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How to cancel the async task with tab host

I am currently working with the tab host and fragment. Currently I set fragment a to download json A and fragment B to download json B , the problem is when I switch fragment, the fragment A onPostExecute function will fall into the fragment B one , is there any way to fix this?
Thanks
Tab host:
tabHost = (FragmentTabHost) findViewById(R.id.tabhost);
tabHost.setup(this, getSupportFragmentManager(), R.id.tabcontent);
tabHost.addTab(
tabHost.newTabSpec("Home").setIndicator("",
res.getDrawable(R.drawable.btn_about)), Home.class,
null);
tabHost.addTab(
tabHost.newTabSpec("About").setIndicator("",
res.getDrawable(R.drawable.btn_about)), About.class,
null);
The async task
public class JSONReader {
public static final String TAG = "JSONReader";
public ArrayList<Record> records;
public Record myRecordObj;
public ArrayList<GalleryImage> images;
public String url;
public int failCount = 0; // retry twice
public Context ctx;
public String readCase;
public JSONReader(String _url, Context _ctx , String _readCase) {
url = _url;
ctx = _ctx;
readCase = _readCase;
}
public void getJSON() {
new JSONDownload().execute(url);
}
private class JSONDownload extends AsyncTask<String, Void, JSONObject> {
// TODO Auto-generated method stub
StringBuilder builder = new StringBuilder();
String temp = "";
String json = ""; // json content
InputStream input = null;
OutputStream output = null;
HttpURLConnection connection = null;
#Override
protected JSONObject doInBackground(String... params) {
// TODO Auto-generated method stub
try {
Log.d(TAG, "Start reading: " + url);
URL url = new URL(params[0]);
connection = (HttpURLConnection) url.openConnection();
connection.connect();
if (connection.getResponseCode() != HttpURLConnection.HTTP_OK)
return null;
// return "Server returned HTTP " + connection.getResponseCode()
// + " " + connection.getResponseMessage();
input = connection.getInputStream();
BufferedReader reader = new BufferedReader(
new InputStreamReader(input));
while ((temp = reader.readLine()) != null) {
builder.append(temp);
}
json = builder.toString();
} catch (Exception e) {
return null;
} finally {
try {
if (input != null)
input.close();
if (output != null)
output.close();
} catch (IOException ignored) {
}
if (connection != null)
connection.disconnect();
}
try {
return new JSONObject(json);
} catch (JSONException e) {
e.printStackTrace();
return null;
}
}
#Override
protected void onPostExecute(JSONObject result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
onJSONDownloaded(result);
}
}
public void onJSONDownloaded(JSONObject result) {
// TODO Auto-generated method stub
if (result != null) {
failCount = 0;
if (readCase.equals("leaderBoard")){
records = new ArrayList<Record>();
try {
JSONObject myRecord = result.getJSONObject("myRecord");
if (myRecord != null) {
myRecordObj = new Record(myRecord.getString("pic"),myRecord.getString("name"),myRecord.getString("score"));
}
JSONArray topRecords = result.getJSONArray("topRecord");
for (int i = 0; i < topRecords.length(); i++) {
JSONObject topRecord = topRecords.getJSONObject(i);
String topName = topRecord.getString("name");
String topPic = topRecord.getString("pic");
String topScore = topRecord.getString("score");
records.add(new Record(topPic, topName, topScore));
}
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
((LeaderBoardDetail)ctx).setData(records,myRecordObj);
} else if (readCase.equals("galleryList")){
images = new ArrayList<GalleryImage>();
try {
JSONArray imageList = result.getJSONArray("images");
for (int i = 0; i < imageList.length(); i++) {
JSONObject image = imageList.getJSONObject(i);
images.add(new GalleryImage(image.getString("url"),image.getString("thumbUrl"),image.getString("category"),image.getString("userPic"),image.getString("name")));
}
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//exception
if (((FragmentActivity) ctx).getSupportFragmentManager().findFragmentById(R.id.tabcontent).getTag().equals("Gallery")) {
PhotoGallery galleryFragment = (PhotoGallery) ((FragmentActivity) ctx).getSupportFragmentManager().findFragmentById(R.id.tabcontent);
galleryFragment.setData(images);
}
}
} else {
if (failCount <= 1) { // check global_conf twice if fail
failCount++;
Log.d(TAG, "No of retry" + failCount);
new JSONDownload().execute(url); // Retry download json
} else {
failCount = 0;
}
}
}
}

I used it in my app this way which works for me and answer is a bit similar to another answer but few additions and more detail. Hope it help you too.
NOTE: This is just a idea, u need to try, it may vary as per your app architect.
At your activity make the task object global [make sure set task obj null once task is finish]
JSDownload js = null;
public void getJSON() {
if(js != null && js.getStatus() == AsyncTask.Status.RUNNING)
{
js.cancel(true);
if(js.isCancelled())
{
js = new JSONDownload();
js.execute(url);
}
else
{
js = new JSONDownload();
js.execute(url);
}
}
At the Async class side....[make sure u take care null result # onpostExcute]
class JSONDownload extends AsyncTask<String, Void, JSONObject>
{
protected JSONObject doInBackground(String... params) {
// TODO Auto-generated method stub
try {
if(!this.isCancelled())
{
//make http connection ..
URL url = new URL(params[0]);
connection = (HttpURLConnection) url.openConnection();
connection.connect();
//as we are in steps in bg check iscancel .. again
//if its a loop here we call break; and return null once only..
if(this.isCancelled())
return null;
// connection status check and get buffer etc .. code here
if(this.isCancelled())
return null;
//read data
return data;
}
} catch (Exception e) {
return null;
}
}
#Override
protected void onCancelled(){
// If you write your own implementation, do not call super.onCancelled(result).
}
}

Use the global async task variable
LongOperation LongOperationOdeme = new LongOperation();
and set:
LongOperationOdeme.cancel(true);
This will stop any async task running at that moment, it's what the back button does

If you are not looking for a simplest answer but for a maybe more interesting and elegant, have a peek at this article, especially if you find functional programming interesting.
It's easier than it looks, i was almost unfamiliar to FP before this article but it covers common problems related to AsyncTask and asynchronicity in Android in general, so I got the gist and consider to use Observables instead of AsyncTask in the future projects myself. It is RxJava and it can solve your problem gracefully: "The fromFragment call transforms the given source observable in such a way that events will only be emitted to the fragment if it’s still alive and attached to its host activity."
One more citation form the article: "What if the user decides to back out of the Activity that triggered the task, and we are holding on to a stale reference. This not only creates a substantial memory leak, but is also worthless because meanwhile it has been detached from the application window. A problem that everyone is well aware of."

App needs to get JSON and display it. How to do it using async thread?

As mentioned I get the above error which I know is because my application is doing networking in UI thread. I went through a lot of stackoverflow questions which advise to use AsyncTask for this purpose. From what I understand asynctask is asynchronous and will run independently in the background. But I need to fetch the data from http and display on the main thread. So basically my UI thread should be blocked till I have the JSON fetched so that I can display it.
My questions are
1) Since I need to run http networking in another thread how do I do it?
2) Do I use an async thread?
3) How do I block my UI thread for the async thread to fetch the result?
4) How do I pass the result from async thread back to UI thread?
This is the current JSON parser class that I use.
public class JSONParser {
static InputStream is = null;
static JSONObject jObj = null;
static String json = "";
// constructor
public JSONParser() {
}
public JSONArray getJSONfromURL(String url) {
// initialize
InputStream is = null;
String result = "";
JSONArray jArray = null;
// http post
try {
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
HttpResponse response = httpclient.execute(httpGet);
HttpEntity entity = response.getEntity();
is = entity.getContent();
} catch (Exception e) {
Log.e("log_tag", "Error in http connection " + e.toString());
return null;
}
// convert response to string
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();
result = sb.toString();
Log.e("log_tag", "JSON data" + result);
} catch (Exception e) {
Log.e("log_tag", "Error converting result " + e.toString());
return null;
}
// try parse the string to a JSON object
try {
jArray = new JSONArray(result);
} catch (JSONException e) {
Log.e("log_tag", "Error parsing data " + e.toString());
return null;
}
return jArray;
}
}
And this is my MainActivity where I call JSONparser to fetch some data that I need to display
JSONArray json = jParser.getJSONfromURL(temp);
if (json == null) {
return -1;
}
for (int i = 0; i < json.length(); i++) {
try {
JSONObject c = json.getJSONObject(i);
// Getting Array of Contacts
// Storing each json item in variable
asr_iq = c.getString("lMAsr");
sunrise_iq = c.getString("lMSunrise");
fajr_iq = c.getString("lMFajr");
isha_iq = c.getString("lMIsha");
dhuhr_iq = c.getString("lMDhuhr");
maghrib_iq = c.getString("lMMaghrib");
} catch (JSONException e) {
e.printStackTrace();
}
}

Load your asynctask on the UI thread.
If you cannot do any network related operation on the UI Thread Honeycomb and later. You will get a NetworkOnMainThread Exception.
new MyTask(url).execute();// can pass parameter to class constructor
// can also pass url to doInBackground.
Async task
class MyTask extends AsyncTask<Void,Void,Void>
{
String url;
public MyTask(String url)
{
this.url =url
}
#Override
protected Void doInBackground(Void... params) {
// all your network related operation
// invoked on the background thread
// all code from getJSONfromURL(param)
// do not update ui here
return null;
}
#Override
protected void onPostExecute(Void result) { // invoked on the ui thread
// TODO Auto-generated method stub
super.onPostExecute(result);
// dismiss progress dialog
// update ui here
}
#Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
// display progress dialog
}
#Override
protected void onProgressUpdate(Void... values) {
// TODO Auto-generated method stub
super.onProgressUpdate(values);
}
}
Detailed information # http://developer.android.com/reference/android/os/AsyncTask.html
Edit:
Use a Handler. Return result in doInBaCkground().
Example in onPostExecute()
Message msg=new Message();
msg.obj=result.getProperty(0).toString();
mHandler.sendMessage(msg);
In your activity onCreate()
Handler mHandler = new Handler() {
#Override public void handleMessage(Message msg) {
//ArrayList s=(ArrayList)msg.obj;
SoapObject s =(SoapObject) msg.obj;
tv.setText("Result = "+s.toString());
}
};
You can also use runonuithread to update ui from doInBackGround()
runOnUiThread(new Runnable() //run on ui thread
{
public void run()
{
_tv.setText("update from doinbackground");
}
});

Using Core Java,
Have your getJson execution logic in a Runnable/Callable(Java concurrency class), submit it via executors so that its an Asynch call.
Then with in your Runnable/Callable once json is retrived call the class which will have logic to display the json, this clas can be designed as a listener and you may publish an even after getting json response

NetworkOnMainThread exception android. fetching server response

I am getting the exception android.os.NetworkOnMainThreadException when I tried to use the following codes:
public class CheckServer extends Activity{
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
Runnable runn = null;
HttpTask.execute(runn);
}
private class HttpTask extends AsyncTask<String, String, String>
{
#Override
protected String doInBackground(String... params) {
// TODO Auto-generated method stub
HttpURLConnection urlConnection = null;
URL theURL = null;
try {
theURL = new URL("http://192.168.2.8/parkme/Client/clientquery.php?ticket=66t");
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
urlConnection = (HttpURLConnection) theURL.openConnection();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
String response = null;
try {
response = readInputStream(urlConnection.getInputStream());
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return response;
}
private String readInputStream(InputStream is) {
// TODO Auto-generated method stub
String line = "";
StringBuilder total = new StringBuilder();
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
try {
while ((line = rd.readLine()) != null) {
total.append(line);
}
} catch (Exception e) {
e.printStackTrace();
}
return total.toString();
}
#Override
protected void onPostExecute(String result) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(), result, Toast.LENGTH_LONG).show();
}
}}
If possible can someone tell me how to use it inside an Async Task and get the output? I tried but can't seem to get anywhere.

NetworkOnMainThread Exception occurs because you are running a network related operation on the main UI Thread.This is only thrown for applications targeting the Honeycomb SDK or higher
You should be using asynctask.
http://developer.android.com/reference/android/os/AsyncTask.html
In onCreate()
new TheTask().execute();
You can also pass parameters like url to the constructor of AsyncTask and use the same in doInBackground()
class TheTask extends AsyncTask<Void,Void,Void>
{
protected void onPreExecute()
{ super.onPreExecute();
//display progressdialog.
}
protected void doInBackground(Void ...params)//return result here
{
//http request. do not update ui here
return null;
}
protected void onPostExecute(Void result)//result of doInBackground is passed a parameter
{
super.onPostExecute(result);
//dismiss progressdialog.
//update ui using the result returned form doInbackground()
}
}
When an asynchronous task is executed, the task goes through 4 steps:
onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.
doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.
onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.
onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Ok, lets do it step by step ...
1) create private class extending AsyncTask
private class HttpUrlConnectionTask extends AsyncTask {
2) Override the doInBackground() method, this will do the heavy load
#Override
protected Object doInBackground(Object... params) {
// your HttpUrlConnection code goes here
return response;
3) Once the job is done and returns, the onPostExecute() method will be called. The result parameter contains the return value of doInBackground() - so response.
#Override
protected void onPostExecute(Object result) {
Within this method you can update your UI.
4) Finally lets have a look onto the HttpUrlConnection code
HttpURLConnection urlConnection = null;
URL theURL = new URL(url);
urlConnection = (HttpURLConnection) theURL.openConnection();
String response = readInputStream(urlConnection.getInputStream());
return response;
Hope this helps. Happy coding!

#Raghunandan comes with a really good explanation of how AsyncTask works
Here you go:
public static class InitializeTask extends MyAsyncTask<String, String, String> {
private Activity activity;
private ProgressDialog dialog;
public InitializeTask(Activity activity) {
this.activity = activity;
}
#Override
protected void onPostExecute(String result) {
Toast.makeText(activity, result, Toast.LENGTH_SHORT).show();
}
#Override
protected String doInBackground(String... params) {
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet("http://192.168.2.8/localhost/parkme/Client/clientquery.php?ticket=");
try {
HttpResponse response = httpclient.execute(httpget);
if(response != null) {
String line = "";
InputStream inputstream = response.getEntity().getContent();
return convertStreamToString(inputstream);
} else {
return "Unable to complete your request";
}
} catch (ClientProtocolException e) {
return "Caught ClientProtocolException";
} catch (IOException e) {
return "Caught IOException";
}
}
private String convertStreamToString(InputStream is) {
String line = "";
StringBuilder total = new StringBuilder();
BufferedReader rd = new BufferedReader(new InputStreamReader(is));
try {
while ((line = rd.readLine()) != null) {
total.append(line);
}
} catch (Exception e) {
return "Stream Exception";
}
return total.toString();
}
}
A little side note, it is generally considered bad code to catch just Exception, since this will catch anything, and you are not accounting for what it is.
To use the AsyncTask in the Activity do this:
InitializeTask task = new InitializeTask(this)
task.execute()

Exactly as it says, network activity isn't allowed on the thread the activity ran in. Moving your code to an Asynctask is the way to do it properly. Though if you're just trying to get your concept working still you can do this...
//lazy workaround with newer than gingerbread
//normally UI thread can't get Internet.
if(Build.VERSION.SDK_INT >= 9){
StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);
}
And then the UI thread actually can. I wouldn't release anything like this however, I haven't even tried infact. It's just my lazy debugging move I use a lot.

Weird NetworkOnMainThread exception

I'm new to android, please halp.
This is a really simple scenario, there's an actionbar, and when a tab is selected, I need to send an http request to get an article list, when the response arrives, update a listview and here's where I got this exception.
I'm aware that I should isolate network operations from UI thread, so I implement the network functions in an AsyncTask class.
The most weird part is, before I try to get the article list, I have actually called a network function once to log in, and that just works fine, no exceptions! And guess what, if I replace the get article list call with log in call, it works fine, too. All the code are just the same, except the arguments differ.
Here's how I do it, since too many modules will use the network module, I use a static wrapper:
UI -> Static Factory -> New AsyncTask -> Response arrives -> Call Static callback function in Static Factory -> Call callback function in UI
AsyncTask doInBackground
#Override
protected HttpResponse doInBackground(String... params) {
// TODO Auto-generated method stub
String uri = params[0];
HttpGet get = new HttpGet(uri);
try {
return _client.execute(get);
}
catch(Exception e) {
e.printStackTrace();
return null;
}
}
AsyncTask post execute
#Override
protected void onPostExecute(HttpResponse result) {
//Do something with result
if (result != null) {
try {
JSONObject json = FormJsonFromResponse();
Command.OnTaskComplete(json);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
static wrapper code:
#Override
public static void OnTaskComplete(JSONObject json) {
// TODO Auto-generated method stub
if(_callback != null) {
_callback.OnCommandComplete(json);
}
}
Here's how I call asynctask in static wrapper:
public static void LogIn(String user, String pass) {
new NetworkTask().execute(_uriPrefix + login, _user, _pass);
}

Revise your AsyncTask following this pattern:
#Override
protected JSONObject doInBackground(String... params) {
// TODO Auto-generated method stub
String uri = params[0];
HttpGet get = new HttpGet(uri);
try {
HttpResponse response = _client.execute(get);
return FormJsonFromResponse();
}
catch(Exception e) {
e.printStackTrace();
return null;
}
}
AsyncTask post execute
#Override
protected void onPostExecute(JSONObject result) {
//Do something with result
if (result != null) {
try {
Command.OnTaskComplete( result );
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

How to make a blocking Android HttpRequest

Here is my issue:
I have a list of values which I retrieve from server.
These values fill ListView in the UI.
I cannot continue loading the View and showing it to user until the list is full.
Since Android forces me to make http calls in separate thread, my question is how do I create 1 class that does the httprequests and in the calling class I wait until I get response from the HttpRequest and only then I proceed loading the View?
Right now I have this class that does the requests:
public class WapConnector extends AsyncTask<String, String, String>{
private static final String TAG = "WapConnector";
private String html = "";
private Handler mHandler;
private String server = "http://....whatever";
private String callUrl = "/api/";
private String params = "login?u=Admin&pw=234234&format=json";
private int _callstate = 1;
#Override
protected String doInBackground(String... uri) {
HttpClient httpclient = new DefaultHttpClient();
HttpResponse response;
String responseString = null;
try {
String fullUrl = "";
Log.i(TAG,fullUrl);
if(params.length() > 0){
fullUrl = server + callUrl + params + "&alf_ticket=" + Globals.getInstance().getTicket() + "&udid=" + Globals.getInstance().udid() + "&phoneNumber=" + Globals.getInstance().phoneNumber();
}
else{
fullUrl = server + callUrl + "?udid=" + Globals.getInstance().udid() + "&alf_ticket=" + Globals.getInstance().getTicket() + "&phoneNumber=" + Globals.getInstance().phoneNumber();
}
Log.i(TAG,fullUrl);
response = httpclient.execute(new HttpGet(fullUrl));
StatusLine statusLine = response.getStatusLine();
if(statusLine.getStatusCode() == HttpStatus.SC_OK){
ByteArrayOutputStream out = new ByteArrayOutputStream();
response.getEntity().writeTo(out);
out.close();
responseString = out.toString();
Log.i(TAG,responseString);
} else{
//Closes the connection.
response.getEntity().getContent().close();
throw new IOException(statusLine.getReasonPhrase());
}
} catch (ClientProtocolException e) {
e.printStackTrace();
//TODO Handle problems..
} catch (IOException e) {
//TODO Handle problems..
e.printStackTrace();
}
return responseString;
}
#Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
Log.i(TAG + "onPostExecute",result);
activity.getClass().handleResponse();
//main load
JSONObject jobj;
JSONObject jvalue;
try {
jobj = new JSONObject(result);
if(_callstate == 1){
jvalue = jobj.getJSONObject("data");
String ticket = jvalue.getString("ticket");
Log.i("loginwap",ticket);
Globals.getInstance().setTicket(ticket);
_callstate = 2;
}
else{
jvalue = jobj.getJSONObject("countries");
JSONArray countries = jvalue.getJSONArray("countries");
}
}
catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
And this is how I make calls from Parent classes:
WapConnector wap = new WapConnector();
wap.setCallUrl("/anyurl/");
wap.callstate(3);
wap.setParams("");
wap.execute("");
Now my issue is that since the request runs in thread, once I call wap.execute(), my Activity continues to load, but I want it to wait until I get response, parse the response and only then continue to load.
thanks everyone for replies.!!!

Pass in a context to your class from the activity you are calling it from. Overload the onPreExecute() to show a ProgressDialog and then overload onPostExecute() to hide the ProgressDialog. This gives you blocking while letting the user you are loading.
There is a kinda hacky way to get more control. If you want to keep the AsyncTask as a separate class but allow it to update UI elements in another Activity, define a Handler in that Activity and then pass it in the constructor of the the AsyncTask. You can then send a message in the onPostExecute() method of your AsyncTask to the handler to tell it to update the UI. You will need to make sure the handler is properly handling the message your AsyncTask is sending back. Could be a little cleaner, but it works and will allow you to reuse an asyncTask that makes a network call across activities.