. } else if (!argv[1].equals("false")) {
24. throw new RuntimeException(argv[0] + USAGE_STRING);
25. }
首先通过registerZygoteSocket()登记端口，接着preloadClasses()装载相关类。这里大概要装载1000多个类，具体装载类见frameworks\base\preloaded-classes。这个文件有WritePreloadedClassFile类自动生成。分析该类的main函数，有如下一段筛选类的代码：
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1. // Preload classes that were loaded by at least 2 processes. Hopefully,
2. // the memory associated with these classes will be shared.
3. for (LoadedClass loadedClass : root.loadedClasses.values()) {
4. Set<String> names = loadedClass.processNames();
5. if (!Policy.isPreloadable(loadedClass)) {
6. continue;
7. }
8.
9. if (names.size() >= MIN_PROCESSES ||
10. (loadedClass.medianTimeMicros() > MIN_LOAD_TIME_MICROS && names.size() > 1)) {
11. toPreload.add(loadedClass);
12. }
13. }
14. int initialSize = toPreload.size();
15. System.out.println(initialSize
16. " classses were loaded by more than one app.");
17. // Preload eligable classes from applications (not long-running
18. // services).
19. for (Proc proc : root.processes.values()) {
20. if (proc.fromZygote() && !Policy.isService(proc.name)) {
21. for (Operation operation : proc.operations) {
22. LoadedClass loadedClass = operation.loadedClass;
23. if (shouldPreload(loadedClass)) {
24. toPreload.add(loadedClass);
25. }
26. }
27. }
28. }
其中MIN_LOAD_TIME_MICROS等于1250，当类的装载时间大于1.25ms，则需要预装载。
Policy.isPreloadable()定于如下：
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1. /**Reports if the given class should be preloaded. */
2. public static boolean isPreloadable(LoadedClass clazz) {
3. return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);
4. }
其中EXCLUDED_CLASSES如下定义：
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1. /**
2. * Classes which we shouldn't load from the Zygote.
3. */
4. private static final Set<String> EXCLUDED_CLASSES
5. new HashSet<String>(Arrays.asList(
6. // Binders
7. "android.app.AlarmManager",
8. "android.app.SearchManager",
9. "android.os.FileObserver",
10. "com.android.server.PackageManagerService$AppDirObserver",
11. // Threads
12. "android.os.AsyncTask",
13. "android.pim.ContactsAsyncHelper",
14. "java.lang.ProcessManager"
15. ));
这几个Binders和Thread是不会被预加载的。
另外还有一些application需要装载，要求满足条件proc.fromZygote()且不是属于常驻内存的服务。SERVICES定义如下：
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1. /**
2. * Long running services. These are restricted in their contribution to the
3. * preloader because their launch time is less critical.
4. */
5. // TODO: Generate this automatically from package manager.
6. private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(
7. "system_server",
8. "com.google.process.content",
9. "android.process.media",
10. "com.android.bluetooth",
11. "com.android.calendar",
12. "com.android.inputmethod.latin",
13. "com.android.phone",
14. "com.google.android.apps.maps.FriendService", // pre froyo
15. "com.google.android.apps.maps:FriendService", // froyo
16. "com.google.android.apps.maps.LocationFriendService",
17. "com.google.android.deskclock",
18. "com.google.process.gapps",
19. "android.tts"
20. ));
preloaded-classes是在下载源码的时候生成，WritePreloadedClassFile类并没有被用到，但可以通过这个类了解Android系统对预加载类的默认要求，参考修改preloaded-classes文件，减少开机初始化时要预加载的类，提高开机速度。
最后(还在ZygoteInit.java的main函数里)来通过startSystemServer()启动SystemServer进程。见如下代码：
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1. /* Hardcoded command line to start the system server */
2. String args[] = {
3. "--setuid=1000",
4. "--setgid=1000",
5. "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",
6. "--capabilities=130104352,130104352",
7. "--runtime-init",
8. "--nice-name=system_server",
9. "com.android.server.SystemServer",
10. };
11.
12. ZygoteConnection.Arguments parsedArgs = null;
13. int pid;
14. try {
15. new ZygoteConnection.Arguments(args);
16. /*
17. * Enable debugging of the system process if *either* the command line flags
18. * indicate it should be debuggable or the ro.debuggable system property
19. * is set to "1"
20. */
21. int debugFlags = parsedArgs.debugFlags;
22. if ("1".equals(SystemProperties.get("ro.debuggable")))
23. debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
24.
25. /* Request to fork the system server process */
26. pid = Zygote.forkSystemServer(
27. parsedArgs.uid, parsedArgs.gid,
28. parsedArgs.gids, debugFlags, null,
29. parsedArgs.permittedCapabilities,
30. parsedArgs.effectiveCapabilities)
Zygote包装了Linux的fork。forkSystemServer()调用forkAndSpecialize()(android4.1中是别的函数)，最终穿过虚拟机调用platform\dalvik\vm\native\dalvik_system_Zygote.cpp中Dalvik_dalvik_system_Zygote_forkAndSpecialize()。由dalvik完成fork新的进程。
main()最后会调用runSelectLoopMode()，进入while循环，由peers创建新的进程。
4. SystemService进程
SystemService用于创建init.rc定义的服务之外的所有服务。在SystemServer.java (frameworks\base\services\java\com\android\server) 的main()的最后有如下代码：
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1. // The system server has to run all of the time, so it needs to be
2. // as efficient as possible with its memory usage.
3. VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
4.
5. System.loadLibrary("android_servers");
6. init1(args);
Init1()是在native空间实现的，用于启动native空间的服务，其实现在com_android_server_SystemServer.cpp中的android_server_SystemServer_init1()：
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1. static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
2. {
3. system_init();
4. }
而system_init()（System_init.cpp (frameworks\base\cmds\system_server\library)文件中）服务初始化创建native层的各个服务：
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1. // Start the sensor service
2. SensorService::instantiate();
3.
4. // On the simulator, audioflinger et al don't get started the
5. // same way as on the device, and we need to start them here
6. if (!proc->supportsProcesses()) {
7. // Start the AudioFlinger
8. AudioFlinger::instantiate();
9.
10. // Start the media playback service
11. MediaPlayerService::instantiate();
12.
13. // Start the camera service
14. CameraService::instantiate();
15.
16. // Start the audio policy service
17. AudioPolicyService::instantiate();
18. }
最后通过如下代码: （android4.1略有区别）
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1. LOGI("System server: starting Android services.\n");
2. runtime->callStatic("com/android/server/SystemServer", "init2");
回到SystemServer.java，调用init2()：
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1. public static final void init2() {
2. "Entered the Android system server!");
3. new ServerThread();
4. "android.server.ServerThread");
5. thr.start();
6. }
Init2启动一个线程，专门用来启动java空间的所有服务。如下代码所示启动部分服务：
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1. // Critical services...
2. try {
3. "Entropy Service");
4. "entropy", new EntropyService());
5.
6. "Power Manager");
7. new PowerManagerService();
8. ServiceManager.addService(Context.POWER_SERVICE, power);
9.
10. "Activity Manager");
11. context = ActivityManagerService.main(factoryTest);
12.
13. "Telephony Registry");
14. "telephony.registry", new TelephonyRegistry(context));
15.
16. AttributeCache.init(context);
17.
18. "Package Manager");
19. // Only run "core" apps if we're encrypting the device.
20. "vold.decrypt");
21. false;
22. if (ENCRYPTING_STATE.equals(cryptState)) {
23. "Detected encryption in progress - only parsing core apps");
24. true;
25. else if (ENCRYPTED_STATE.equals(cryptState)) {
26. "Device encrypted - only parsing core apps");
27. true;
28. }
29.
30. pm = PackageManagerService.main(context,
31. factoryTest != SystemServer.FACTORY_TEST_OFF,
32. onlyCore);
33. false;
34. try {
35. firstBoot = pm.isFirstBoot();
36. catch (RemoteException e) {
37. }
38.
39. ActivityManagerService.setSystemProcess();
40.
41. mContentResolver = context.getContentResolver();
42.
43. // The AccountManager must come before the ContentService
44. try {
45. "Account Manager");
46. ServiceManager.addService(Context.ACCOUNT_SERVICE,
47. new AccountManagerService(context));
48. catch (Throwable e) {
49. "Failure starting Account Manager", e);
50. }
51.
52. "Content Manager");
53. ContentService.main(context,
54. factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);
55.
56. "System Content Providers");
57. ActivityManagerService.installSystemProviders();
58.
59. "Lights Service");
60. new LightsService(context);
61.
62. "Battery Service");
63. new BatteryService(context, lights);
64. "battery", battery);
65.
66. "Vibrator Service");
67. "vibrator", new VibratorService(context));
68.
69. // only initialize the power service after we have started the
70. // lights service, content providers and the battery service.
71. power.init(context, lights, ActivityManagerService.self(), battery);
72.
73. "Alarm Manager");
74. new AlarmManagerService(context);
75. ServiceManager.addService(Context.ALARM_SERVICE, alarm);
76.
77. "Init Watchdog");
78. Watchdog.getInstance().init(context, battery, power, alarm,
79. ActivityManagerService.self());
80.
81. "Window Manager");
82. wm = WindowManagerService.main(context, power,
83. factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL,
84. !firstBoot);
85. ServiceManager.addService(Context.WINDOW_SERVICE, wm);
86.
87. ActivityManagerService.self().setWindowManager(wm);
88.
89. // Skip Bluetooth if we have an emulator kernel
90. // TODO: Use a more reliable check to see if this product should
91. // support Bluetooth - see bug 988521
92. if (SystemProperties.get("ro.kernel.qemu").equals("1")) {
93. "No Bluetooh Service (emulator)");
94. else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {
95. "No Bluetooth Service (factory test)");
96. else {
97. "Bluetooth Service");
98. new BluetoothService(context);
99. ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);
100. bluetooth.initAfterRegistration();
101. new BluetoothA2dpService(context, bluetooth);
102. ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,
103. bluetoothA2dp);
104. bluetooth.initAfterA2dpRegistration();
105.
106. int airplaneModeOn = Settings.System.getInt(mContentResolver,
107. Settings.System.AIRPLANE_MODE_ON, 0);
108. int bluetoothOn = Settings.Secure.getInt(mContentResolver,
109. Settings.Secure.BLUETOOTH_ON, 0);
110. if (airplaneModeOn == 0 && bluetoothOn != 0) {
111. bluetooth.enable();
112. }
113. }
114.
115. } catch (RuntimeException e) {
116. "System", "******************************************");
117. "System", "************ Failure starting core service", e);
118. }
并且把这些服务添加到ServiceManager中，以便管理和进程间通讯。
在该线程后半部分，ActivityManagerService会等待AppWidget、WallPaper、IMM等systemReady后调用自身的systemReady()。
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1. Slog.i(TAG, "Content Manager");
2. ContentService.main(context,
3. factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);
4.
5. ((ActivityManagerService)ServiceManager.getService("activity"))
6. .setWindowManager(wm);
7.
8.
9. // Skip Bluetooth if we have an emulator kernel
10. // TODO: Use a more reliable check to see if this product should
11. // support Bluetooth - see bug 988521
12. if (SystemProperties.get("ro.kernel.qemu").equals("1")) {
13. "Registering null Bluetooth Service (emulator)");
14. ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);
15. else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {
16. "Registering null Bluetooth Service (factory test)");
17. ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);
18. else {
19. "Bluetooth Service");
20. new BluetoothService(context);
21. ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);
22. bluetooth.initAfterRegistration();
23. new BluetoothA2dpService(context, bluetooth);
24. ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,
25. bluetoothA2dp);
26.
27. int bluetoothOn = Settings.Secure.getInt(mContentResolver,
28. Settings.Secure.BLUETOOTH_ON, 0);
29.
30. if (bluetoothOn > 0) {
31. bluetooth.enable();
32. }
33. }
而在ActivityManagerService的systemReady()最后会执行如下代码：
- mMainStack.resumeTopActivityLocked(null);
由于Activity管理栈为空，因此启动Launcher。
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1. // Find the first activity that is not finishing.
2. ActivityRecord next = topRunningActivityLocked(null);
3.
4. // Remember how we'll process this pause/resume situation, and ensure
5. // that the state is reset however we wind up proceeding.
6. final boolean userLeaving = mUserLeaving;
7. mUserLeaving = false;
8.
9. if (next == null) {
10. // There are no more activities! Let's just start up the
11. // Launcher...
12. if (mMainStack) {
13. return mService.startHomeActivityLocked();
14. }
15. }
在startHomeActivityLocked()中创建一个带Category为CATEGORY_HOME的Intent，由此去启动相应Activity，即Launcher。
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1. Intent intent = new Intent(
2. mTopAction,
3. mTopData != null ? Uri.parse(mTopData) : null);
4. intent.setComponent(mTopComponent);
5.
6. if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
7. intent.addCategory(Intent.CATEGORY_HOME);
8. }
这样，Android系统便启动起来进入到待机界面。
