我正在尝试使用AsyncTask发出HTTP请求并返回一些JSON，但由于某种原因，我的Async任务的doInBackground方法永远不会运行。 我已经在这个问题上看到了一些关于SO的解决方案，但似乎没有任何帮助 - 我也尝试使用常规线程而不是Async Task，但它似乎也有类似的问题。
我的片段类包含AsyncTask类：
public class ChallengeListFragment extends ListFragment {
...
private ArrayList<Challenge> mChallenges;
// Creating JSON Parser object
JSONParser jParser = new JSONParser();
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getActivity().setTitle(R.string.challenges_list_fragment_title);
mChallenges = new ArrayList<Challenge>();
new JSONRetriever().execute("URL_HERE");
...
}
// Async Task class to perform HTTP requests in a separate thread:
private class JSONRetriever extends AsyncTask<String, Void, JSONObject>{
JSONArray challengesJsonArray;
JSONObject challengesJsonObject;
@Override
protected void onPreExecute() {
super.onPreExecute();
Log.i(LOG_TAG, "onPreExecute()!!!");
}
@Override
protected JSONObject doInBackground(String... url) {
challengesJsonObject = jParser.getJSONFromUrl(url[0]);
return challengesJsonObject;
}
@Override
protected void onPostExecute(JSONObject jsonObject) {
super.onPostExecute(jsonObject);
Log.i(LOG_TAG, jsonObject.toString());
}
}
}
...和我的JSONParser课程，取自常用的指南或答案：
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
DefaultHttpClient httpClient = new DefaultHttpClient();
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;
}
}
我已多次执行该程序，并且在onPreExecute（）运行时，doInBackground（）从不运行。在发生这种情况时，我没有运行任何其他异步任务。
任何建议都非常感谢，谢谢。
更新 根据评论中的建议，我尝试删除doInBackground（）中的代码，并在其中放置一个日志语句，看起来它实际上正在运行，尽管在我执行程序时似乎从未进入过该方法。
发生这种情况时，完整的LogCat是：
850-850/com.ethanml.proveit I/PROVEIT﹕ onPreExecute()!!!
12-08 19:01:45.952 850-850/com.ethanml.proveit D/AndroidRuntime﹕ Shutting down VM
12-08 19:01:45.952 850-878/com.ethanml.proveit D/PROVEIT﹕ DO IN BACKGROUND
12-08 19:01:45.962 850-850/com.ethanml.proveit W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41465700)
12-08 19:01:45.992 850-850/com.ethanml.proveit E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ethanml.proveit/com.ethanml.proveit.ChallengeListActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
at android.app.ActivityThread.access$600(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5103)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.ethanml.proveit.ChallengeListFragment.onCreate(ChallengeListFragment.java:55)
at android.app.Fragment.performCreate(Fragment.java:1673)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:854)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1057)
at android.app.BackStackRecord.run(BackStackRecord.java:682)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1435)
at android.app.Activity.performStart(Activity.java:5142)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
at android.app.ActivityThread.access$600(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5103)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
日志中引用的第55行的ChallengeListFragment上的NPE指的是我用来逐步执行JSON数组的for循环，该数组应该在异步任务的onPostExecute中填充 - 因为这似乎永远不会被执行， JSONArray为null，程序崩溃。