public class ActivityBackStack implements Application.ActivityLifecycleCallbacks { 
  private Stack<Activity> mActivityStack; 
  public ActivityBackStack() { 
    mActivityStack = new Stack<>(); 
  } 
  @Override 
  public void onActivityCreated(Activity activity, Bundle savedInstanceState) { 
    mActivityStack.add(activity); 
  } 
  @Override 
  public void onActivityStarted(Activity activity) { 
  } 
  @Override 
  public void onActivityResumed(Activity activity) { 
  } 
  @Override 
  public void onActivityPaused(Activity activity) { 
  } 
  @Override 
  public void onActivityStopped(Activity activity) { 
  } 
  @Override 
  public void onActivitySaveInstanceState(Activity activity, Bundle outState) { 
  } 
  @Override 
  public void onActivityDestroyed(Activity activity) { 
    if (activity != null) { 
      mActivityStack.remove(activity); 
    } 
  } 
  /** 
   * 获取栈顶的Activity 
   * 
   * @return 
   */ 
  public Activity getLastActivity() { 
    return mActivityStack.lastElement(); 
  } 
  /** 
   * 获取倒数第二个Activity 
   * 
   * @return 
   */ 
  public Activity getPreActivty() { 
    int size = mActivityStack.size(); 
    if (size < 2) { 
      return null; 
    } 
    return mActivityStack.get(size - 2); 
  } 
  public int getSize() { 
    return mActivityStack.size(); 
  } 
}

registerActivityLifecycleCallbacks(mBackStack);