AsyncTask Android example

Question

I was reading about AsyncTask  and I tried the simple program below  But it does not seem to work  How can I make it work   public class AsyncTaskActivity extends Activity        Button btn           Called when the activity is first created          Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout main            btn    Button  findViewById R id button1           btn setOnClickListener  OnClickListener  this              public void onClick View view           new LongOperation   execute                 private class LongOperation extends AsyncTask lt String  Void  String gt             Override         protected String doInBackground String    params                for int i 0 i lt 5 i                      try                       Thread sleep 1000                     catch  InterruptedException e                           TODO Auto-generated catch block                     e printStackTrace                                                TextView txt    TextView  findViewById R id output               txt setText  Executed                return null                      Override         protected void onPostExecute String result                        Override         protected void onPreExecute                         Override         protected void onProgressUpdate Void    values                        I am just trying to change the label after 5 seconds in the background process   This is my main xml    lt  xml version  1 0  encoding  utf-8   gt   lt LinearLayout xmlns android  http   schemas android com apk res android                android layout width  fill parent                android layout height  fill parent                android orientation  vertical   gt       lt ProgressBar         android id    id progressBar          style   android attr progressBarStyleHorizontal          android layout width  match parent          android layout height  wrap content          android indeterminate  false          android max  10          android padding  10dip  gt       lt  ProgressBar gt       lt Button         android id    id button1          android layout width  wrap content          android layout height  wrap content          android text  Start Progress   gt       lt  Button gt       lt TextView android id    id output          android layout width  match parent          android layout height  wrap content          android text  Replace   gt   lt  LinearLayout gt

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private class AsyncTaskDemo extends AsyncTask lt Void  Void  Void gt          Override     protected void onPreExecute             super onPreExecute               Showing progress dialog         progressDialog   new ProgressDialog this           progressDialog setMessage  Loading               progressDialog setCancelable false           progressDialog show                Override     protected Void doInBackground Void    arg0                Do code here          return null              Override     protected void onPostExecute Void result            super onPostExecute result              Dismiss the progress dialog         if  progressDialog isShowing                  progressDialog dismiss                          Override     protected void onCancelled              super onCancelled            progressDialog dismiss            Toast toast   Toast makeText                            getActivity                               An error is occurred due to some problem                             Toast LENGTH LONG           toast setGravity Gravity TOP  25  400           toast show

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While working with AsyncTask  it is necessary to create a class-successor and in it to register the implementation of methods necessary for us  In this lesson we will look at three methods   doInBackground - will be executed in a new thread  and here we solve all our difficult tasks  Because a non-primary thread does not have access to the UI   onPreExecute - executed before doInBackground and has access to the UI  onPostExecute - executed after doInBackground  does not work if AsyncTask was canceled - about this in the next lessons  and has access to the UI   This is the MyAsyncTask class   class MyAsyncTask extends AsyncTask lt Void  Void  Void gt        Override   protected void onPreExecute         super onPreExecute        tvInfo setText  Start            Override   protected Void doInBackground Void    params           Your background method     return null          Override   protected void onPostExecute Void result        super onPostExecute result       tvInfo setText  Finish            And this is how to call in your Activity or Fragment   MyAsyncTask myAsyncTask   new MyAsyncTask    myAsyncTask execute

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Sample AsyncTask example with progress  import android animation ObjectAnimator  import android os AsyncTask  import android support v7 app AppCompatActivity  import android os Bundle  import android util Log  import android view View  import android view animation AccelerateDecelerateInterpolator  import android view animation DecelerateInterpolator  import android view animation LinearInterpolator  import android widget Button  import android widget ProgressBar  import android widget TextView   public class AsyncTaskActivity extends AppCompatActivity implements View OnClickListener        Button btn      ProgressBar progressBar        Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity main           btn    Button  findViewById R id button1           btn setOnClickListener this           progressBar    ProgressBar findViewById R id pbar              public void onClick View view            switch  view getId                  case R id button1                  new LongOperation   execute                      break                       private class LongOperation extends AsyncTask lt String  Integer  String gt              Override         protected String doInBackground String    params                Log d  AsyncTask    doInBackground                for  int i   0  i  lt  5  i                      try                       Log d  AsyncTask    task    i   1                        publishProgress i   1                       Thread sleep 1000                     catch  InterruptedException e                        Thread interrupted                                                return  Completed                       Override         protected void onPostExecute String result                Log d  AsyncTask    onPostExecute                TextView txt    TextView  findViewById R id output               txt setText result               progressBar setProgress 0                       Override         protected void onPreExecute                 Log d  AsyncTask    onPreExecute                TextView txt    TextView  findViewById R id output               txt setText  onPreExecute                progressBar setMax 500               progressBar setProgress 0                       Override         protected void onProgressUpdate Integer    values                Log d  AsyncTask    onProgressUpdate   values 0                TextView txt    TextView  findViewById R id output               txt setText  onProgressUpdate   values 0                 ObjectAnimator animation   ObjectAnimator ofInt progressBar   progress   100   values 0                animation setDuration 1000               animation setInterpolator new LinearInterpolator                 animation start

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When you are in the worker thread  you can not directly manipulate UI elements on Android   When you are using AsyncTask please understand the callback methods   For example   public class MyAyncTask extends AsyncTask lt Void  Void  Void gt         Override     protected void onPreExecute                Here you can show progress bar or something on the similar lines             Since you are in a UI thread here          super onPreExecute                Override     protected void onPostExecute Void aVoid            super onPostExecute aVoid              After completing execution of given task  control will return here             Hence if you want to populate UI elements with fetched data  do it here              Override     protected void onProgressUpdate Void    values            super onProgressUpdate values              You can track you progress update here             Override     protected Void doInBackground Void    params               Here you are in the worker thread and you are not allowed to access UI thread from here             Here you can perform network operations or any heavy operations you want          return null            FYI  To access the UI thread from a worker thread  you either use runOnUiThread   method or post method on your view   For instance   runOnUiThread new Runnable         textView setText  something          or     yourview post new Runnable         yourview setText  something          This will help you know the things better  Hence in you case  you need to set your textview in the onPostExecute   method

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Concept and code here   I have created a simple example for using AsyncTask of Android  It starts with onPreExecute    doInBackground    publishProgress   and finally onProgressUpdate     In this  doInBackground   works as a background thread  while other works in the UI Thread  You can t access an UI element in doInBackground    The sequence is the same as I have mentioned   However  if you need to update any widget from doInBackground  you can publishProgress from doInBackground which will call onProgressUpdate to update your UI widget   class TestAsync extends AsyncTask lt Void  Integer  String gt        String TAG   getClass   getSimpleName         protected void onPreExecute             super onPreExecute            Log d TAG     PreExceute   On pre Exceute                     protected String doInBackground Void   arg0            Log d TAG     DoINBackGround    On doInBackground                for  int i 0  i lt 10  i                 Integer in   new Integer i               publishProgress i                     return  You are at PostExecute              protected void onProgressUpdate Integer   a            super onProgressUpdate a           Log d TAG     onProgressUpdate    You are in progress update         a 0               protected void onPostExecute String result            super onPostExecute result           Log d TAG     onPostExecute        result             Call it like this in your activity   new TestAsync   execute      Developer Reference Here

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ASync Task  public class MainActivity extends AppCompatActivity     private String ApiUrl  quot your api quot        Override    protected void onCreate Bundle savedInstanceState         super onCreate savedInstanceState        setContentView R layout activity main        MyTask myTask new MyTask         try            String result myTask execute ApiUrl  get             Toast makeText getApplicationContext   result Toast LENGTH SHORT  show           catch  ExecutionException e             e printStackTrace           catch  InterruptedException e            e printStackTrace                     public class MyTask extends AsyncTask lt String Void String gt          Override     protected String doInBackground String    strings            String result  quot  quot           HttpURLConnection httpURLConnection null          URL url           try               url new URL strings 0                httpURLConnection  HttpURLConnection  url openConnection                InputStream inputStream httpURLConnection getInputStream                InputStreamReader reader new InputStreamReader inputStream               result getData reader             catch  MalformedURLException e                e printStackTrace              catch  IOException e                e printStackTrace                      return result             public String getData InputStreamReader reader  throws IOException          String result  quot  quot           int data reader read            while  data  -1               char now  char  data              result  data              data reader read                      return result

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if you open AsyncTask class you can see below code   public abstract class AsyncTask lt Params  Progress  Result gt         WorkerThread     protected abstract Result doInBackground Params    params        MainThread     protected void onPreExecute                SuppressWarnings   UnusedDeclaration         MainThread     protected void onPostExecute Result result              AsyncTask features   AsyncTask is abstract class AsyncTask is have 3 generic params  AsyncTask has abstract method of doInBackground  onPreExecute  onPostExecute doInBackground is WorkerThread  you can t update UI  onPreExecute is MainThread onPostExecute is MainThread  you can update UI    example   Override protected void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       setContentView R layout activity login       mEmailView    AutoCompleteTextView  findViewById R id email        AsyncTask lt Void  Void  Post gt  asyncTask   new AsyncTask lt Void  Void  Post gt               Override         protected Post doInBackground Void    params                try                   ApiClient defaultClient   Configuration getDefaultApiClient                    String authorization    eyJhbGciOiJIUzI1NiJ9 eyJ1c2VyX2lkIjoxLCJleHAiOjE1ODIxMzM4MTB9 bA3Byc SuB6jzqUGAY4Pyt4oBNg0VfDRctZ8-PcPlYg      String   JWT token for Authorization                 ApiKeyAuth Bearer    ApiKeyAuth  defaultClient getAuthentication  Bearer                    Bearer setApiKey authorization                   PostApi apiInstance   new PostApi                    String id    1      String   id                 Integer commentPage   1     Integer   Page number for Comment                 Integer commentPer   10     Integer   Per page number For Comment                 Post result                  try                       result   apiInstance apiV1PostsIdGet id  authorization  commentPage  commentPer                     catch  ApiException e                        e printStackTrace                        result   new Post                                      return result                catch  Exception e                    e printStackTrace                    return new Post                                      Override         protected void onPostExecute Post post                super onPostExecute post               if  post    null                    mEmailView setText post getBody                     System out print post                                      asyncTask execute

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Background   Theory  AsyncTask allows you to run a task on a background thread  while publishing results to the UI thread      The user should always able to interact with the app so it is important   to avoid blocking the main  UI  thread with tasks such as   downloading content from the web       This is why we use an AsyncTask       It offers a straightforward interface by wrapping the UI thread message queue and handler that allow you to send and process runnable objects and messages from other threads    Implementation  AsyncTask is a generic class    It takes parameterized types in its constructor    It uses these three generic types      Params - the type of the parameters sent to the task upon execution   Progress - the type of the progress units published during the background computation   Result - the type of the result of the background computation      Not all types are always used by an asynchronous task  To mark a type as unused  simply use the type Void       private class MyTask extends AsyncTask lt Void  Void  Void gt            These three parameters correspond to three primary functions you can override in AsyncTask    doInBackground Params     onProgressUpdate Progress     onPostExecute Result    To execute AsyncTask   Call execute   with parameters to be sent to the background task    What Happens   On main UI thread  onPreExecute   is called    To initialize something in this thread   E g  show a progress bar on the user interface    On a background thread  doInBackground Params     is called     Params were passed via execute   Where the long-running task should happen  Must override at least doInBackground   to use AsyncTask  Call publishProgress Progress     to update the user interface with a display of progress  e g  UI animation or log text printed  while the background computation is still executing    Causes onProgressUpdate   to be called    On the background thread a result is returned from doInBackground       This triggers the next step    On main UI thread  onPostExecute   is called with the returned result    Examples  In both examples the  blocking task  is a download from the web    Example A downloads an image and displays it in an ImageView  while Example B downloads some files    Example A  The doInBackground   method downloads the image and stores it in an object of type BitMap   The onPostExecute   method takes the bitmap and places it in the ImageView   class DownloadImageTask extends AsyncTask lt String  Void  Bitmap gt        ImageView bitImage       public DownloadImageTask ImageView bitImage            this bitImage   bitImage             protected Bitmap doInBackground String    urls            String urldisplay   urls 0           Bitmap mBmp   null          try               InputStream in   new java net URL urldisplay  openStream                mBmp   BitmapFactory decodeStream in             catch  Exception e                Log e  Error   e getMessage                 e printStackTrace                      return mBmp             protected void onPostExecute Bitmap result            bitImage setImageBitmap result             Example B   private class DownloadFilesTask extends AsyncTask lt URL  Integer  Long gt         protected Long doInBackground URL    urls             int count   urls length           long totalSize   0           for  int i   0  i  lt  count  i                   totalSize    Downloader downloadFile urls i                 publishProgress  int    i    float  count    100                    Escape early if cancel   is called              if  isCancelled    break                      return totalSize               protected void onProgressUpdate Integer    progress             setProgressPercent progress 0                 protected void onPostExecute Long result             showDialog  Downloaded     result     bytes                Example B execution  new DownloadFilesTask   execute url1  url2  url3

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I would recommend making your life easier by using this library for background works   https   github com Arasthel AsyncJobLibrary  It s this simple     AsyncJob doInBackground new AsyncJob OnBackgroundJob           Override     public void doOnBackground             startRecording

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Change your code as given below    Override protected void onPostExecute String result         runOnUiThread new Runnable             public void run                 TextView txt    TextView  findViewById R id output               txt setText  Executed

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How to memorize the parameters used in AsyncTask      Don t   If you are new to AsyncTask then it is very common to get confused while writing an AsyncTask  The main culprits are the parameters used in the AsyncTask  i e   AsyncTask lt A  B  C gt   Based on the A  B  C  arguments  signature of the methods differs which makes things even more confusing   Keep it simple   The key is don t memorize  If you can visualize what your task really needs to do then writing the AsyncTask with the correct signature at the first attempt would be a piece of cake  Just figure out what your Input  Progress  and Output are  and you will be good to go   So what is an AsyncTask   AsyncTask is a background task that runs in the background thread  It takes an Input  performs Progress and gives an Output      I e   AsyncTask lt Input  Progress  Output gt     For example     What is the relationship with methods      Between AsyncTask and doInBackground          doInBackground   and onPostExecute   onProgressUpdate    are also   related     How to write that in the code   DownloadTask extends AsyncTask lt String  Integer  String gt           Always same signature      Override     public void onPreExecute                Override     public String doInbackGround String    parameters                   Download code         int downloadPerc      Calculate that         publish downloadPerc            return  Download Success               Override     public void onPostExecute String result                super onPostExecute result               Override     public void onProgressUpdate Integer    parameters                   Show in spinner  and access UI elements            How will you run this Task   new DownLoadTask   execute  Paradise mp3

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Ok  you are trying to access the GUI via another thread  This  in the main  is not good practice   The AsyncTask executes everything in doInBackground   inside of another thread  which does not have access to the GUI where your views are   preExecute   and postExecute   offer you access to the GUI before and after the heavy lifting occurs in this new thread  and you can even pass the result of the long operation to postExecute   to then show any results of processing   See these lines where you are later updating your TextView   TextView txt   findViewById R id output   txt setText  Executed      Put them in onPostExecute     You will then see your TextView text updated after the doInBackground completes   I noticed that your onClick listener does not check to see which View has been selected  I find the easiest way to do this is via switch statements  I have a complete class edited below with all suggestions to save confusion   import android app Activity  import android os AsyncTask  import android os Bundle  import android provider Settings System  import android view View  import android widget Button  import android widget TextView  import android view View OnClickListener   public class AsyncTaskActivity extends Activity implements OnClickListener        Button btn      AsyncTask lt         gt  runningTask        Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout main           btn   findViewById R id button1               Because we implement OnClickListener  we only            have to pass  this   much easier          btn setOnClickListener this               Override     public void onClick View view               Detect the view that was  clicked          switch  view getId              case R id button1              if  runningTask    null                  runningTask cancel true               runningTask   new LongOperation                runningTask execute                break                        Override     protected void onDestroy             super onDestroy               Cancel running task s  to avoid memory leaks         if  runningTask    null              runningTask cancel true              private final class LongOperation extends AsyncTask lt Void  Void  String gt              Override         protected String doInBackground Void    params                for  int i   0  i  lt  5  i                      try                       Thread sleep 1000                     catch  InterruptedException e                           We were cancelled  stop sleeping                                              return  Executed                       Override         protected void onPostExecute String result                TextView txt    TextView  findViewById R id output               txt setText  Executed       txt setText result                  You might want to change  executed  for the returned string                passed into onPostExecute    but that is up to you

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Simply   LongOperation MyTask   new LongOperation    MyTask execute

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Update  March 2020  According to Android developer official documentation  AsyncTask is now deprecated    It s recommended to use kotlin corourines instead  Simply  it allows you to write asynchronous tasks in a sequential style

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When an asynchronous task is executed  the task goes through four steps    onPreExecute   doInBackground Params     onProgressUpdate Progress     onPostExecute Result    Below is a demo example   private class DownloadFilesTask extends AsyncTask lt URL  Integer  Long gt         protected Long doInBackground URL    urls            int count   urls length          long totalSize   0          for  int i   0  i  lt  count  i                  totalSize    Downloader downloadFile urls i                publishProgress  int    i    float  count    100                    Escape early if cancel   is called             if  isCancelled                    break                    return totalSize             protected void onProgressUpdate Integer    progress            setProgressPercent progress 0               protected void onPostExecute Long result            showDialog  Downloaded     result     bytes               And once you created  a task is executed very simply   new DownloadFilesTask   execute url1  url2  url3

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Shortest example for just doing something asynchronously   class MyAsyncTask extends android os AsyncTask        Override     protected Object doInBackground Object   objects               Do something asynchronously         return null            To run it    new MyAsyncTask    execute

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Sample Async Task with POST request   List lt NameValuePair gt  params   new ArrayList lt NameValuePair gt     params add new BasicNameValuePair  key1    value1     params add new BasicNameValuePair  key1    value2     new WEBSERVICEREQUESTOR URL  params  execute     class WEBSERVICEREQUESTOR extends AsyncTask lt String  Integer  String gt        String URL      List lt NameValuePair gt  parameters       private ProgressDialog pDialog       public WEBSERVICEREQUESTOR String url  List lt NameValuePair gt  params                this URL   url          this parameters   params              Override     protected void onPreExecute                 pDialog   new ProgressDialog LoginActivity this           pDialog setMessage  Processing Request               pDialog setIndeterminate false           pDialog setCancelable false           pDialog show            super onPreExecute                Override     protected String doInBackground String    params                try                       DefaultHttpClient httpClient   new DefaultHttpClient                HttpEntity httpEntity   null              HttpResponse httpResponse   null               HttpPost httpPost   new HttpPost URL                if  parameters    null                                httpPost setEntity new UrlEncodedFormEntity parameters                              httpResponse   httpClient execute httpPost                httpEntity   httpResponse getEntity                return EntityUtils toString httpEntity               catch  Exception e                               return                 Override     protected void onPostExecute String result                pDialog dismiss             try                              catch  Exception e                               super onPostExecute result

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I m sure it is executing properly  but you re trying to change the UI elements in the background thread and that won t do   Revise your call and AsyncTask as follows   Calling Class  Note  I personally suggest using onPostExecute   wherever you execute your AsyncTask thread and not in the class that extends AsyncTask itself  I think it makes the code easier to read especially if you need the AsyncTask in multiple places handling the results slightly different   new LongThread          Override public void onPostExecute String result            TextView txt    TextView  findViewById R id output           txt setText result           execute        LongThread class  extends AsyncTask     Override protected String doInBackground String    params        for  int i   0  i  lt  5  i              try               Thread sleep 1000             catch  InterruptedException e                e printStackTrace                         return  Executed

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You need to declare the button onclicklistener  Once clicked  it calls AsyncTask class DownloadJson   The process will be shown below    Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout main            btn    Button  findViewById R id button1            btn setOnClickListener new View OnClickListener                  Override             public void onClick View v                    new DownloadJson   execute                                               DownloadJSON AsyncTask     private class DownloadJson extends AsyncTask lt Void  Void  Void gt             Override         protected void onPreExecute                 super onPreExecute                        Override         protected Void doInBackground Void    params                newlist   new ArrayList lt HashMap lt String  String gt  gt                 json   jsonParser makeHttpRequest json   POST                try                   newarray   new JSONArray json                       for  int i   0  i  lt  countdisplay  i                              HashMap lt String  String gt  eachnew   new HashMap lt String  String gt                             newobject   newarray getJSONObject i                           eachnew put  id   newobject getString  ID                             eachnew put  name   newobject getString  Name                             newlist add eachnew                                                         catch  JSONException e                    Log e  Error   e getMessage                     e printStackTrace                              return null                      Override         protected void onPostExecute Void args                newlisttemp addAll newlist               NewAdapterpager newadapterpager   new NewAdapterpager ProcesssActivitypager this  newlisttemp               newpager setAdapter newadapterpager

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My full answer is here  but here is an explanatory image to supplement the other answers on this page  For me  understanding where all the variables were going was the most confusing part in the beginning

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Move these two lines   TextView txt    TextView  findViewById R id output   txt setText  Executed      out of your AsyncTask s doInBackground method and put them in the onPostExecute method  Your AsyncTask should look something like this   private class LongOperation extends AsyncTask lt String  Void  String gt          Override     protected String doInBackground String    params            try               Thread sleep 5000      no need for a loop           catch  InterruptedException e                Log e  LongOperation    Interrupted   e               return  Interrupted                     return  Executed                     Override     protected void onPostExecute String result                           TextView txt    TextView  findViewById R id output           txt setText result

[android] AsyncTask Android example

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