如何使用 BackupAgentHelper

有一个代码：

public class BackupHelper extends BackupAgentHelper {

    public static final Object[] DATA_LOCK = new Object[0];

    @Override
    public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
                         ParcelFileDescriptor newState) throws IOException {

        synchronized (DATA_LOCK) {
            super.onBackup(oldState, data, newState);
        }
        Log.d("BackupHelper", "onBackup()");
    }

    @Override
    public void onCreate(){
        FileBackupHelper db = new FileBackupHelper(this, DatabaseHelper.DATABASE_NAME);
        addHelper(DatabaseHelper.DATABASE_NAME, db);
        Log.d("BackupHelper", "onCreate()");
    }

    @Override
    public File getFilesDir(){
        File path = getDatabasePath(DatabaseHelper.DATABASE_NAME);
        return path.getParentFile();
    }

    @Override
    public void onRestore(BackupDataInput data, int appVersionCode, ParcelFileDescriptor newState)
            throws IOException {
        Log.d("BackupHelper", "onRestore()");
        synchronized (DATA_LOCK) {
            super.onRestore(data, appVersionCode, newState);
        }
    }
}

清单说：

android:allowBackup="true"
android:fullBackupOnly="true"
android:backupInForeground="true"
android:backupAgent=".app.db.BackupHelper"

我这样称呼：

private BackupManager backupManager;
backupManager = new BackupManager(context);

但即使是来自 BackupHelper 的日志也不会显示。也许我做错了什么？请帮助，我已经挣扎了几天。