Android URL连接错误：使用XAMPP的数据库连接错误

我目前正在建立一个需要管理历史交易数据库的项目。

我按照此网站的说明进行操作，以便将我的数据库连接到该项目：

如何将Android应用程序连接到MySQL数据库

之后，我尝试使用中的代码构建项目Fragment_History，但是，发生了此错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mind.loginregisterapps, PID: 6912
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
    at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:121)
    at org.json.JSONTokener.nextValue(JSONTokener.java:98)
    at org.json.JSONArray.<init>(JSONArray.java:94)
    at org.json.JSONArray.<init>(JSONArray.java:110)
    at com.mind.loginregisterapps.Fragment_History.loadIntoListView(Fragment_History.java:83)
    at com.mind.loginregisterapps.Fragment_History.access$000(Fragment_History.java:24)
    at com.mind.loginregisterapps.Fragment_History$1DownloadJSON.onPostExecute(Fragment_History.java:55)
    at com.mind.loginregisterapps.Fragment_History$1DownloadJSON.onPostExecute(Fragment_History.java:42)
    at android.os.AsyncTask.finish(AsyncTask.java:771)
    at android.os.AsyncTask.access$900(AsyncTask.java:199)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:788)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

我检查了代码，意识到由于downloadJSON中的URL连接失败而传递了空值。

因此，我添加了res/xml/network_security_config.xml：

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartextTrafficPermitted="true">
        <domain includeSubdomains="true">localhost</domain>
    </domain-config>
</network-security-config>

并编辑清单：

<application
        ...
        android:networkSecurityConfig="@xml/network_security_config"
        android:usesCleartextTraffic="true"
</application>

但是，当使用时HTTPUrlConnection，会发生此错误，我迷迷糊糊：

Failed to connect to localhost/127.0.0.1:80

我试图在downloadJSON和中将localhost更改为127.0.0.1，network_security_config.xml错误变成：

Failed to connect to /127.0.0.1:80

如何解决此问题？请告诉我。

非常感谢！

这是以下代码：

Fragment_History.java

public class Fragment_History extends Fragment {

ListView listView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_history, container, false);

    listView = (ListView)view.findViewById(R.id.list_history);
    downloadJSON("http://localhost/hist_login.php");

    return view;
}

private void downloadJSON(final String urlWebService) {

    class DownloadJSON extends AsyncTask<Void, Void, String> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }


        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            Toast.makeText(getActivity(), s, Toast.LENGTH_SHORT).show();
            try {
                loadIntoListView(s);
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }

        @Override
        protected String doInBackground(Void... voids) {
            try {
                URL url = new URL(urlWebService);
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                StringBuilder sb = new StringBuilder();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
                String json;
                while ((json = bufferedReader.readLine()) != null) {
                    sb.append(json + "\n");
                }
                return sb.toString().trim();
            } catch (Exception e) {
                return null;
            }
        }
    }
    DownloadJSON getJSON = new DownloadJSON();
    getJSON.execute();
}

private void loadIntoListView(String json) throws JSONException {
    JSONArray jsonArray = new JSONArray(json);
    ArrayList<History> list = new ArrayList<History>();
    for (int i = 0; i < jsonArray.length(); i++) {
        JSONObject obj = jsonArray.getJSONObject(i);
        list.add(new History(obj.getString("h_rName"),obj.getDouble("h_amount")));
    }
    HistoryAdapter adapter = new HistoryAdapter(list, getActivity());
    listView.setAdapter(adapter);
}

}

hist_login.php：

<?php
$db = "test";
$user = "root";
$pass = "";
$host = "localhost";

// Create connection
$con=mysqli_connect($host,$user,$pass,$db);
 
// Check connection
if (mysqli_connect_errno())
{
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
 
// Select all of our stocks from table 'history'
$sql = "SELECT * FROM history";
 
// Confirm there are results
if ($result = mysqli_query($con, $sql))
{
    // We have results, create an array to hold the results
        // and an array to hold the data
    $resultArray = array();
    $tempArray = array();
 
    // Loop through each result
    while($row = $result->fetch_object())
    {
        // Add each result into the results array
        $tempArray = $row;
        array_push($resultArray, $tempArray);
    }
 
    // Encode the array to JSON and output the results
    echo json_encode($resultArray);
} else
{
    echo "Not connected...";
}
 
// Close connections
mysqli_close($con);
?>