//main() method in Init.c
int main(int argc, char **argv)
{
    //Parsing execution init.rc configuration file
    init_parse_config_file("/init.rc");
}

2. Execute the instructions defined in the configuration file init.rc(system\core\rootdir) to initialize the environment; execute many bin instructions to start system services

//Start the incubator process and execute app ﹣ process (a compiled executable)
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

3. Find app main.cpp in the app process file and check the main() method

int main(int argc, const char* const argv[])
{
    //Start a system service: ZygoteInit
  runtime.start("com.android.internal.os.ZygoteInit",startSystemServer);
}

4. In ZygoteInit.java, look at the main() method

public static void main(String argv[]) {
    //Classes required for preloading Android system
    preloadClasses();
    if (argv[1].equals("true")) {
        //Call method to start system service
        startSystemServer();
     }
}

preloadClasses() to load the Android runtime environment

/**
* The name of a resource file that contains classes to preload.
*/
//The "preloaded classes" file contains all the full package names that need to be loaded in Android
private static final String PRELOADED_CLASSES = "preloaded-classes";

private static void preloadClasses() {
    //Use class loader to load resources as an input stream according to the file name
    InputStream is = ZygoteInit.class.getClassLoader().getResourceAsStream(PRELOADED_CLASSES);
    BufferedReader br = new BufferedReader(new InputStreamReader(is), 256);
    String line;
    //Read full package name line by line
    while ((line = br.readLine()) != null) {
    //Load classes by full package name
      Class.forName(line);
    }  

}

startSystemServer(), which starts system services

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",
     //Open system server service
     "com.android.server.SystemServer",
};

5. Start system server service

//init1() method in C library
native public static void init1(String[] args);

public static void main(String[] args) {
    //Loading C Library
    System.loadLibrary("android_servers");
    //Execute init1 method in C library
    init1(args);
}

In the OM Android server systemserver.cpp file, find the init1() method

static JNINativeMethod gMethods[] = {
    /* name, signature, funcPtr */
    //Mapping a pointer to the init1 method, finally calling the system_init() method, while system_init() has no method body.
    { "init1", "([Ljava/lang/String;)V", (void*)   
    android_server_SystemServer_init1 },
};

static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
    system_init();
}

extern "C" int system_init();

Find the system init() method in the system init.cpp file

extern "C" status_t system_init()
{
    //In this method, hardware management service is mainly enabled
    SensorService::instantiate();
    AudioFlinger::instantiate();
    MediaPlayerService::instantiate();
    CameraService::instantiate();

    //Execute init2() method in SystemServer class
    runtime->callStatic("com/android/server/SystemServer", "init2");
}

Go back to the init2() method in the SystemServer class

public static final void init2() {
    //Create system service thread
    Thread thr = new ServerThread();
    thr.setName("android.server.ServerThread");
    thr.start();
}

6. In the run() method of the ServerThread class, start other services of the system

//Prepare message poller
Looper.prepare();

//Start a large number of system services and add them to service manager one by one
ServiceManager.addService(Context.WINDOW_SERVICE, wm);

//Call the systemReady() method in the ActivityManagerService class to create the first activity
((ActivityManagerService)ActivityManagerNative.getDefault()).systemReady(new Runnable() {});

7. In ActivityManagerService.java, find systemReady() method

public ActivityStack mMainStack;

public void systemReady(final Runnable goingCallback) {
    //Launch laucher
    mMainStack.resumeTopActivityLocked(null);
}

Find the resumeTopActivityLocked() method in ActivityStack.java

final boolean resumeTopActivityLocked(ActivityRecord prev) {
    ActivityRecord next = topRunningActivityLocked(null);

    //Judge whether there is activity on the top of the stack. If not, start the laucher directly
    if (next == null) {
    // There are no more activities!  Let's just start up the Launcher...
        if (mMainStack) {
            return mService.startHomeActivityLocked();
        }
    }
}

So far, Laucher has been started and Android system has been started;

Declaration: This article only does the study exchange, welcome everybody to raise each kind of question, lets us progress together!!!

Finally, attach a time sequence diagram