boolean startActivitySafely(Intent intent, Object tag) { 
  intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
  try { 
   startActivity(intent); 
   return true; 
  } catch (ActivityNotFoundException e) {} 
} 

@Override 
 public void startActivity(Intent intent, @Nullable Bundle options) { 
  if (options != null) { 
   startActivityForResult(intent, -1, options); 
  } else { 
   startActivityForResult(intent, -1); 
  } 
 } 

public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) { 
  if (mParent == null) { //一般的Activity其mParent都为null 
   Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(this, 
      mMainThread.getApplicationThread(), mToken, this,intent, requestCode, options); 
   if (ar != null) { //发送结果，即onActivityResult会被调用 
    mMainThread.sendActivityResult(mToken, mEmbeddedID, requestCode, ar.getResultCode(), 
      ar.getResultData()); 
   } 
   if (requestCode >= 0) { 
    mStartedActivity = true; 
   } 
 
   final View decor = mWindow != null ? mWindow.peekDecorView() : null; 
   if (decor != null) { 
    decor.cancelPendingInputEvents(); 
   } 
  } else { //在ActivityGroup内部的Activity，内部处理逻辑和上面是类似的 
   if (options != null) { 
    mParent.startActivityFromChild(this, intent, requestCode, options); 
   } else { 
    mParent.startActivityFromChild(this, intent, requestCode); 
   } 
  } 
  if (options != null && !isTopOfTask()) { 
   mActivityTransitionState.startExitOutTransition(this, options); 
  } 
 } 

public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target, 
   Intent intent, int requestCode, Bundle options) { 
  IApplicationThread whoThread = (IApplicationThread) contextThread; 
  if (mActivityMonitors != null) { 
   synchronized (mSync) { 
    final int N = mActivityMonitors.size(); 
    for (int i=0; i<N; i++) { //先查找一遍看是否存在这个activity 
     final ActivityMonitor am = mActivityMonitors.get(i); 
     if (am.match(who, null, intent)) { 
      am.mHits++; 
      if (am.isBlocking()) { 
       return requestCode >= 0 ? am.getResult() : null; 
      } 
      break; 
     } 
    } 
   } 
  } 
  try { 
   intent.migrateExtraStreamToClipData(); 
   intent.prepareToLeaveProcess(); 
   int result = ActivityManagerNative.getDefault().startActivity(whoThread, who.getBasePackageName(), intent, 
      intent.resolveTypeIfNeeded(who.getContentResolver()),token, target != null ? target.mEmbeddedID : null, 
      requestCode, 0, null, options); //这里才是真正打开activity的地方，其核心功能在whoThread中完成。 
   checkStartActivityResult(result, intent); // 处理各种异常，如ActivityNotFound 
  } catch (RemoteException e) { 
  } 
  return null; 
 } 

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident, 
    ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo, 
    String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state, 
    PersistableBundle persistentState, List<ResultInfo> pendingResults, 
    List<ReferrerIntent> pendingNewIntents, boolean notResumed, boolean isForward, 
    ProfilerInfo profilerInfo) { 
 
   updateProcessState(procState, false); 
 
   ActivityClientRecord r = new ActivityClientRecord(); 
 
   r.token = token; 
   r.ident = ident; 
   r.intent = intent; 
   r.referrer = referrer; 
   r.voiceInteractor = voiceInteractor; 
   r.activityInfo = info; 
   r.compatInfo = compatInfo; 
   r.state = state; 
   r.persistentState = persistentState; 
 
   r.pendingResults = pendingResults; 
   r.pendingIntents = pendingNewIntents; 
 
   r.startsNotResumed = notResumed; 
   r.isForward = isForward; 
 
   r.profilerInfo = profilerInfo; 
 
   updatePendingConfiguration(curConfig); 
 
   sendMessage(H.LAUNCH_ACTIVITY, r); 
  } 

switch (msg.what) { // 消息类型 
  case LAUNCH_ACTIVITY: { 
   Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); 
   final ActivityClientRecord r = (ActivityClientRecord) msg.obj; 
 
   r.packageInfo = getPackageInfoNoCheck( 
     r.activityInfo.applicationInfo, r.compatInfo); 
   handleLaunchActivity(r, null); // 处理消息 
   Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); 
  } break; 
  case RELAUNCH_ACTIVITY: { 
   Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart"); 
   ActivityClientRecord r = (ActivityClientRecord)msg.obj; 
   handleRelaunchActivity(r); 
   Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); 
  } break; 
  case PAUSE_ACTIVITY: 
   Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); 
   handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2, 
     (msg.arg1&2) != 0); 
   maybeSnapshot(); 
   Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); 
   break; 
  ... ... 
} 

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) { 
  unscheduleGcIdler(); 
  mSomeActivitiesChanged = true; 
 
  if (r.profilerInfo != null) { 
   mProfiler.setProfiler(r.profilerInfo); 
   mProfiler.startProfiling(); 
  } 
 
  handleConfigurationChanged(null, null); 
 
  if (localLOGV) Slog.v( 
   TAG, "Handling launch of " + r); 
 
  WindowManagerGlobal.initialize(); 
 
  Activity a = performLaunchActivity(r, customIntent); //performLaunchActivity真正完成了activity的调起,Activity被实例化，onCreate被调用 
 
  if (a != null) { 
   r.createdConfig = new Configuration(mConfiguration); 
   Bundle oldState = r.state; 
   handleResumeActivity(r.token, false, r.isForward, // 再调用Activity实例的Resume(用户界面可见) 
     !r.activity.mFinished && !r.startsNotResumed); 
 
   if (!r.activity.mFinished && r.startsNotResumed) { 
    try { 
     r.activity.mCalled = false; 
     mInstrumentation.callActivityOnPause(r.activity); // finish的时候先调onPause 
     if (r.isPreHoneycomb()) { 
      r.state = oldState; 
     } 
     if (!r.activity.mCalled) { 
      throw new SuperNotCalledException( 
       "Activity " + r.intent.getComponent().toShortString() + 
       " did not call through to super.onPause()"); 
     } 
 
    } catch (SuperNotCalledException e) { 
     throw e; 
 
    } catch (Exception e) { 
     if (!mInstrumentation.onException(r.activity, e)) { 
      throw new RuntimeException( 
        "Unable to pause activity " 
        + r.intent.getComponent().toShortString() 
        + ": " + e.toString(), e); 
     } 
    } 
    r.paused = true; 
   } 
  } else { 
   try { 
    ActivityManagerNative.getDefault() // finishActivity 一样的原理 
     .finishActivity(r.token, Activity.RESULT_CANCELED, null, false); 
   } catch (RemoteException ex) { 
   } 
  } 
 } 

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { 
  ActivityInfo aInfo = r.activityInfo; 
  if (r.packageInfo == null) { 
   r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo, 
     Context.CONTEXT_INCLUDE_CODE); 
  } 
  //从intent中取出目标activity的启动参数（包名、类名等） 
  ComponentName component = r.intent.getComponent(); 
  if (component == null) { 
   component = r.intent.resolveActivity( 
    mInitialApplication.getPackageManager()); 
   r.intent.setComponent(component); 
  } 
 
  if (r.activityInfo.targetActivity != null) { 
   component = new ComponentName(r.activityInfo.packageName, 
     r.activityInfo.targetActivity); 
  } 
 
  Activity activity = null; 
  try { 
   java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); // 将Activity类文件加载到内存中 
   activity = mInstrumentation.newActivity( // 创建Activity实例 
     cl, component.getClassName(), r.intent); 
   StrictMode.incrementExpectedActivityCount(activity.getClass()); 
   r.intent.setExtrasClassLoader(cl); 
   r.intent.prepareToEnterProcess(); 
   if (r.state != null) { 
    r.state.setClassLoader(cl); 
   } 
  } catch (Exception e) { 
   if (!mInstrumentation.onException(activity, e)) { 
    throw new RuntimeException( 
     "Unable to instantiate activity " + component 
     + ": " + e.toString(), e); 
   } 
  } 
 
  try { 
   Application app = r.packageInfo.makeApplication(false, mInstrumentation); 
 
   if (localLOGV) Slog.v(TAG, "Performing launch of " + r); 
   if (localLOGV) Slog.v( 
     TAG, r + ": app=" + app 
     + ", appName=" + app.getPackageName() 
     + ", pkg=" + r.packageInfo.getPackageName() 
     + ", comp=" + r.intent.getComponent().toShortString() 
     + ", dir=" + r.packageInfo.getAppDir()); 
 
   if (activity != null) { 
    Context appContext = createBaseContextForActivity(r, activity); // 初始化Context对象，作为Activity的上下文 
    CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); 
    Configuration config = new Configuration(mCompatConfiguration); 
    if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity " 
      + r.activityInfo.name + " with config " + config); 
    activity.attach(appContext, this, getInstrumentation(), r.token, 
      r.ident, app, r.intent, r.activityInfo, title, r.parent, 
      r.embeddedID, r.lastNonConfigurationInstances, config, 
      r.referrer, r.voiceInteractor); 
 
    if (customIntent != null) { 
     activity.mIntent = customIntent; 
    } 
    r.lastNonConfigurationInstances = null; 
    activity.mStartedActivity = false; 
    int theme = r.activityInfo.getThemeResource(); 
    if (theme != 0) { 
     activity.setTheme(theme); 
    } 
 
    activity.mCalled = false; 
    if (r.isPersistable()) { //下面就是调用到acitivity的onCreate方法了 
     mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState); 
    } else { 
     mInstrumentation.callActivityOnCreate(activity, r.state); 
    } // 至此，Activity启动过程就结束了，其生命周期由ApplicationThread来管理 
    if (!activity.mCalled) { 
     throw new SuperNotCalledException( 
      "Activity " + r.intent.getComponent().toShortString() + 
      " did not call through to super.onCreate()"); 
    } 
    r.activity = activity; 
    r.stopped = true; 
    if (!r.activity.mFinished) { 
     activity.performStart(); 
     r.stopped = false; 
    } 
    if (!r.activity.mFinished) { 
     if (r.isPersistable()) { 
      if (r.state != null || r.persistentState != null) { 
       mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state, 
         r.persistentState); 
      } 
     } else if (r.state != null) { 
      mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state); 
     } 
    } 
    if (!r.activity.mFinished) { 
     activity.mCalled = false; 
     if (r.isPersistable()) { 
      mInstrumentation.callActivityOnPostCreate(activity, r.state, 
        r.persistentState); 
     } else { 
      mInstrumentation.callActivityOnPostCreate(activity, r.state); 
     } 
     if (!activity.mCalled) { 
      throw new SuperNotCalledException( 
       "Activity " + r.intent.getComponent().toShortString() + 
       " did not call through to super.onPostCreate()"); 
     } 
    } 
   } 
   r.paused = true; 
   mActivities.put(r.token, r); // 将ActivityRecord对象保存在ActivityThread的mActivities中 
  } catch (SuperNotCalledException e) { 
   throw e; 
  } catch (Exception e) { 
   if (!mInstrumentation.onException(activity, e)) { 
    throw new RuntimeException( 
     "Unable to start activity " + component 
     + ": " + e.toString(), e); 
   } 
  } 
  return activity; 
 }