这个问题在这里已有答案:
我在MainActivity的一个单独文件中的另一个类中有一个AsyncTask类
get warehouse sales.Java
public class GetWarehouseSales extends Activity {
private String TAG = MainActivity.class.getSimpleName();
private ListView lv;
private ProgressDialog pDialog;
//URL to get JSON details
private static String url = "http://192.168.0.1/mycc/retrieve_ws.php";
ArrayList<HashMap<String,String>> sales_details;
public GetWarehouseSales(){
}
public void executeGWS(){
new RetrieveWarehouseSalesTask().execute();
}
public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,Void>{
public RetrieveWarehouseSalesTask(){
}
@Override
protected void onPreExecute(){
super.onPreExecute();
pDialog = new ProgressDialog(GetWarehouseSales.this);
pDialog.setMessage("Getting you the best warehouse sales...");
pDialog.setCancelable(false);
pDialog.show();
}
@Override
protected Void doInBackground(Void... arg0){
HttpHandler sh = new HttpHandler();
//making a request to URL and getting response
String jsonStr = sh.makeServiceCall(url);
Log.e(TAG, "Response from url: " + jsonStr);
if(jsonStr != null){
try{
JSONObject jsonObj = new JSONObject(jsonStr);
//Getting JSON Array Node
JSONArray sales = jsonObj.getJSONArray("Result");
//looping through all results
for(int i = 0; i<sales.length();i++){
JSONObject s = sales.getJSONObject(i);
String title = s.getString("title");
String description = s.getString("description");
HashMap<String,String> salesDetails = new HashMap<>();
//adding each child node to HashMap key =>value
salesDetails.put("title",title);
salesDetails.put("description",description);
//adding to array list
sales_details.add(salesDetails);
}
Log.d("TAG",sales_details.toString());
}catch(final JSONException e){
Log.e(TAG, "JSON parsing error: " + e.getMessage());
runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(getApplicationContext(),"JSON parsing error: " + e.getMessage(),Toast.LENGTH_LONG).show();
}
});
}
}else{
Log.e(TAG,"Couldn't get json from server");
runOnUiThread(new Runnable() {
@Override
public void run() {
Toast.makeText(getApplicationContext(), "Check logcat", Toast.LENGTH_LONG ).show();
}
});
}
return null;
}
@Override
protected void onPostExecute(Void result){
super.onPostExecute(result);
if(pDialog.isShowing()){
pDialog.dismiss();
}
//update parsed JSON data into ListView
ListAdapter adapter = new SimpleAdapter(GetWarehouseSales.this, sales_details,R.layout.item_listview, new String[]{
"title","description"}, new int[]{R.id.text,R.id.description});
lv.setAdapter(adapter);
}
}
}
main activity.Java
@Override
protected void onCreate(Bundle savedInstanceState) {
GetWarehouseSales gws = new GetWarehouseSales();
gws.executeGWS();
}
错误:
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
错误指向此行:
pDialog = new ProgressDialog(GetWarehouseSales.this);
我想执行AsyncTask RetrieveWarehouseSalesTask
,它可以在GetWarehouseSales.java
的MainActivity.java
中找到,但是会出现NPE错误。
尝试传递MainActivity上下文..
@Override
protected void onCreate(Bundle savedInstanceState) {
GetWarehouseSales gws = new GetWarehouseSales(MainActivity.this);
gws.executeGWS();
}
检索上下文:
private Activity activityContext;
public GetWarehouseSales(Activity context){
this.activityContext=context;
}
现在使用该上下文初始化您的ProgressDialog
@Override
protected void onPreExecute(){
super.onPreExecute();
pDialog = new ProgressDialog(activityContext);
pDialog.setMessage("Getting you the best warehouse sales...");
pDialog.setCancelable(false);
pDialog.show();
}
或..使用
pDialog = new ProgressDialog(getApplicationContext());
这是错误的原因:
GetWarehouseSales gws = new GetWarehouseSales();
gws.executeGWS();
您创建了一个活动实例,当AsyncTask
正在执行时,尝试访问Context
。
看起来最简单的解决方法是将qazxsw poi放入qazxsw poi,例如:
RetrieveWarehouseSalesTask
希望对你有帮助..
MainActivity
检索上下文:
class MainActivity ... {
...
@Override
protected void onCreate(Bundle savedInstanceState) {
...
new RetrieveWarehouseSalesTask().execute();
}
public class RetrieveWarehouseSalesTask extends AsyncTask<Void,Void,Void>{
...
}
现在使用该上下文初始化ProgressDialog
@Override
protected void onCreate(Bundle savedInstanceState) {
GetWarehouseSales gws = new GetWarehouseSales(this);
gws.executeGWS();
}