java - Android java JSON 解析 'null' 作为输出

Question

我们一直在使用this code在我们的应用程序中从在线数据库获取数据，但无论我们尝试什么，它总是显示“null”作为专辑和持续时间的输出。这可能是一个非常小的细节，但由于我们是初级开发人员，所以我们对 JSON 的了解并不是很多。有人有什么建议吗？

package com.example.androidhive;

import java.util.ArrayList;
import java.util.List;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.text.Html;
import android.util.Log;
import android.widget.TextView;

import com.example.androidhive.helper.AlertDialogManager;
import com.example.androidhive.helper.ConnectionDetector;
import com.example.androidhive.helper.JSONParser;

public class SingleTrackActivity extends Activity {
    // Connection detector
    ConnectionDetector cd;

    // Alert dialog manager
    AlertDialogManager alert = new AlertDialogManager();

    // Progress Dialog
    private ProgressDialog pDialog;

    // Creating JSON Parser object
    JSONParser jsonParser = new JSONParser();

    // tracks JSONArray
    JSONArray albums = null;

    // Album id
    String album_id = null;
    String song_id = null;

    String album_name, song_name, duration;

    // single song JSON url
    // GET parameters album, song
    private static final String URL_SONG = "http://api.androidhive.info/songs/track.php";

    // ALL JSON node names
    private static final String TAG_NAME = "name";
    private static final String TAG_DURATION = "duration";
    private static final String TAG_ALBUM = "album";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_single_track);

        cd = new ConnectionDetector(getApplicationContext());

        // Check if Internet present
        if (!cd.isConnectingToInternet()) {
            // Internet Connection is not present
            alert.showAlertDialog(SingleTrackActivity.this, "Internet Connection Error",
                    "Please connect to working Internet connection", false);
            // stop executing code by return
            return;
        }

        // Get album id, song id
        Intent i = getIntent();
        album_id = i.getStringExtra("album_id");
        song_id = i.getStringExtra("song_id");

        // calling background thread
        new LoadSingleTrack().execute();
    }

    /**
     * Background Async Task to get single song information
     * */
    class LoadSingleTrack extends AsyncTask<String, String, String> {

        /**
         * Before starting background thread Show Progress Dialog
         * */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pDialog = new ProgressDialog(SingleTrackActivity.this);
            pDialog.setMessage("Loading song ...");
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(false);
            pDialog.show();
        }

        /**
         * getting song json and parsing
         * */
        protected String doInBackground(String... args) {
            // Building Parameters
            List<NameValuePair> params = new ArrayList<NameValuePair>();

            // post album id, song id as GET parameters
            params.add(new BasicNameValuePair("album", album_id));
            params.add(new BasicNameValuePair("song", song_id));

            // getting JSON string from URL
            String json = jsonParser.makeHttpRequest(URL_SONG, "GET",
                    params);

            // Check your log cat for JSON reponse
            Log.d("Single Track JSON: ", json);

            try {
                JSONObject jObj = new JSONObject(json);
                if(jObj != null){
                    song_name = jObj.getString(TAG_NAME);
                    album_name = jObj.getString(TAG_ALBUM);
                    duration = jObj.getString(TAG_DURATION);                    
                }           

            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;
        }

        /**
         * After completing background task Dismiss the progress dialog
         * **/
        protected void onPostExecute(String file_url) {
            // dismiss the dialog after getting song information
            pDialog.dismiss();

            // updating UI from Background Thread
            runOnUiThread(new Runnable() {
                public void run() {

                    TextView txt_song_name = (TextView) findViewById(R.id.song_title);
                    TextView txt_album_name = (TextView) findViewById(R.id.album_name);
                    TextView txt_duration = (TextView) findViewById(R.id.duration);

                    // displaying song data in view
                    txt_song_name.setText(song_name);
                    txt_album_name.setText(Html.fromHtml("<b>Album:</b> " + album_name));
                    txt_duration.setText(Html.fromHtml("<b>Duration:</b> " + duration));

                    // Change Activity Title with Song title
                    setTitle(song_name);
                }
            });

        }

    }
}

Answer 1

查看您链接到的教程，请求看起来如下所示:

http://api.androidhive.info/songs/track.php?album=3&song=1

响应是:

{"id":1,"name":"Born to Die","duration":"4:46","album_id":"3","album":"Lana Del Rey - Born to Die"}

鉴于...您的 JSON 解析没有任何问题。

您需要验证“专辑”(album_id) 和“歌曲”(song_id) 参数是否是您执行 GET 时所认为的参数。

如果 PHP 脚本为空，则不会返回任何数据(例如 http://api.androidhive.info/songs/track.php?album=&song=)，只返回 no album 如果它们不是它识别的东西。看起来这就是正在发生的情况，因此程序的输出值是 null。