Android Networking Tutorial: Getting Started

Eunice Obugyei

android-networking-feature

Networking has played a huge role in Android apps since API level 1. Most apps don’t work in isolation; rather, they connect to an online service to retrieve data or perform other networking functions.

In this Android networking tutorial, you will create a simple app which connects to the GitHub API to retrieve and display a list of repositories.

In the process, you will learn about the following:

  • How to check your network connection status.
  • How to perform network operations.
  • How to leverage open source libraries to perform network operations.
Note: This tutorial assumes you’re already familiar with the basics of Android development. If you are completely new to Android development, read through our Android Tutorial for Beginners to familiarize yourself with the basics.

Getting Started

Download the starter project for this tutorial and extract the project. Open the starter project in Android Studio by selecting Open an existing Android Studio project from the Quick Start menu:

quick_start_menu

You can also use File\Open. Navigate to and select the starter project folder.

Open Util.java and look inside; this file contains a helper method which you’ll use to convert a JSON string to a list of repository objects. Repository.java is your model class that represents a Github repository.

Build and run the project to see what you have to work with:

intro_to_net_1

Required Permissions

To perform network operations in Android, your application must include certain permissions.

Open manifest/AndroidManifest.xml, and add the following permissions just before the application tag:

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.INTERNET" />

The ACCESS_NETWORK_STATE permission is required for your application to check the network state of the device, while the INTERNET permission is required for your application to access the Internet.

Before you attempt to perform any network operations, it is advisable to check first that the device is connected to a network! :]

Checking the Network Connection

Before adding any Java code, you’ll need to configure Android Studio to automatically insert import statements to save you from having to add each one manually.

Go to Android Studio\Preferences\Editor\General\Auto Import, select the Add unambiguous imports on the fly checkbox and click OK.

Open MainActivity.java, and add the following method to the class:

private boolean isNetworkConnected() {
  ConnectivityManager connMgr = (ConnectivityManager)
   getSystemService(Context.CONNECTIVITY_SERVICE); // 1
  NetworkInfo networkInfo = connMgr.getActiveNetworkInfo(); // 2
  return networkInfo != null && networkInfo.isConnected(); // 3
}

isNetworkConnected() checks that the device has an active Internet connection as follows:

  1. Retrieves an instance of the ConnectivityManager class from the current application context.
  2. Retrieves an instance of the NetworkInfo class that represents the current network connection. This will be null if no network is available.
  3. Check if there is an available network connection and the device is connected.

It’s a good idea to show an alert that prompts the user to connect to a network if the device isn’t already connected.

You will need to manually add the following import statements to the top of the file since Android Studio can’t automatically determine which packages to include:

import android.content.DialogInterface;
import android.support.v7.app.AlertDialog;

Replace the showListFragment(new ArrayList()); line in onCreate() with the following code:

if (isNetworkConnected()) {
  mProgressDialog = new ProgressDialog(this);
  mProgressDialog.setMessage("Please wait...");
  mProgressDialog.setCancelable(false);
  mProgressDialog.show();

  startDownload();
} else {
  new AlertDialog.Builder(this)
    .setTitle("No Internet Connection")
    .setMessage("It looks like your internet connection is off. Please turn it " +
    "on and try again")
    .setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int which) {
    }
  }).setIcon(android.R.drawable.ic_dialog_alert).show();
}

This displays a progress dialog and calls startDownload() if the device is connected to a network; if it isn’t connected, it displays an alert to the user instead.

Build and run your app; if your device is connected to a network, the app will show a ProgressDialog like so:

intro_to_net_2

Now turn off all network connections on your device and start the app again. The app should now show an alert with the Internet connection error message:

intro_to_net_3

Note: If using the emulator, check out the Android Emulator 2.0 section of our tutorial What’s New in Android Studio 2 for tips on mocking different network states.

Determining the Connection Type

When you have an app that retrieves huge amounts of data, you might want to restrict network connections to particular network types, such as Wi-Fi. You can do this using getType() on the NetworkInfo object.

Add the following method to MainActivity:

private boolean isWifiConnected() {
  ConnectivityManager connMgr = (ConnectivityManager)
              getSystemService(Context.CONNECTIVITY_SERVICE);
  NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
  return networkInfo != null && (ConnectivityManager.TYPE_WIFI == networkInfo.getType()) && networkInfo.isConnected();
}

The above method returns true if the device is connected to a Wi-Fi network.

Replace the isNetworkConnected() call in onCreate() with the following:

isWifiConnected()

Build and run your app; if your device is connected to a wifi network, the app will display the following progress dialog:

intro_to_net_2

Now turn off the Wi-Fi connection on the device (leaving packet data active) and restart the app. You’ll see an alert with the Internet connection error message:

intro_to_net_3

Performing Network Operations

Android best practice is to handle any long-running tasks that may hang your application in a thread separate from the user interface thread. Network operations fall into this category of tasks.

Since Android 3.0 (Honeycomb), the system has been configured to crash with a NetworkOnMainThreadException exception if the network is accessed in the user interface thread.

The AsyncTask class lets you perform asynchronous tasks in the background and publish results on the UI thread without any thread manipulation.

Since AsyncTask is an abstract class, you must subclass it before you can use it.

To create a new class, select the package in which you want to create the class – in this case, reposearch. Next, select File/New/Java Class:

intro_to_net_4.png

In the Create New Class dialog, enter DownloadRepoTask as the name of the class and click OK:

intro_to_net_5

Replace the class declaration with the following code:

public class DownloadRepoTask extends AsyncTask<String, Void, String> {
  // 1
  @Override
  protected String doInBackground(String... params) {
    return null;
  }

  // 2
  @Override
  protected void onPostExecute(String result) {

  }
}

Here’s what’s going on in the code above:

AsyncTask makes it possible to perform (long-running) operations in the background and publish results on the UI thread without having to write threading code. The declaration for DownloadRepoTask contains the keywords extends AsyncTask. The first parameter in the diamond operator determines the type of the input parameter to the doInBackground method while the third parameter in the diamond operator determines the type of the return value of the onPostExecute method. The second parameter determines the type of the return value of the onProgressUpdate method which is not used here so it is set to Void.

  1. doInBackground() is where your background task executes – in this case, the data download task.
  2. When your background task is done, onPostExecute() is called is called with the results.

Add the following code to startDownload() in MainActivity.java:

new DownloadRepoTask().execute("https://api.github.com/repositories");

This calls an instance of the DownloadRepoTask class, with the parameters required to perform the task passed in the execute call.

Connecting to a Network URL

Open DownloadRepoTask.java and add the following method:

private String downloadData(String urlString) throws IOException {
  InputStream is = null;
  try {
    URL url = new URL(urlString);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setRequestMethod("GET");
    conn.connect();

    return null;
  } finally {
    if (is != null) {
      is.close();
    }
  }
}

The above code creates and opens a connection to the resource referred to by the URL.

Replace the return statement in doInBackground() with the following:

try {
  return downloadData(params[0]);
} catch (IOException e) {
  e.printStackTrace();
  return null;
}

This calls downloadData() and passes to it the URL you passed in from the task execution.

Retrieving Repositories

Now that you’ve created a connection to the resource, you can retrieve the data. You get the data as an InputStream by calling getInputStream() method on the HttpURLConnection object.

Add the following code to the DownloadRepoTask.java class, just before the return statement:

is = conn.getInputStream();

An InputStream is a readable source of bytes; in this case, it’s the JSON returned from GitHub that you’ll have to decode.

Add the following method to DownloadRepoTask.java:

private String convertToString(InputStream is) throws IOException {
  BufferedReader r = new BufferedReader(new InputStreamReader(is));
  StringBuilder total = new StringBuilder();
  String line;
  while ((line = r.readLine()) != null) {
    total.append(line);
  }
  return new String(total);
}

convertToString() uses the StringBuilder class to build a string from the input stream.

Replace the return statement in downloadData() with the following:

return convertToString(is);

You need to pass the downloaded data on to the Activity that started the task. You can use a listener, a type of interface, for this.

To create a new interface, select the package in which you want to create the interface – in this case, the reposearch package. Next, select File/New/Java Class.

In the Create New Class dialog, enter the name of the interface as DownloadCompleteListener, select the Interface option in the Kind select box and click OK.

Add the following method declaration inside DownloadCompleteListener:

void downloadComplete(ArrayList<Repository> repositories);

Add implements DownloadCompleteListener to the beginning of the class declaration of MainActivity.java so it looks like the following:

public class MainActivity extends AppCompatActivity implements DownloadCompleteListener

Now implement downloadComplete() in MainActivity.java like so:

@Override
public void downloadComplete(ArrayList<Repository> repositories) {

}

Add the following DownloadCompleteListener to DownloadRepoTask.java:

DownloadCompleteListener mDownloadCompleteListener;

Next, add a new constructor that takes a DownloadCompleteListener parameter and uses it to set the DownloadCompleteListener field in the class:

public DownloadRepoTask(DownloadCompleteListener downloadCompleteListener) {
  this.mDownloadCompleteListener = downloadCompleteListener;
}

Next, add the following code to onPostExecute:

try {
  mDownloadCompleteListener.downloadComplete(Util.retrieveRepositoriesFromResponse(result));
  }
catch (JSONException e) {
  e.printStackTrace();
}

This converts and passes the results to the DownloadCompleteListener using downloadComplete().

Open MainActivity.java and replace the code in startDownload() with the following:

new DownloadRepoTask(this).execute("https://api.github.com/repositories");

This will start the AsyncTask that you made and direct it to the github repositories URL.

Next, add the following code to downloadComplete():

showListFragment(repositories);
if (mProgressDialog != null) {
  mProgressDialog.hide();
}

The above code receives a list of Repository objects, displays them to the user, then hides the progress dialog.

Make sure the device has a working Internet connection, then build and run:

intro_to_net_6

Cool – your app connected to the GitHub API and retrieved a list of repositories for your perusal!

Performing network operations in Android can be a bit tedious; you have to open and close connections, handle InputStream conversions and ensure you’re performing the network operation in a background thread.

As usual, the geniuses of the open source community have come to the rescue in the form of open source libraries that take care of the tedious details for you.

Open Source To The Rescue

There are a number of open source libraries that simplify a lot of Android networking operations. I’ve put together a few notes on the three most popular ones below.

OkHttp

OkHttp is an efficient HTTP client which supports synchronous and asynchronous calls. It handles the opening and closing of connections along with InputStream-to-string conversion. It’s compatible with Android 2.3 and above.

Implementing OkHttp is pretty simple and reduces the overall amount of code needed to make network calls – but there’s a catch.

Since OkHttp is built as a Java library, and not an Android library, it doesn’t take into consideration the Android Framework limitations of only permitting view updates on the main UI thread. To update any views, or post data back to the main thread, you will need to use runOnUiThread() provided in the Android Activity superclass.

To use OkHttp, you have to first add it as a dependency to your project.

Add the following dependency to build.gradle of the app module:

dependencies {
  ...
  compile 'com.squareup.okhttp3:okhttp:3.1.2'
}

Your build.gradle file should look something like the one shown below:
intro_to_net_7

Now rebuild your project:

Open MainActivity.java and add the following method:

private void makeRequestWithOkHttp(String url) {
  OkHttpClient client = new OkHttpClient();   // 1
  okhttp3.Request request = new okhttp3.Request.Builder().url(url).build();  // 2

  client.newCall(request).enqueue(new okhttp3.Callback() { // 3
    @Override
    public void onFailure(okhttp3.Call call, IOException e) {
      e.printStackTrace();
    }

    @Override
    public void onResponse(okhttp3.Call call, okhttp3.Response response)
    throws IOException {
      final String result = response.body().string();  // 4

      MainActivity.this.runOnUiThread(new Runnable() {
        @Override
        public void run() {
          try {
            downloadComplete(Util.retrieveRepositoriesFromResponse(result));  // 5
          } catch (JSONException e) {
            e.printStackTrace();
          }
        }
      });
    }
  });
}

The code above does the following:

  1. Creates an OkHttpClient object.
  2. Builds an OkHttpClient request with the URL you want to connect to.
  3. Queues the request call.
  4. Retrieves the response as a string.
  5. Converts and passes the results to the main thread.

Replace the code in the startDownload() method with the following:

makeRequestWithOkHttp("https://api.github.com/repositories");

Build and run your app, and everything should work as before:

intro_to_net_6

Pretty simple, right?

Volley

Volley is an Android library for making fast and easy network calls. Just like OkHttp, it does all the dirty work for you. One of its advantages over OkHttp is that it’s compatible with Android 1.6 and above.

By default, Volley makes all calls in an asynchronous manner. Since it’s an Android library, it doesn’t require any extra work to return data to the main thread or update views.

To use Volley, add the following dependency to build.gradle of the app module and rebuild the project:

dependencies {
  ...
  compile 'com.android.volley:volley:1.0.0'
}

Open MainActivity.java and add the following method:

private void makeRequestWithVolley(String url) {

  RequestQueue queue = Volley.newRequestQueue(this); // 1

  StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
              new com.android.volley.Response.Listener<String>() { // 2
    @Override
    public void onResponse(String response) {
      try {
        downloadComplete(Util.retrieveRepositoriesFromResponse(response)); // 3
      } catch (JSONException e) {
        e.printStackTrace();
      }
    }
  }, new com.android.volley.Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
    }
  });
  queue.add(stringRequest);  // 4

}

makeRequestWithVolley does the following:

  1. Creates a new request queue.
  2. Creates a new instance of StringRequest with the URL you want to connect to.
  3. Converts and passes the results to downloadComplete().
  4. Adds the string request to the request queue.

Replace the code in startDownload() with the following code:

makeRequestWithVolley("https://api.github.com/repositories");

Build and run your project; once again, the network requests should work as before:

intro_to_net_6

Retrofit

Retrofit is an Android and Java library which is great at retrieving and uploading structured data such as JSON and XML. Retrofit makes HTTP requests using OkHttp. Unlike the case of Volley, where you have to convert a JSON string to a Repository object, Retrofit does that conversion for you. Retrofit also lets you specify any of the following libraries for the data conversion:

  1. Gson
  2. Jackson
  3. Moshi
  4. Protobuf
  5. Wire
  6. Simple XML
  7. Scalars (primitives, boxed, and String)

To use Retrofit, add the following dependencies to build.gradle of the app module and rebuild the project:

dependencies {
  ...
  compile 'com.squareup.retrofit2:retrofit:2.0.0-beta4'
  compile 'com.squareup.retrofit2:converter-gson:2.0.0-beta4'
}

Then create a new interface and name it RetrofitAPI; this will define the HTTP operations.

Add the following code to RetrofitAPI.java:

@GET("/repositories")
Call<ArrayList<Repository>> retrieveRepositories();

This specifies the request method and the URL for the request call.

Add the following imports to MainActivity.java:

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

Then add the following method:

private void makeRetrofitCalls() {
  Retrofit retrofit = new Retrofit.Builder()
   .baseUrl("https://api.github.com") // 1
   .addConverterFactory(GsonConverterFactory.create()) // 2
   .build();

  RetrofitAPI retrofitAPI = retrofit.create(RetrofitAPI.class); // 3

  Call<ArrayList<Repository>> call = retrofitAPI.retrieveRepositories(); // 4

  call.enqueue(new Callback<ArrayList<Repository>>() {  // 5
    @Override
    public void onResponse(Call<ArrayList<Repository>> call,
                                 Response<ArrayList<Repository>> response) {
      downloadComplete(response.body());  // 6
    }

    @Override
    public void onFailure(Call<ArrayList<Repository>> call, Throwable t) {
      Toast.makeText(MainActivity.this, t.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
    }
  });
}

makeRetrofitCalls() does the following:

  1. Specifies the base URL
  2. Specifies GsonConverterFactory as the converter which uses Gson for its deserialization.
  3. Generates an implementation of the RetrofitAPI.
  4. Generates the request call.
  5. Queues the request call.
  6. Passes the results to downloadComplete().

Replace the code in startDownload() with the following:

makeRetrofitCalls();

Build and run your project, and the list of repositories shows up as before:

intro_to_net_6

Where to Go From Here?

You’ve explored (and survived!) a crash-course on network operations in Android. :] You can download the final project from this tutorial here.

For more details on the open source projects used in this Android networking tutorial, check out the OkHttp and Retrofit pages on Square’s GitHub pages and the Volley page on the Android developer site.

You can also check out the Networking Operations page on the Android developer site.

I hope you enjoyed this tutorial; if you have any questions or comments, please join the forum discussion below!

Eunice Obugyei

Eunice is a software engineer at DreamOval, currently focusing on Mobile development.

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