public int checkAddPermission(WindowManager.LayoutParams attrs) {
  int type = attrs.type;

  if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW
      || type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {
    return WindowManagerImpl.ADD_OKAY;
  }
  String permission = null;
  switch (type) {
    case TYPE_TOAST:
      // XXX right now the app process has complete control over
      // this... should introduce a token to let the system
      // monitor/control what they are doing.
      break;
    case TYPE_INPUT_METHOD:
    case TYPE_WALLPAPER:
      // The window manager will check these.
      break;
    case TYPE_PHONE:
    case TYPE_PRIORITY_PHONE:
    case TYPE_SYSTEM_ALERT:
    case TYPE_SYSTEM_ERROR:
    case TYPE_SYSTEM_OVERLAY:
      permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;
      break;
    default:
      permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;
  }
  if (permission != null) {
    if (mContext.checkCallingOrSelfPermission(permission)
        != PackageManager.PERMISSION_GRANTED) {
      return WindowManagerImpl.ADD_PERMISSION_DENIED;
    }
  }
  return WindowManagerImpl.ADD_OKAY;
}

android.Manifest.permission.INTERNAL_SYSTEM_WINDOW

/**
 * Window type: phone. These are non-application windows providing
 * user interaction with the phone (in particular incoming calls).
 * These windows are normally placed above all applications, but behind
 * the status bar.
 * In multiuser systems shows on all users' windows.
 */
public static final int TYPE_PHONE       = FIRST_SYSTEM_WINDOW+2;

/**
 * Window type: priority phone UI, which needs to be displayed even if
 * the keyguard is active. These windows must not take input
 * focus, or they will interfere with the keyguard.
 * In multiuser systems shows on all users' windows.
 */
public static final int TYPE_PRIORITY_PHONE   = FIRST_SYSTEM_WINDOW+7;

/**
 * Window type: system window, such as low power alert. These windows
 * are always on top of application windows.
 * In multiuser systems shows only on the owning user's window.
 */
public static final int TYPE_SYSTEM_ALERT = FIRST_SYSTEM_WINDOW+3;

/**
 * Window type: system overlay windows, which need to be displayed
 * on top of everything else. These windows must not take input
 * focus, or they will interfere with the keyguard.
 * In multiuser systems shows only on the owning user's window.
 */
public static final int TYPE_SYSTEM_OVERLAY = FIRST_SYSTEM_WINDOW+6;

/**
 * Window type: internal system error windows, appear on top of
 * everything they can.
 * In multiuser systems shows only on the owning user's window.
 */
public static final int TYPE_SYSTEM_ERROR = FIRST_SYSTEM_WINDOW+10;

protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState); 
  final Window win = getWindow(); 
  win.addFlags(WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED 
      | WindowManager.LayoutParams.FLAG_DISMISS_KEYGUARD 
      | WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON 
      | WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON); 
}

<activity android:name=".LockScreenActivity" 
     android:launchMode="singleInstance" 
     android:excludeFromRecents="true" 
     android:theme="@android:style/Theme.Wallpaper.NoTitleBar"/>

KeyguardManager km = (KeyguardManager) context.getSystemService(Context.KEYGUARD_SERVICE);
if (km.inKeyguardRestrictedInputMode()) {
  // 处于锁屏状态
}