[android] AsyncTask Android example

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<String, Void, String> {
        @Override
        protected String doInBackground(String... params) {
            for(int i=0;i<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:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical" >
    <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">
    </ProgressBar>
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start Progress" >
    </Button>
    <TextView android:id="@+id/output"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Replace"/>
</LinearLayout>

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<Void, Integer, String> {
    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<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

private class AsyncTaskDemo extends AsyncTask<Void, Void, Void> {

    @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();
    }

}

if you open AsyncTask class you can see below code.

public abstract class AsyncTask<Params, Progress, Result> {
    @WorkerThread
    protected abstract Result doInBackground(Params... params);
    @MainThread
    protected void onPreExecute() {
    }
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }
}

AsyncTask features

1. AsyncTask is abstract class
2. AsyncTask is have 3 generic params.
3. AsyncTask has abstract method of doInBackground, onPreExecute, onPostExecute
4. doInBackground is WorkerThread (you can't update UI)
5. onPreExecute is MainThread
6. 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<Void, Void, Post> asyncTask = new AsyncTask<Void, Void, Post>() {
        @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();
}

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<A, B, C>. 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<Input, Progress, Output>.

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<String, Integer, String>{

    // 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");

ASync Task;

public class MainActivity extends AppCompatActivity {

 private String ApiUrl="your_api";

   @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<String,Void,String>{


    @Override
    protected String doInBackground(String... strings) {
        String result="";
        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="";
        int data=reader.read();
        while (data!=-1){
            char now=(char) data;
            result+=data;
            data=reader.read();
        }
        return result;
    }
  }
}

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<Void, Void, Void> { ... }

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

1. On main/UI thread, onPreExecute() is called.

   • To initialize something in this thread. (E.g. show a progress bar on the user interface.)

2. 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.

3. On the background thread a result is returned from doInBackground().

   • (This triggers the next step.)

4. 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<String, Void, Bitmap> {
    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<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < 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);

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");
        }
    });
}

Sample Async Task with POST request:

List<NameValuePair> params = new ArrayList<NameValuePair>();
params.add(new BasicNameValuePair("key1", "value1"));
params.add(new BasicNameValuePair("key1", "value2"));
new WEBSERVICEREQUESTOR(URL, params).execute();

class WEBSERVICEREQUESTOR extends AsyncTask<String, Integer, String>
{
    String URL;
    List<NameValuePair> parameters;

    private ProgressDialog pDialog;

    public WEBSERVICEREQUESTOR(String url, List<NameValuePair> 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);
    }
}

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 < 5; i++) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    return "Executed";
}      

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<Void, Void, Void>{

    @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.

Simply:

LongOperation MyTask = new LongOperation();
MyTask.execute();

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();
    }
});

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();

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.

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<Void, Void, Void> {
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Void... params) {
            newlist = new ArrayList<HashMap<String, String>>();
            json = jsonParser.makeHttpRequest(json, "POST");
            try {
                newarray = new JSONArray(json);
                    for (int i = 0; i < countdisplay; i++) {
                        HashMap<String, String> eachnew = new HashMap<String, String>();
                        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);
        }
    }

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<Void, Void, Void> {

  @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();

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<String, Void, String> {

    @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);
    }
}

When an asynchronous task is executed, the task goes through four steps:

1. onPreExecute()
2. doInBackground(Params...)
3. onProgressUpdate(Progress...)
4. onPostExecute(Result)

Below is a demo example:

private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {

    protected Long doInBackground(URL... urls) {
        int count = urls.length;
        long totalSize = 0;
        for (int i = 0; i < 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);

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.

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<String, Integer, String> {

        @Override
        protected String doInBackground(String... params) {
            Log.d("AsyncTask", "doInBackground");
            for (int i = 0; i < 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();
        }
    }
}