这是我的问题。
我尝试从 url
获取 JSONObject
。这是我的解析器:
public class JSONParser {
static InputStream is = null;
static JSONObject jObj = null;
static String json = "";
// constructor
public JSONParser() {
}
public JSONObject getJSONFromUrl(String url) {
// Making HTTP request
try {
// defaultHttpClient
HttpParams params = new BasicHttpParams();
params.setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1);
HttpClient httpClient = new DefaultHttpClient(params);
HttpPost httpPost = new HttpPost(url);
HttpResponse httpResponse = httpClient.execute(httpPost);
HttpEntity httpEntity = httpResponse.getEntity();
is = httpEntity.getContent();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (ClientProtocolException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
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();
json = sb.toString();
} catch (Exception e) {
Log.e("Buffer Error", "Error converting result " + e.toString());
}
// try parse the string to a JSON object
try {
jObj = new JSONObject(json);
} catch (JSONException e) {
Log.e("JSON Parser", "Error parsing data " + e.toString());
}
// return JSON String
return jObj;
}
}
这是我的主要 Activity :
public class MainActivity extends ActionBarActivity {
private TextView tv;
private static final String urllisteemplois = "the url that contains the json";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.text);
JParse jParse = new JParse(urllisteemplois);
jParse.execute();
String result = jParse.concat();
tv.setText(result);
}
}
这是我的 ASynctask :
public class JParse extends AsyncTask<String, String, JSONObject> {
private String url;
private JSONObject json;
public JParse(String url) {
this.url=url;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
}
@Override
protected JSONObject doInBackground(String... args) {
JSONParser jParser = new JSONParser();
JSONObject json2 = jParser.getJSONFromUrl(url);
return json2;
}
@Override
protected void onPostExecute(JSONObject json2) {
this.json=json2;
}
public String concat()
{
String result = this.json.toString()+"hello";
return result;
}
}
我收到这个错误:
01-20 03:18:43.797 2298-2298/com.aziz.bdk.ryadencule E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.aziz.bdk.ryadencule, PID: 2298
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.aziz.bdk.ryadencule/com.aziz.bdk.ryadencule.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONObject.toString()' on a null object reference
我尝试了一些东西,我知道在 onPostExecute
中,this.json
不是 null
并且包含我解析的 JSON
,在 concat()
方法中它是 null
。
此外,我需要在 doInBackground(..)
中添加 onPostExecute(json2)
,它不会自动执行。
感谢您的帮助!
最佳答案
onCreate
中的这段代码是问题所在:
jParse.execute();
String result = jParse.concat();
tv.setText(result);
因为 AsyncTask
是异步的,当您调用 jParse.concat() 并且 json 对象仍然为空时,对 jParse.execute() 的调用仍在运行。
您应该只在 onCreate
中执行第一行,然后将后两行移至您的 JParse 对象中的 onPostExecute
。
关于android - 异步任务并从 url NullPointerException 解析 JSONObject,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28037363/