1. This is a normal phenomenon.

It seems that, on mobile phones, the running Activities, that are in Landscape mode, are switched into Portrait mode upon screen lock is a normal behaviour. At least it is true on the tested phones so far. So we have to make sure our lifecycle functions can take care this change elegantly always.

2. Guess this is because of the "default orientation" in the locked screen.

We do not have documentation or many resource talking about this issue. But the assumption that the running Activities switching back to the device's "default orientation" upon screen lock, as in most devices the locked screen is in Portrait, is quite logical.

Further study:

I just wonder how the Activities behave if we are having a landscape locked screen?

You can stop the activity from restarting on orientation change but this is generally a pretty bad idea.

The Android documentation has a section on handling runtime changes with this note:

Note: Handling the configuration change yourself can make it much more difficult to use alternative resources, because the system does not automatically apply them for you. This technique should be considered a last resort when you must avoid restarts due to a configuration change and is not recommended for most applications.

Android generally only recommends you supress recreating on rotation if you don't need alternate resources and, more importantly, have a performance requirement. A well-designed app shouldn't need to do this in most cases.

If you insist on going down the path of supressing default Android behavior, I'd modify Raghunandan's code and include a screen size attribute as well. As of API level 13, the screen size changes upon orientation change. So you must include screenSize unless you are only targeting API 12 and below.

<activity android:name=".MyActivity"
      android:configChanges="orientation|screenSize"
      android:label="@string/app_name">

To avoid activity from restarting

<activity android:name=".MyActivity"
      android:configChanges="orientation|keyboardHidden"//add tthis in manifest
      android:label="@string/app_name">

http://developer.android.com/guide/topics/resources/runtime-changes.html.

In normal circumstances when your screen is locked your activity is paused and when screen is unlocked activity resumes.

An issue when screen locked is: Current Activity may be stopped forcefully by the system if it finds shortage of memory, instead of moving Activity to background. In such a case, we should have to save (all the necessary data) the current state of the Activity.

Save you data in onSaveInstanceState() and restore data onRestoreInstanceState().

@Override
 public void  onSaveInstanceState(Bundle outState)
 {
  Log.v("$````$", "In Method: onSaveInstanceState()");
  //if necessary,set a flag to check whether we have to restore or not
  //handle necessary savings…
 }

@Override
public void onRestoreInstanceState(Bundle inState)
{
  Log.v("$````$", "In Method: onRestoreInstanceState()");
  //if any saved state, restore from it…
}

In your onCreate()

IntentFilter filter = new IntentFilter(Intent.ACTION_SCREEN_ON);
filter.addAction(Intent.ACTION_SCREEN_OFF);
filter.addAction(Intent.ACTION_USER_PRESENT);

mReceiver = new ScreenReceiver();
registerReceiver(mReceiver, filter); //register


public class ScreenReceiver extends BroadcastReceiver {

  @Override
  public void onReceive(Context context, Intent intent)
   {
        if (intent.getAction().equals(Intent.ACTION_SCREEN_OFF))
        {    
              Log.v("$$$$$$", "In Method:  ACTION_SCREEN_OFF");
              // onPause() will be called.
        }
        else if (intent.getAction().equals(Intent.ACTION_SCREEN_ON))
        {
              Log.v("$$$$$$", "In Method:  ACTION_SCREEN_ON");
              //onResume() will be called.
              //Better check for whether the screen was already locked
              // if locked, do not take any resuming action in onResume()
              //Suggest you, not to take any resuming action here.       
        }
        else if(intent.getAction().equals(Intent.ACTION_USER_PRESENT))
        {
              Log.v("$$$$$$", "In Method:  ACTION_USER_PRESENT"); 
              //Handle resuming events
        }
  }

In your onDestroy

  @Override
  public void onDestroy()
  {
        super.onDestroy();
        Log.v("$$$$$$", "In Method: onDestroy()");

        if (mReceiver != null)
        {
              unregisterReceiver(mReceiver); //unregister 
              mReceiver = null;
        }          

  }

来源：https://stackoverflow.com/questions/15645611/activity-return-to-portrait-when-screen-is-locked

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