当引导程序启动Linux内核后，会加载各种驱动和数据结构，当有了驱动以后，开始启动Android系统同时会加载用户级别的第一个进程init（system\core\init.c）代码如下：

int main(int argc, char **argv)
{// 创建文件夹 挂载mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");mkdir("/dev/pts", 0755);// 打卡日志log_init();INFO("reading config file\n");// 加载init.rc配置文件init_parse_config_file("/init.rc");}

加载init.rc文件，会启动一个Zygote进程，此进程是Android系统的一个母进程，用来启动Android的其他服务进程，代码：

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

从c++代码调到java代码：

int main(int argc, const char* const argv[])
{...// Android运行时环境AppRuntime runtime;...// Next arg is startup classname or "--zygote"if (i < argc) {arg = argv[i++];if (0 == strcmp("--zygote", arg)) {bool startSystemServer = (i < argc) ? strcmp(argv[i], "--start-system-server") == 0 : false;setArgv0(argv0, "zygote");set_process_name("zygote");// 启动java代码runtime.start("com.android.internal.os.ZygoteInit",...}

ZygoteInit.java 代码：

public static void main(String argv[]) { try { VMRuntime.getRuntime().setMinimumHeapSize(5 * 1024 * 1024);

// 加载Android依赖的类preloadClasses();//cacheRegisterMaps();preloadResources();...if (argv[1].equals("true")) {// 启动系统服务startSystemServer();} else if (!argv[1].equals("false")) {...
}private static boolean startSystemServer()...args = new String[] {"--setuid=1000","--setgid=1000","--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006","--capabilities=130104352,130104352","--rlimit=8,","--runtime-init","--nice-name=system_server","com.android.server.SystemServer",.../* Request to fork the system server process */// 母进程开始分叉服务 启动SystemServerpid = Zygote.forkSystemServer(parsedArgs.uid, parsedArgs.gid,parsedArgs.gids, debugFlags, rlimits,parsedArgs.permittedCapabilities,parsedArgs.effectiveCapabilities);..
}

SystemServer.java 代码

public static void main(String[] args) {... // 加载jni库System.loadLibrary("android_servers");// 调用native方法init1(args);
}
native public static void init1(String[] args);

SystemServer 对应的c++代码 comandroidserver_SystemServer.cpp 代码如下：

// 类似java的抽象方法
extern "C" int system_init();static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{   // 转调system_init();
}/** JNI registration.*/
static JNINativeMethod gMethods[] = {/* name, signature, funcPtr */ // 函数指针 把init1方法映射到android_server_SystemServer_init1{ "init1", "([Ljava/lang/String;)V", (void*) android_server_SystemServer_init1 },
};

systeminit 的实现方法在Systeminit.cpp 代码如下：

extern "C" status_t system_init()
{...// 启动硬件的服务if (strcmp(propBuf, "1") == 0) {// Start the SurfaceFlingerSurfaceFlinger::instantiate();}AndroidRuntime* runtime = AndroidRuntime::getRuntime();LOGI("System server: starting Android services.\n");// 启动完硬件服务后，又回到Systemserver的init2方法runtime->callStatic("com/android/server/SystemServer", "init2");...
}

SystemServer 的init2方法代码：

public static final void init2() {Slog.i(TAG, "Entered the Android system server!");Thread thr = new ServerThread();thr.setName("android.server.ServerThread");thr.start();}

ServerThread的run方法：

public void run() { ... // 开启Android各种服务并且添加到ServiceManager去管理 Slog.i(TAG, "Device Policy"); devicePolicy = new DevicePolicyManagerService(context); ServiceManager.addService(Context.DEVICEPOLICYSERVICE, ottle =

...// We now tell the activity manager it is okay to run third party// code.  It will call back into us once it has gotten to the state// where third party code can really run (but before it has actually// started launching the initial applications), for us to complete our// initialization.// 各种服务开启后调用ActivityManagerService.systemReady((ActivityManagerService)ActivityManagerNative.getDefault()).systemReady(new Runnable() {public void run() {Slog.i(TAG, "Making services ready");

ActivityMangerService的systemReady的方法：

public void systemReady(final Runnable goingCallback) {...// 打开第一个ActivitymMainStack.resumeTopActivityLocked(null);}}

ActivityStack的resumeTopActivityLocked方法

final boolean resumeTopActivityLocked(ActivityRecord prev) {// Find the first activity that is not finishing.// 没有已经打开的Activity next为 nullActivityRecord 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) {// 启动lucher应用的锁屏界面return mService.startHomeActivityLocked();}}

Android系统启动完成，打开了Luncher应用的Home界面。