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當引導程序啟動Linux內核后，會加載各種驅動和數據結構，當有了驅動以后，開始啟動Android系統同時會加載用戶級別的第一個進程init（system\core\init.c）代碼如下：

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

加載init.rc文件，會啟動一個Zygote進程，此進程是Android系統的一個母進程，用來啟動Android的其他服務進程，代碼：

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

從c++代碼調到java代碼：

int main(int argc, const char* const argv[]) {...// Android運行時環境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");// 啟動java代碼runtime.start("com.android.internal.os.ZygoteInit",...}

ZygoteInit.java 代碼：

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

// 加載Android依賴的類preloadClasses();//cacheRegisterMaps();preloadResources();...if (argv[1].equals("true")) {// 啟動系統服務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 */// 母進程開始分叉服務 啟動SystemServerpid = Zygote.forkSystemServer(parsedArgs.uid, parsedArgs.gid,parsedArgs.gids, debugFlags, rlimits,parsedArgs.permittedCapabilities,parsedArgs.effectiveCapabilities);.. }

SystemServer.java 代碼

public static void main(String[] args) {... // 加載jni庫System.loadLibrary("android_servers");// 調用native方法init1(args); } native public static void init1(String[] args);

SystemServer 對應的c++代碼 comandroidserver_SystemServer.cpp 代碼如下：

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

systeminit 的實現方法在Systeminit.cpp 代碼如下：

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

SystemServer 的init2方法代碼：

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() { ... // 開啟Android各種服務并且添加到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.// 各種服務開啟后調用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) {...// 打開第一個ActivitymMainStack.resumeTopActivityLocked(null);}}

ActivityStack的resumeTopActivityLocked方法

final boolean resumeTopActivityLocked(ActivityRecord prev) {// Find the first activity that is not finishing.// 沒有已經打開的Activity next為 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) {// 啟動lucher應用的鎖屏界面return mService.startHomeActivityLocked();}}

Android系統啟動完成，打開了Luncher應用的Home界面。

轉載于:https://my.oschina.net/u/2550627/blog/610855

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