This lesson teaches you to
- Choose an HTTP Client
- Check the Network Connection
- Perform Network Operations on a Separate Thread
- Connect and Download Data
- Convert the InputStream to a String
You should also read
This lesson shows you how to implement a simple application that connects to the network. It explains some of the best practices you should follow in creating even the simplest network-connected app.
Note that to perform the network operations described in this lesson, your application manifest must include the following permissions:
<uses-permission android:name="android.permission.INTERNET" /> <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
Choose an HTTP Client
Most network-connected Android apps use HTTP to send and receive data.
Android includes two HTTP clients: HttpURLConnection
and Apache
HttpClient
. Both support HTTPS, streaming uploads and downloads, configurable
timeouts, IPv6, and connection pooling. We recommend using HttpURLConnection
for applications targeted at Gingerbread and higher. For
more discussion of this topic, see the blog post Android's HTTP Clients.
Check the Network Connection
Before your app attempts to connect to the network, it should check to see whether a
network connection is available using
getActiveNetworkInfo()
and isConnected()
.
Remember, the device may be out of range of a
network, or the user may have disabled both Wi-Fi and mobile data access.
For more discussion of this topic, see the lesson Managing Network
Usage.
public void myClickHandler(View view) { ... ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); NetworkInfo networkInfo = connMgr.getActiveNetworkInfo(); if (networkInfo != null && networkInfo.isConnected()) { // fetch data } else { // display error } ... }
Perform Network Operations on a Separate Thread
Network operations can involve unpredictable delays. To prevent this from
causing a poor user experience, always perform network operations on a separate
thread from the UI. The AsyncTask
class provides one of the
simplest ways to fire off a new task from the UI thread. For more discussion of
this topic, see the blog post Multithreading For Performance.
In the following snippet, the myClickHandler()
method invokes new
DownloadWebpageTask().execute(stringUrl)
. The
DownloadWebpageTask
class is a subclass of AsyncTask
. DownloadWebpageTask
implements the following
AsyncTask
methods:
doInBackground()
executes the methoddownloadUrl()
. It passes the web page URL as a parameter. The methoddownloadUrl()
fetches and processes the web page content. When it finishes, it passes back a result string.onPostExecute()
takes the returned string and displays it in the UI.
public class HttpExampleActivity extends Activity { private static final String DEBUG_TAG = "HttpExample"; private EditText urlText; private TextView textView; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); urlText = (EditText) findViewById(R.id.myUrl); textView = (TextView) findViewById(R.id.myText); } // When user clicks button, calls AsyncTask. // Before attempting to fetch the URL, makes sure that there is a network connection. public void myClickHandler(View view) { // Gets the URL from the UI's text field. String stringUrl = urlText.getText().toString(); ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); NetworkInfo networkInfo = connMgr.getActiveNetworkInfo(); if (networkInfo != null && networkInfo.isConnected()) { new DownloadWebpageTask().execute(stringUrl); } else { textView.setText("No network connection available."); } } // Uses AsyncTask to create a task away from the main UI thread. This task takes a // URL string and uses it to create an HttpUrlConnection. Once the connection // has been established, the AsyncTask downloads the contents of the webpage as // an InputStream. Finally, the InputStream is converted into a string, which is // displayed in the UI by the AsyncTask's onPostExecute method. private class DownloadWebpageTask extends AsyncTask<String, Void, String> { @Override protected String doInBackground(String... urls) { // params comes from the execute() call: params[0] is the url. try { return downloadUrl(urls[0]); } catch (IOException e) { return "Unable to retrieve web page. URL may be invalid."; } } // onPostExecute displays the results of the AsyncTask. @Override protected void onPostExecute(String result) { textView.setText(result); } } ... }
The sequence of events in this snippet is as follows:
- When users click the button that invokes
myClickHandler()
, the app passes the specified URL to theAsyncTask
subclassDownloadWebpageTask
. - The
AsyncTask
methoddoInBackground()
calls thedownloadUrl()
method. - The
downloadUrl()
method takes a URL string as a parameter and uses it to create aURL
object. - The
URL
object is used to establish anHttpURLConnection
. - Once the connection has been established, the
HttpURLConnection
object fetches the web page content as anInputStream
. - The
InputStream
is passed to thereadIt()
method, which converts the stream to a string. - Finally, the
AsyncTask
'sonPostExecute()
method displays the string in the main activity's UI.
Connect and Download Data
In your thread that performs your network transactions, you can use
HttpURLConnection
to perform a GET
and download your data.
After you call connect()
, you can get an InputStream
of the data
by calling getInputStream()
.
In the following snippet, the doInBackground()
method calls the method downloadUrl()
. The
downloadUrl()
method takes the given URL and uses it to connect to
the network via HttpURLConnection
. Once a connection has been
established, the app uses the method getInputStream()
to retrieve
the data as an InputStream
.
// Given a URL, establishes an HttpUrlConnection and retrieves // the web page content as a InputStream, which it returns as // a string. private String downloadUrl(String myurl) throws IOException { InputStream is = null; // Only display the first 500 characters of the retrieved // web page content. int len = 500; try { URL url = new URL(myurl); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setReadTimeout(10000 /* milliseconds */); conn.setConnectTimeout(15000 /* milliseconds */); conn.setRequestMethod("GET"); conn.setDoInput(true); // Starts the query conn.connect(); int response = conn.getResponseCode(); Log.d(DEBUG_TAG, "The response is: " + response); is = conn.getInputStream(); // Convert the InputStream into a string String contentAsString = readIt(is, len); return contentAsString; // Makes sure that the InputStream is closed after the app is // finished using it. } finally { if (is != null) { is.close(); } } }
Note that the method getResponseCode()
returns the connection's
status code. This is
a useful way of getting additional information about the connection. A status
code of 200 indicates success.
Convert the InputStream to a String
An InputStream
is a readable source of bytes. Once you get an InputStream
,
it's common to decode or convert it into a
target data type. For example, if you were downloading image data, you might
decode and display it like this:
InputStream is = null; ... Bitmap bitmap = BitmapFactory.decodeStream(is); ImageView imageView = (ImageView) findViewById(R.id.image_view); imageView.setImageBitmap(bitmap);
In the example shown above, the InputStream
represents the text of a
web page. This is how the example converts the InputStream
to
a string so that the activity can display it in the UI:
// Reads an InputStream and converts it to a String. public String readIt(InputStream stream, int len) throws IOException, UnsupportedEncodingException { Reader reader = null; reader = new InputStreamReader(stream, "UTF-8"); char[] buffer = new char[len]; reader.read(buffer); return new String(buffer); }