In AsyncTask InputStream get android.os.NetworkOnMainThreadException

Issue

I'm getting a crash in an Android device which shows an exception about android.os.NetworkOnMainThreadException in AsyncTask. Why do I get this exception? The exception is on line 60 in onPreExecute.

I send context with WeakReference. Please explain the advantage of using WeakReference here.

public class MainActivity extends AppCompatActivity {

    RecyclerView recyclerView;
    RecyclerView.LayoutManager layoutManager;
    RecyclerViewAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView = findViewById(R.id.rview);
        layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(true);
        MyTask task = new MyTask(this);
        task.execute();
    }

    static class MyTask extends AsyncTask {
        BufferedReader reader;
        private WeakReference < MainActivity > contextRef;

        MyTask(MainActivity mainActivity) {
            contextRef = new WeakReference < >(mainActivity);
        }

        @Override
        protected void onPreExecute() {
            try {
                URL url = new URL("https://api.myjson.com/bins/1fi1zm");

                URLConnection connection = url.openConnection();
                InputStream stream = connection.getInputStream();
                reader = new BufferedReader(new InputStreamReader(stream));

            } catch(MalformedURLException e) {
                e.printStackTrace();
            } catch(IOException e) {
                e.printStackTrace();
            }
        }

        @Override
        protected Object doInBackground(Object[] objects) {
            return null;
        }

        @Override
        protected void onPostExecute(Object o) {
            MainActivity context = contextRef.get();
            if (context != null) {
                Gson gson = new Gson();
                List < Contact > list;
                try {
                    list = Arrays.asList(gson.fromJson(reader.readLine(), Contact[].class));
                    context.adapter = new RecyclerViewAdapter(list);
                    context.recyclerView.setAdapter(context.adapter);
                } catch(IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Solution

The issue is for the code in onPreExecute(). It runs on Main Thread and you cannot do background task here. Try this

static class MyTask extends AsyncTask {

BufferedReader reader;
private WeakReference<MainActivity> contextRef;

MyTask(MainActivity mainActivity) {
    contextRef = new WeakReference<>(mainActivity);
}


@Override
protected void onPreExecute() {

}

@Override
protected Object doInBackground(Object[] objects) {
    try {
        URL url = new URL("https://api.myjson.com/bins/1fi1zm");

        URLConnection connection = url.openConnection();
        InputStream stream = connection.getInputStream();
        reader = new BufferedReader(new InputStreamReader(stream));

    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    return null;
}

@Override
protected void onPostExecute(Object o) {
    MainActivity context = contextRef.get();

    if (context != null) {
        Gson gson = new Gson();
        List<Contact> list;
        try {
            list = Arrays.asList(gson.fromJson(reader.readLine(), Contact[].class));
            context.adapter = new RecyclerViewAdapter(list);
            context.recyclerView.setAdapter(context.adapter);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

}

Let me know if it solved your issue.

Answered By - Hari N Jha