我试图从 AsyncTask 类中获取值以返回到主 UI，即使在 AsyncTask 类完成之后也是如此。来自 onPostExectute() 的字符串值将保存到json_string中以供以后使用。但是，当我尝试运行此代码时，它给了我NullPointerException。
我已经尽力了 3 天了，仍然不知道如何解决这个问题:( 请有人帮忙。我非常感谢你的帮助。
PS：我按照http://www.androidhive.info/2012/01/android-json-parsing-tutorial/的教程进行操作，但是我制作了这个简单的版本进行测试。
public class ParsingJSONDataSample extends Activity {
// url to make request
private static String url = "http://api.androidhive.info/contacts/";
// JSON Node names
private static final String TAG_CONTACTS = "contacts";
private static final String TAG_ID = "id";
private static final String TAG_NAME = "name";
static TextView textId, textName;
// contacts JSONArray
static JSONArray contacts = null;
static InputStream is = null;
static JSONObject jObj = null;
static String json_string;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.json_listview_sample);
textId = (TextView) findViewById(R.id.textView1);
textName = (TextView) findViewById(R.id.textView2);
new ReadJSONWebServiceFeedFromURL().execute(url);
jObj = new JSONObject(json_string);
// Getting Array of Contacts
contacts = jObj.getJSONArray(TAG_CONTACTS);
JSONObject c = contacts.getJSONObject(0);
textId.setText(c.getString(TAG_ID));
textName.setText(c.getString(TAG_NAME));
}
}
//get the JSON webservice from URL
public String getJSONFromUrl(String url) {
StringBuilder sb = new StringBuilder();
String json;
// Making HTTP request
try {
// defaultHttpClient
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
HttpResponse httpResponse = httpClient.execute(httpGet);
StatusLine statusLine = httpResponse.getStatusLine();
int statuscode = statusLine.getStatusCode();
if (statuscode == 200){
HttpEntity httpEntity = httpResponse.getEntity();
is = httpEntity.getContent();
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
String line = null;
while ((line = reader.readLine()) != null) {
sb.append(line + "\n");
}
is.close();
json = sb.toString();
}
else{
Log.d("readJSONFeed", "Failed to download file");
}
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
// return JSON String
return json;
}
//Asynctask class to read the JSON webservice in background
private class ReadJSONWebServiceFeedFromURL extends AsyncTask<String, Void, String>{
protected String doInBackground(String... urls) {
// TODO Auto-generated method stub
return getJSONFromUrl(urls[0]);
}
protected void onPostExecute(String result){
json_string = result;
}
}
}