从我上一篇文章中得知，我需要使用异步任务从url解析json，做了同样的事情并附在下面，
public class ReadJson extends ListActivity {
private static String url = "http://docs.blackberry.com/sampledata.json";
private static final String TAG_VTYPE = "vehicleType";
private static final String TAG_VCOLOR = "vehicleColor";
private static final String TAG_FUEL = "fuel";
private static final String TAG_TREAD = "treadType";
ArrayList<HashMap<String, String>> jsonlist = new ArrayList<HashMap<String, String>>();
ListView lv ;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_read_json);
new ProgressTask(ReadJson.this).execute();
}
private class ProgressTask extends AsyncTask<String, Void, Boolean> {
private ProgressDialog dialog;
// private List<Message> messages;
public ProgressTask(ListActivity activity) {
context = activity;
dialog = new ProgressDialog(context);
}
/** progress dialog to show user that the backup is processing. */
/** application context. */
private Context context;
protected void onPreExecute() {
this.dialog.setMessage("Progress start");
this.dialog.show();
}
@Override
protected void onPostExecute(final Boolean success) {
if (dialog.isShowing()) {
dialog.dismiss();
}
ListAdapter adapter = new SimpleAdapter(context, jsonlist,
R.layout.list_item, new String[] { TAG_VTYPE, TAG_VCOLOR,
TAG_FUEL, TAG_TREAD }, new int[] {
R.id.vehicleType, R.id.vehicleColor, R.id.fuel,
R.id.treadType });
setListAdapter(adapter);
// selecting single ListView item
lv = getListView();
}
protected Boolean doInBackground(final String... args) {
JSONParser jParser = new JSONParser();
JSONArray json = jParser.getJSONFromUrl(url);
for (int i = 0; i < json.length(); i++) {
try {
JSONObject c = json.getJSONObject(i);
String vtype = c.getString(TAG_VTYPE);
String vcolor = c.getString(TAG_VCOLOR);
String vfuel = c.getString(TAG_FUEL);
String vtread = c.getString(TAG_TREAD);
HashMap<String, String> map = new HashMap<String, String>();
map.put(TAG_VTYPE, vtype);
map.put(TAG_VCOLOR, vcolor);
map.put(TAG_FUEL, vfuel);
map.put(TAG_TREAD, vtread);
jsonlist.add(map);
} catch (JSONException e) {
e.printStackTrace();
}
}
return null;
}
}
}
当我执行此操作时，我在行中的asyc背景中执行错误时得到空指针异常，因为（int i = 0; i＆lt; json.length（）; i ++），尝试了几件事但没有工作，任何帮助将受到赞赏!!
编辑1：添加了解析器代码
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.StatusLine;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.util.Log;
public class JSONParser {
static InputStream is = null;
static JSONArray jarray = null;
static String json = "";
// constructor
public JSONParser() {
}
public JSONArray getJSONFromUrl(String url) {
StringBuilder builder = new StringBuilder();
HttpClient client = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
try {
HttpResponse response = client.execute(httpGet);
StatusLine statusLine = response.getStatusLine();
int statusCode = statusLine.getStatusCode();
if (statusCode == 200) {
HttpEntity entity = response.getEntity();
InputStream content = entity.getContent();
BufferedReader reader = new BufferedReader(new InputStreamReader(content));
String line;
while ((line = reader.readLine()) != null) {
builder.append(line);
}
} else {
Log.e("==>", "Failed to download file");
}
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
// try parse the string to a JSON object
try {
jarray = new JSONArray( builder.toString());
} catch (JSONException e) {
Log.e("JSON Parser", "Error parsing data " + e.toString());
}
// return JSON String
return jarray;
}
}
答案 0 :(得分：0)
我认为你正在使用this JSONParser。似乎正在发生的事情是你有一个没有生成有效JSON的URL，这会导致Exception被抛出并在整个过程中的某个类中被捕获 - 最有可能出现在jObj = new JSONObject(json);线。最终结果是返回的变量仍为null。因此，当您在循环中调用json.length()时，您尝试在length()对象上调用null。您应该在进入循环之前对其进行检查，以确保它不是null。
答案 1 :(得分：0)
我认为，您没有从服务器获取有效的JSON格式。在解析之前检查从服务器获取的响应，这里是代码片段，这里传递url并在logcat中获取yourReponseInJSONStr检查响应字符串是否采用正确的JSON格式，然后进行解析操作。
try {
HttpClient httpclient = new DefaultHttpClient();
HttpPost httppost = new HttpPost(
url);
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
ResponseHandler<String> responseHandler = new BasicResponseHandler();
String yourReponseInJSONStr = httpclient.execute(httppost,
responseHandler);
Log.d("yourReponseInJSONStr ", yourReponseInJSONStr );
JSONObject yourJsonObj = new JSONObject(yourReponseInJSONStr);
} catch (Exception e) {
e.printStackTrace();
}
你也可以继续解析这段代码，希望这对你有所帮助，
答案 2 :(得分：0)
尝试获取json
public static String getJSONString(String url) {
String jsonString = null;
HttpURLConnection linkConnection = null;
try {
URL linkurl = new URL(url);
linkConnection = (HttpURLConnection) linkurl.openConnection();
int responseCode = linkConnection.getResponseCode();
if (responseCode == HttpURLConnection.HTTP_OK) {
InputStream linkinStream = linkConnection.getInputStream();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
int j = 0;
while ((j = linkinStream.read()) != -1) {
baos.write(j);
}
byte[] data = baos.toByteArray();
jsonString = new String(data);
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if (linkConnection != null) {
linkConnection.disconnect();
}
}
return jsonString;
}
public static boolean isNetworkAvailable(Activity activity) {
ConnectivityManager connectivity = (ConnectivityManager) activity
.getSystemService(Context.CONNECTIVITY_SERVICE);
if (connectivity == null) {
return false;
} else {
NetworkInfo[] info = connectivity.getAllNetworkInfo();
if (info != null) {
for (int i = 0; i < info.length; i++) {
if (info[i].getState() == NetworkInfo.State.CONNECTED) {
return true;
}
}
}
}
return false;
}
使用isNetworkAvailable检查连接
我用这种方式解析
try {
JSONObject jObjectm = new JSONObject(result);
JSONObject jObject=jObjectm.getJSONObject("items");
if(jObject!=null)
{
Iterator<?> iterator1=jObject.keys();
LinkedHashMap<String,LinkedHashMap<String, Object> > inneritem = new LinkedHashMap<String, LinkedHashMap<String, Object> >();
while (iterator1.hasNext() ){
Item hashitem=new Item();
String key1 = (String)iterator1.next();
JSONObject jObject1=jObject.getJSONObject(key1);
Iterator<?> iterator=jObject1.keys();
LinkedHashMap<String, Object> inneritem1 = new LinkedHashMap<String, Object>();
while (iterator.hasNext() ){
String key =(String) iterator.next();
inneritem1.put(key, jObject1.get(key));
}
hashitem.setItem(key1,inneritem1);
inneritem.put(key1,inneritem1);
arrayOfList.add(hashitem);
}
}
} catch (JSONException e) {
System.out.println("NO Json data found");
}