public class Launcher extends Activity
  implements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks,
     View.OnTouchListener, PageSwitchListener, LauncherProviderChangeListener {
 ...
 /**
  * Launches the intent referred by the clicked shortcut.
  *
  * @param v The view representing the clicked shortcut.
  */
 public void onClick(View v) {
  // Make sure that rogue clicks don't get through while allapps is launching, or after the
  // view has detached (it's possible for this to happen if the view is removed mid touch).
  if (v.getWindowToken() == null) {
   return;
  }

  ...

  Object tag = v.getTag();
  if (tag instanceof ShortcutInfo) {
   onClickAppShortcut(v);
  } else if (tag instanceof FolderInfo) {
   ...
  } else if (v == mAllAppsButton) {
   onClickAllAppsButton(v);
  } else if (tag instanceof AppInfo) {
   startAppShortcutOrInfoActivity(v);
  } else if (tag instanceof LauncherAppWidgetInfo) {
   ...
  }
 }
 
 private void startAppShortcutOrInfoActivity(View v) {
  ...
  boolean success = startActivitySafely(v, intent, tag);
  ...
 }
 
 boolean startActivitySafely(View v, Intent intent, Object tag) {
  ...
  try {
   success = startActivity(v, intent, tag);
  } catch (ActivityNotFoundException e) {
   ...
  }
  return success;
 }
 
 boolean startActivity(View v, Intent intent, Object tag) {
  intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
  try {
   ...

   if (user == null || user.equals(UserHandleCompat.myUserHandle())) {
    // Could be launching some bookkeeping activity
    startActivity(intent, optsBundle);
   } else {
    ...
   }
   return true;
  } catch (SecurityException e) {
   ...
  }
  return false;
 }
}

public class Instrumentation {
 ...
 public ActivityResult execStartActivity(
   Context who, IBinder contextThread, IBinder token, Activity target,
   Intent intent, int requestCode, Bundle options) {
  IApplicationThread whoThread = (IApplicationThread) contextThread;
  ...
  try {
   ...
   int result = ActivityManagerNative.getDefault()
    .startActivity(whoThread, who.getBasePackageName(), intent,
      intent.resolveTypeIfNeeded(who.getContentResolver()),
      token, target != null ? target.mEmbeddedID : null,
      requestCode, 0, null, options);
   ...
  } catch (RemoteException e) {
  }
  return null;
 }
 ...
}

public final class ActivityStackSupervisor implements DisplayListener {
 ...
 final int startActivityUncheckedLocked(ActivityRecord r, ActivityRecord sourceRecord,
   IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor, 
 int startFlags, boolean doResume, Bundle options, TaskRecord inTask) {
 final Intent intent = r.intent;
 final int callingUid = r.launchedFromUid;
 ...
 final boolean launchSingleTop = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP;
 final boolean launchSingleInstance = r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE;
 final boolean launchSingleTask = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK; 

 int launchFlags = intent.getFlags();
 ...
 // We'll invoke onUserLeaving before onPause only if the launching
 // activity did not explicitly state that this is an automated launch.
 mUserLeaving = (launchFlags & Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;
 ...
   
   ActivityRecord notTop =
    (launchFlags & Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP) != 0 ? r : null;

 // If the onlyIfNeeded flag is set, then we can do this if the activity
 // being launched is the same as the one making the call... or, as
 // a special case, if we do not know the caller then we count the
 // current top activity as the caller.
 if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {
 ...
 }
   ...
   // If the caller is not coming from another activity, but has given us an
 // explicit task into which they would like us to launch the new activity,
 // then let's see about doing that.
 if (sourceRecord == null && inTask != null && inTask.stack != null) {
 final Intent baseIntent = inTask.getBaseIntent();
 final ActivityRecord root = inTask.getRootActivity();
 ...
 // If this task is empty, then we are adding the first activity -- it
 // determines the root, and must be launching as a NEW_TASK.
 if (launchSingleInstance || launchSingleTask) {
 ...
 }
 ...
 }
   ...
   if (inTask == null) {
 if (sourceRecord == null) {
 // This activity is not being started from another... in this
 // case we -always- start a new task.
 if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0 && inTask == null) {
  Slog.w(TAG, "startActivity called from non-Activity context; forcing " +
       "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
  launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
 }
 } else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
 // The original activity who is starting us is running as a single
 // instance... this new activity it is starting must go on its
 // own task.
 launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
 } else if (launchSingleInstance || launchSingleTask) {
 // The activity being started is a single instance... it always
 // gets launched into its own task.
 launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
 }
 }
   
 ...
   
   // We may want to try to place the new activity in to an existing task. We always
 // do this if the target activity is singleTask or singleInstance; we will also do
 // this if NEW_TASK has been requested, and there is not an additional qualifier telling
 // us to still place it in a new task: multi task, always doc mode, or being asked to
 // launch this as a new task behind the current one.
 if (((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
    (launchFlags & Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
    || launchSingleInstance || launchSingleTask) {
 // If bring to front is requested, and no result is requested and we have not
 // been given an explicit task to launch in to, and
 // we can find a task that was started with this same
 // component, then instead of launching bring that one to the front.
 if (inTask == null && r.resultTo == null) {
 // See if there is a task to bring to the front. If this is
 // a SINGLE_INSTANCE activity, there can be one and only one
 // instance of it in the history, and it is always in its own
 // unique task, so we do a special search.
 ActivityRecord intentActivity = !launchSingleInstance ?
  findTaskLocked(r) : findActivityLocked(intent, r.info);
 if (intentActivity != null) {
  ...
 }
 }
 }
  
 ...
   
   if (r.packageName != null) {
   // If the activity being launched is the same as the one currently
   // at the top, then we need to check if it should only be launched
   // once.
   ActivityStack topStack = getFocusedStack();
   ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(notTop);
   if (top != null && r.resultTo == null) {
    if (top.realActivity.equals(r.realActivity) && top.userId == r.userId) {
     ...
    }
   }
   } else{
   ...
   }
  
 boolean newTask = false;
 boolean keepCurTransition = false;

 TaskRecord taskToAffiliate = launchTaskBehind && sourceRecord != null ?
    sourceRecord.task : null;

 // Should this be considered a new task?
 if (r.resultTo == null && inTask == null && !addingToTask
    && (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
 ...
 if (reuseTask == null) {
    r.setTask(targetStack.createTaskRecord(getNextTaskId(),
      newTaskInfo != null ? newTaskInfo : r.info,
      newTaskIntent != null ? newTaskIntent : intent,
      voiceSession, voiceInteractor, !launchTaskBehind /* toTop */),
      taskToAffiliate);
    ...
   } else {
    r.setTask(reuseTask, taskToAffiliate);
   }
    ...
 } else if (sourceRecord != null) {
   
 } else if (!addingToTask &&
     (launchFlags&Intent.FLAG_ACTIVITY_REORDER_TO_FRONT) != 0) {
      
 } else if (inTask != null){
   
 } else {
   
 }
  
 ...
  
 targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options);
   
 ...
   return ActivityManager.START_SUCCESS;
 }
 ...
}

final class ActivityStack {
 ...
 final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping, boolean resuming,
   boolean dontWait) {
 if (mPausingActivity != null) {
  ...
 } 
 ActivityRecord prev = mResumedActivity;
 if (prev == null) {
  ...
 }
 ...
 mResumedActivity = null;
 mPausingActivity = prev;
 mLastPausedActivity = prev;
 mLastNoHistoryActivity = (prev.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_HISTORY) != 0
    || (prev.info.flags & ActivityInfo.FLAG_NO_HISTORY) != 0 ? prev : null;
 prev.state = ActivityState.PAUSING;
 ...
 
 if (prev.app != null && prev.app.thread != null) {
  try {
 ...
 prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing,
      userLeaving, prev.configChangeFlags, dontWait);
  } catch (Exception e) {
 ...
  }
 } else {
  ...
 }
 ...
 }
 ...
}

public final class ActivityThread {
 ...
 private void handlePauseActivity(IBinder token, boolean finished,
   boolean userLeaving, int configChanges, boolean dontReport) {
  ActivityClientRecord r = mActivities.get(token);
  if (r != null) {
   ...
   performPauseActivity(token, finished, r.isPreHoneycomb());

   // Make sure any pending writes are now committed.
   if (r.isPreHoneycomb()) {
    QueuedWork.waitToFinish();
   }

   // Tell the activity manager we have paused.
   if (!dontReport) {
    try {
     ActivityManagerNative.getDefault().activityPaused(token);
    } catch (RemoteException ex) {
    }
   }
   ...
  }
 }
 ...
}

public final class ActivityManagerService extends ActivityManagerNative
  implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
 ...
 private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
 ...
 try {
  ...
  // Start the process. It will either succeed and return a result containing
  // the PID of the new process, or else throw a RuntimeException.
  boolean isActivityProcess = (entryPoint == null);
  if (entryPoint == null) entryPoint = "android.app.ActivityThread";
  Process.ProcessStartResult startResult = Process.start(entryPoint,
     app.processName, uid, uid, gids, debugFlags, mountExternal,
     app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
     app.info.dataDir, entryPointArgs);
  ...
 } catch () {
  ...
 }
 }
 ...
}

public final class ActivityThread {
 ...
 public static void main(String[] args) {
  ...
  Looper.prepareMainLooper();

  ActivityThread thread = new ActivityThread();
  thread.attach(false);

  ...

  Looper.loop();
  
  ...
 }
 
 private void attach(boolean system) {
  ...
  if (!system) {
   ...
   final IActivityManager mgr = ActivityManagerNative.getDefault();
   try {
    mgr.attachApplication(mAppThread);
   } catch (RemoteException ex) {
    // Ignore
   }
  } else {
   ...
  }
  ...
 }
 ...
}

public final class ActivityStackSupervisor implements DisplayListener {
 ...
 final boolean realStartActivityLocked(ActivityRecord r,
   ProcessRecord app, boolean andResume, boolean checkConfig)
   throws RemoteException {
  ...
  r.app = app;
  ...
   
  try {
  ...
  app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
     System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
     r.compat, r.launchedFromPackage, r.task.voiceInteractor, app.repProcState,
     r.icicle, r.persistentState, results, newIntents, !andResume,
     mService.isNextTransitionForward(), profilerInfo);
  ...
  } catch (RemoteException e) {
  ...
  }
  ...
 }
 ...
}