android - 如何解决 java.net.UnknownHostException

Question

我在 list 文件中获得了互联网许可。我的移动网络连接也正常。主要代码:

/**
 * Created by Yogesh on 5/07/2016.
 */
public class All_Item_Fragment extends android.support.v4.app.ListFragment {
    // Declare Variables
    JSONArray jsonarray = null;
    ListView list;
    ListViewAdapter adapter;
    ArrayList<HashMap<String, String>> itemlist;
    static String NAME = "name";
    static String DESCRIPTION = "Description";
    static String PRICE = "price";
    static String IMAGE = "image_path";

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.all_item_layout, container, false);
        itemlist = new ArrayList<HashMap<String, String>>();
        new ReadJSON().execute();
        list = (ListView) view.findViewById(android.R.id.list);

        return view;
    }

    private class ReadJSON extends AsyncTask<Void, Void, Void> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Void... params) {
            // Create an array
            itemlist = new ArrayList<HashMap<String, String>>();
            // Retrieve JSON Objects from the given URL address
            JSONObject jsonobject = JSONfunctions.getJSONfromURL("http://mahatiffin.com/web/selectallmenu.php");

            try {
                // Locate the array name in JSON
                jsonarray = jsonobject.getJSONArray("AllMenu");

                for (int i = 0; i < jsonarray.length(); i++) {
                    HashMap<String, String> map = new HashMap<String, String>();
                    jsonobject = jsonarray.getJSONObject(i);
                    // Retrive JSON Objects
                    map.put("name", jsonobject.getString("name"));
                    map.put("Description", jsonobject.getString("Description"));
                    map.put("price", jsonobject.getString("price"));
                    map.put("image_path", jsonobject.getString("image_path"));
                    // Set the JSON Objects into the array
                    itemlist.add(map);
                }
            } catch (JSONException e) {
                Log.e("Error", e.getMessage());
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void args) {
            // Pass the results into ListViewAdapter.java
            adapter = new ListViewAdapter(getActivity(), itemlist);
            // Set the adapter to the ListView
            list.setAdapter((ListAdapter) adapter);
        }
    }
}


public class JSONfunctions {
    public static JSONObject getJSONfromURL(String url) {
        InputStream is = null;
        String result = "";
        JSONObject jArray = null;

        // Download JSON data from URL
        try {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(url);
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();

        } catch (Exception e) {
            Log.e("log_tag", "Error in http connection " + e.toString());
        }

        // Convert response to string
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            result = sb.toString();
        } catch (Exception e) {
            Log.e("log_tag", "Error converting result " + e.toString());
        }

        try {

            jArray = new JSONObject(result);
        } catch (JSONException e) {
            Log.e("log_tag", "Error parsing data " + e.toString());
        }

        return jArray;
    }
}

在可能的 fragment 页面中，不会显示任何内容，只会显示空白页面。

日志输出:

>05-18 17:43:11.730 27019-27019/com.androidbelieve.MahaTiffin D/dalvikvm: VFY: replacing opcode 0x6f at 0x0000
    05-18 17:43:11.740 27019-27082/com.androidbelieve.MahaTiffin E/log_tag: Error converting result java.lang.NullPointerException
    05-18 17:43:11.740 27019-27082/com.androidbelieve.MahaTiffin E/log_tag: Error parsing data org.json.JSONException: End of input at character 0 of 
    05-18 17:43:11.770 27019-27082/com.androidbelieve.MahaTiffin W/dalvikvm: threadid=12: thread exiting with uncaught exception (group=0x40e11378)
    05-18 17:43:11.770 27019-27082/com.androidbelieve.MahaTiffin E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
    java.lang.RuntimeException: An error occured while executing doInBackground()
    at android.os.AsyncTask$3.done(AsyncTask.java:299)
    at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
    at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
    at java.util.concurrent.FutureTas...(FutureTask.java:137)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
    at java.lang.Thread.run(Thread.java:856)
    Caused by: java.lang.NullPointerException
    at com.androidbelieve.MahaTiffin.All_Item_Fragment$ReadJSON.doInBackground(All_Item_Fragment.java:58)
    at com.androidbelieve.MahaTiffin.All_Item_Fragment$ReadJSON.doInBackground(All_Item_Fragment.java:43)
    at android.os.AsyncTask$2.call(AsyncTask.java:287)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
    at java.util.concurrent.FutureTas...(FutureTask.java:137) 
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) 
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076) 
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569) 
    at java.lang.Thread.run(Thread.java:856) 
    05-18 17:43:12.420 27019-27019/com.androidbelieve.MahaTiffin W/FragmentManager: moveToState: Fragment state for Veg_Item_Fragment{415d7030 #1 id=0x7f0c00a0 android:switcher:2131493024:1} not updated inline; expected state 3 found 2
    05-18 17:43:12.530 27019-27104/com.androidbelieve.MahaTiffin E/log_tag: Error in http connection java.net.UnknownHostException: Unable to resolve host "mahatiffin.com": No address associated with hostname
    05-18 17:43:12.540 27019-27104/com.androidbelieve.MahaTiffin E/log_tag: Error converting result java.lang.NullPointerException
    05-18 17:43:12.540 27019-27104/com.androidbelieve.MahaTiffin E/log_tag: Error parsing data org.json.JSONException: End of input at character 0 of 
    05-18 17:43:12.570 27019-27104/com.androidbelieve.MahaTiffin W/dalvikvm: threadid=14: thread exiting with uncaught exception (group=0x40e11378)
    05-18 17:43:12.570 27019-27104/com.androidbelieve.MahaTiffin I/Process: Sending signal. PID: 27019 SIG: 9

我该如何解决这个问题？

Answer 1

原因:通常，当您无法解析您提供的 URL 的 DNS 记录时，会触发 UnknownHostException。该操作有一个合理的超时时间，但如果您的 Wi-Fi 连接信号较弱或者您的设备上没有足够的信号，通信可能会在发送请求和接收响应之间中断，因此您的设备没有收到响应，因此它认为这是 DNS 超时。

长话短说，异常主要有两个原因:

1. 如果您的 Wi-Fi 信号较弱。
2. 如果您的设备连接到 Wi-Fi 但没有互联网可用。

解决方案:在请求之前检查互联网的可用性

public static boolean isInternetOn(Context context) {

    if (isMobileOrWifiConnectivityAvailable(context)) {
        try {
            HttpURLConnection urlc = (HttpURLConnection) (new URL("http://www.google.com").openConnection());
            urlc.setRequestProperty("User-Agent", "Test");
            urlc.setRequestProperty("Connection", "close");
            urlc.setConnectTimeout(1500);
            urlc.connect();
            return (urlc.getResponseCode() == 200);
        } catch (Exception e) {
          DebugLog.console("Couldn't check internet connection Exception is : " + e);
        }
    } else {
        DebugLog.console("Internet not available!");
    }
    return false;
}


public static boolean isMobileOrWifiConnectivityAvailable(Context ctx) {
    boolean haveConnectedWifi = false;
    boolean haveConnectedMobile = false;


    try {
        ConnectivityManager cm = (ConnectivityManager) ctx.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo[] netInfo = cm.getAllNetworkInfo();
        for (NetworkInfo ni : netInfo) {
            if (ni.getTypeName().equalsIgnoreCase("WIFI"))
                if (ni.isConnected()) {
                    haveConnectedWifi = true;
                }
            if (ni.getTypeName().equalsIgnoreCase("MOBILE"))
                if (ni.isConnected()) {
                    haveConnectedMobile = true;
                }
        }
    } catch (Exception e) {
        DebugLog.console("[ConnectionVerifier] inside isInternetOn() Exception is : " + e.toString());
    }
    return haveConnectedWifi || haveConnectedMobile;
}