如何在 AsyncTask 中更新 ArrayList 的内容？
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【中文标题】如何在 AsyncTask 中更新 ArrayList 的内容？【英文标题】：How to update contents of ArrayList in AsyncTask? 【发布时间】：2014-05-14 19:14:43 【问题描述】：我要做的是从我的 json 文件中获取名称列表并将其存储到 ArrayList 中。我使用 getPointsList() 方法实现了一个名为 ServiceHandler 的类。所有的值和对象传递似乎都是正确的，所以我想知道这是否是我运行 AsyncTask 的问题。错误日志似乎返回 NullPointerException 语句 这是服务处理程序
public class ServiceHandler
private static String pt_url="full url name here";
private static final String TAG_NAME = "name";
private static final String TAG_LON = "gps_lon";
private static final String TAG_LAT = "gps_lat";
ArrayList<String> PList;
static String response = null;
public final static int GET = 1;
public final static int POST = 2;
static InputStream is= null;
static JSONObject jObj=null;
ProgressDialog pDialog;
Activity A;
JSONArray json;
public ServiceHandler(Activity A)
this.A=A;
/**
* Making service call
* @url - url to make request
* @method - http request method
* */
public String makeServiceCall(String url, int method)
return this.makeServiceCall(url, method, null);
/**
* Making service call
* @url - url to make request
* @method - http request method
* @params - http request params
* */
public String makeServiceCall(String url, int method,
List<NameValuePair> params)
try
// http client
DefaultHttpClient httpClient = new DefaultHttpClient();
HttpEntity httpEntity = null;
HttpResponse httpResponse = null;
// Checking http request method type
if (method == POST)
HttpPost httpPost = new HttpPost(url);
// adding post params
if (params != null)
httpPost.setEntity(new UrlEncodedFormEntity(params));
httpResponse = httpClient.execute(httpPost);
else if (method == GET)
// appending params to url
if (params != null)
String paramString = URLEncodedUtils
.format(params, "utf-8");
url += "?" + paramString;
HttpGet httpGet = new HttpGet(url);
httpResponse = httpClient.execute(httpGet);
httpEntity = httpResponse.getEntity();
response = EntityUtils.toString(httpEntity);
catch (UnsupportedEncodingException e)
e.printStackTrace();
catch (ClientProtocolException e)
e.printStackTrace();
catch (IOException e)
e.printStackTrace();
return response;
public ArrayList<String> getPointsList() throws JSONException
GetPoints AT = new GetPoints();
pDialog = new ProgressDialog(A);
pDialog.setMessage("Retreiving points...");
pDialog.setCancelable(false);
pDialog.show();
AT.execute(pt_url);
PList = new ArrayList<String>();
int i;
Log.d("",""+AT.getStatus());
如果我删除 !它永远不会通过 if 语句，问题在于 AsyncTask 连续运行
if(!(AT.getStatus().equals(AsyncTask.Status.FINISHED)))
if (pDialog.isShowing()) pDialog.dismiss();
Log.d("Status: ",AT.getStatus().toString());
for (i = 0; i < json.length(); i++)
JSONObject c = json.getJSONObject(i);
Log.i("Name of Object: "+i,c.getString(TAG_NAME));
PList.add(c.getString(TAG_NAME));
//LatList.add(c.getDouble(TAG_LAT));
//LonList.add(c.getDouble(TAG_LON));
return PList;
public class GetPoints extends AsyncTask<String, Void, JSONArray>
@Override
protected void onPreExecute()
super.onPreExecute();
/*pDialog = new ProgressDialog(A);
pDialog.setMessage("Retreiving points...");
pDialog.setCancelable(false);
pDialog.show(); */
@Override
protected JSONArray doInBackground(String... arg)
//ServiceHandler sh = new ServiceHandler();
//Log.i("arg[0]",arg[0]);
// Making a request to url and getting response
String ptStr = makeServiceCall(arg[0], GET);
Log.d("Response: ", "> " + ptStr);
if (ptStr != null)
try
json = new JSONArray(ptStr);
//Log.d("New json: ", json.toString());
catch (JSONException e)
e.printStackTrace();
else
Log.e("ServiceHandler", "Couldn't get any data from the url");
//PList.add("some text");
return json;
@Override
protected void onPostExecute(JSONArray result)
super.onPostExecute(result);
//Log.d("New json: ", json.toString());
// Dismiss the progress dialog
/*if (pDialog.isShowing())
pDialog.dismiss();*/
//for(String entry: PList)
// Log.i("GETPOINTS", entry);
//
/* Adapters are updated here, so change this to fit whatever we use in UI
stAdapter = new ArrayAdapter<String>(context, android.R.layout.simple_spinner_item, FList);
festAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
inputFest.setAdapter(festAdapter);
*/
这是错误日志
04-03 02:22:37.480: E/AndroidRuntime(18824): FATAL EXCEPTION: main
04-03 02:22:37.480: E/AndroidRuntime(18824): java.lang.NullPointerException
04-03 02:22:37.480: E/AndroidRuntime(18824): at com.example.demo.ServiceHandler.getPointsList(ServiceHandler.java:123)
04-03 02:22:37.480: E/AndroidRuntime(18824): at com.example.demo.MainMenuActivity$2.onClick(MainMenuActivity.java:70)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.view.View.performClick(View.java:4198)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.view.View$PerformClick.run(View.java:17158)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.os.Handler.handleCallback(Handler.java:615)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.os.Handler.dispatchMessage(Handler.java:92)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.os.Looper.loop(Looper.java:137)
04-03 02:22:37.480: E/AndroidRuntime(18824): at android.app.ActivityThread.main(ActivityThread.java:4918)
04-03 02:22:37.480: E/AndroidRuntime(18824): at java.lang.reflect.Method.invokeNative(Native Method)
04-03 02:22:37.480: E/AndroidRuntime(18824): at java.lang.reflect.Method.invoke(Method.java:511)
04-03 02:22:37.480: E/AndroidRuntime(18824): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
04-03 02:22:37.480: E/AndroidRuntime(18824): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
04-03 02:22:37.480: E/AndroidRuntime(18824): at dalvik.system.NativeStart.main(Native Method)
04-03 02:22:37.680: D/Response:(18824): > ["id":"5","name":"The Hendrix Center","gps_lat":"34.67605277777778","gps_lon":"-82.831825","campus_id":"1","id":"7","name":"Red Fern","gps_lat":"34.67575","gps_lon":"-82.83358055555556","campus_id":"1","id":"9","name":"McAdams Hall","gps_lat":"34.67559166666666","gps_lon":"-82.83461388888888","campus_id":"1","id":"10","name":"Edwards Hall","gps_lat":"34.67681666666667","gps_lon":"-82.83379166666666","campus_id":"1","id":"11","name":"Vickory Hall","gps_lat":"34.677375","gps_lon":"-82.83379722222222","campus_id":"1","id":"12","name":"Daniel Hall","gps_lat":"34.67714722222222","gps_lon":"-82.8351611111111","campus_id":"1","id":"13","name":"Kinard Hall","gps_lat":"34.677524999999996","gps_lon":"-82.83515","campus_id":"1","id":"14","name":"Martin Hall","gps_lat":"34.67805833333333","gps_lon":"-82.83553055555555","campus_id":"1"]
【问题讨论】：
第 123 行是什么？ 你调试应用程序并找到你在哪里得到空指针 问题在于第 123 行的 for 循环，因为 AsyncTask 似乎永远不会完成，这意味着 json 保持为空。所以问题一定是在 AsyncTask 没有结束。 不清楚您的 AT.getStatus 检查代码在哪里执行？您不只是使用 postExecute 方法运行该代码的任何原因（然后您不需要 AT 状态检查，因为该方法在 toInBackground 完成后调用 【参考方案1】：在 getPointsList() 中你这样做：
for (i = 0; i < json.length(); i++)
JSONObject c = json.getJSONObject(i);
变量 json 为空，因为 ASyncTask 尚未完成。
【讨论】：
是的，这就是我尝试使用 (AT.getStatus().equals(AsyncTask.Status.FINISHED)) 的原因 AsyncTask 以某种方式持续运行，但是当我打印到登录 onPostExecute() 时它没有问题。所以我的假设是 AsyncTask 正在执行它的最终方法。以上是关于如何在 AsyncTask 中更新 ArrayList 的内容？的主要内容，如果未能解决你的问题，请参考以下文章
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