为什么 ArrayList 会将其值重置为零？

有一个方法 initData(urisImg) 应该创建一个 Cover 对象列表。输入是一个ArrayList urisImg，在onCreate方法中初始化，每次通过FileLoader()方法加载文件时填充值。我不明白为什么上传文件后我的 urisImg 列表大小变为零。也就是initData方法没有开始工作，因为urisImg列表是空的……请帮我找出错误。下面的代码

import android.net.Uri;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.widget.Toast;

import com.thin.downloadmanager.DownloadRequest;
import com.thin.downloadmanager.ThinDownloadManager;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.File;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private String filename;
    private String fileUrl;
    private ArrayList<Uri> urisImg;
    private ArrayList<String> urls;

    private static final String LOG_TAG = "my_log";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        recyclerView = (RecyclerView) findViewById(R.id.recyclerview);
        LinearLayoutManager llm = new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
        recyclerView.setLayoutManager(llm);
        recyclerView.setHasFixedSize(true);

        urisImg = new ArrayList<>();

        File file = new File(this.getCacheDir().getPath() + File.separator + "booklist.json");

        if (!file.exists()){
            new ParseBookCovers().execute();
        }
        else {
            CoverFormer(); //посмотреть потом, будет ли успевать выполнять код при большом количестве обложек
        }

        initializeAdapter();

    }

    private List<Cover> initData(ArrayList<Uri> urisImg) {
        ArrayList<Cover> covers = new ArrayList<>();
        for (int i=0; i<urisImg.size();i++){
            covers.add(new Cover(urisImg.get(i)));
        }
        return covers;
    }

    private void initializeAdapter() {
        RecyclerAdapter recyclerAdapter = new RecyclerAdapter(initData(urisImg));
        recyclerView.setAdapter(recyclerAdapter);
    }

    private void FileLoader(String fileUrl, String filename) {

        ThinDownloadManager downloadManager = new ThinDownloadManager(5); //количество потоков загрузки
        Uri downloadUri = Uri.parse(fileUrl);
        Uri destinationUri = Uri.parse(filename);
        DownloadRequest downloadRequest = new DownloadRequest(downloadUri).setDestinationURI(destinationUri);
        downloadManager.add(downloadRequest);
        urisImg.add(destinationUri);

    }

    private class ParseBookCovers extends AsyncTask<Void, Void, String> {

        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;
        String resultJson = "";

        @Override
        protected String doInBackground(Void... params) {
            // получаем данные с внешнего ресурса
            try {
                URL url = new URL("http://******.ru/todbook/booklist.json");

                urlConnection = (HttpURLConnection) url.openConnection();
                urlConnection.setRequestMethod("GET");
                urlConnection.connect();

                InputStream inputStream = urlConnection.getInputStream();
                StringBuffer buffer = new StringBuffer();

                reader = new BufferedReader(new InputStreamReader(inputStream));

                String line;
                while ((line = reader.readLine()) != null) {
                    buffer.append(line);
                }

                resultJson = buffer.toString();
                MyJSON.saveData(getApplicationContext(), resultJson);


            } catch (Exception e) {
                e.printStackTrace();
            }
            return resultJson;
        }

        @Override
        protected void onPostExecute(String strJson) {
            CoverFormer();

        }
    }

    private ArrayList<String> GetListUrlCovers(String strJson) {

        JSONObject dataJsonObj;

        ArrayList<String> urls = new ArrayList<>();

        try {
            dataJsonObj = new JSONObject(strJson);
            JSONArray books = dataJsonObj.getJSONArray("books");

            for (int i = 0; i < books.length(); i++) {
                JSONObject book = books.getJSONObject(i);
                String url_book = book.getString("coverUrl");
                urls.add(url_book);// пишу урлы в ArrayList urls
                Log.d(LOG_TAG, "coverUrl: " + url_book);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return urls;
    }

    private void CoverFormer(){
        MyJSON.getData(getApplicationContext()); //read file json
        urls = GetListUrlCovers(MyJSON.getData(getApplicationContext())); // parse urls + write to array

        for (int i = 0; i < urls.size(); i++) {
            int d = i + 1;
            fileUrl = urls.get(i);
            filename = this.getFilesDir() + File.separator + "bookcover_" + d + ".jpg";
            FileLoader(fileUrl, filename);
        }
    }

}