basics/network-ops/connecting.jd

page.title=Connecting to the Network
parent.title=Performing Network Operations
parent.link=index.html

trainingnavtop=true
next.title=Managing Network Usage
next.link=managing.html

@jd:body

<div id="tb-wrapper">
<div id="tb">


<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#http-client">Choose an HTTP Client</a></li>
  <li><a href="#connection">Check the Network Connection</a></li>
  <li><a href="#AsyncTask">Perform Network Operations on a Separate Thread</a></li>
  <li><a href="#download">Connect and Download Data</a></li>
  <li><a href="#stream">Convert the InputStream to a String</a></li>

</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}training/monitoring-device-state/index.html">Optimizing Battery Life</a></li>
  <li><a href="{@docRoot}training/efficient-downloads/index.html">Transferring Data Without Draining the Battery</a></li>
  <li><a href="{@docRoot}guide/webapps/index.html">Web Apps Overview</a></li>
  <li><a href="{@docRoot}guide/components/fundamentals.html">Application Fundamentals</a></li>
</ul>

</div>
</div>

<p>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.</p>

<p>Note that to perform the network operations described in this lesson, your
application manifest must include the following permissions:</p>

<pre>&lt;uses-permission android:name=&quot;android.permission.INTERNET&quot; /&gt;
&lt;uses-permission android:name=&quot;android.permission.ACCESS_NETWORK_STATE&quot; /&gt;</pre>


<h2 id="http-client">Choose an HTTP Client</h2>

<p>Most network-connected Android apps  use HTTP to send and receive  data.
Android includes two HTTP clients: {@link java.net.HttpURLConnection} and Apache
 {@link org.apache.http.client.HttpClient}. Both support HTTPS, streaming uploads and downloads,  configurable
timeouts, IPv6, and connection pooling. We recommend using {@link
java.net.HttpURLConnection} for applications targeted at Gingerbread and higher. For
more discussion of this topic, see the blog post <a
href="http://android-developers.blogspot.com/2011/09/androids-http-clients.html"
>Android's HTTP Clients</a>.</p>

<h2 id="connection">Check the Network Connection</h2>

<p>Before your app attempts to connect to the network, it should check to see whether a
network connection is available using
{@link android.net.ConnectivityManager#getActiveNetworkInfo getActiveNetworkInfo()}
and {@link android.net.NetworkInfo#isConnected 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 <a
href="{@docRoot}training/basics/network-ops/managing.html">Managing Network
Usage</a>.</p>

<pre>
public void myClickHandler(View view) {
    ...
    ConnectivityManager connMgr = (ConnectivityManager)
        getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
    if (networkInfo != null &amp;&amp; networkInfo.isConnected()) {
        // fetch data
    } else {
        // display error
    }
    ...
}</pre>

<h2 id="AsyncTask">Perform Network Operations on a Separate Thread</h2>

<p>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 {@link android.os.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 <a
href="http://android-developers.blogspot.com/2010/07/multithreading-for-
performance.html">Multithreading For Performance</a>.</p>


<p>In the following snippet, the <code>myClickHandler()</code> method invokes <code>new
DownloadWebpageTask().execute(stringUrl)</code>. The
<code>DownloadWebpageTask</code> class is a subclass of {@link
android.os.AsyncTask}. <code>DownloadWebpageTask</code> implements the following
{@link android.os.AsyncTask} methods:</p>

    <ul>

      <li>{@link android.os.AsyncTask#doInBackground doInBackground()} executes
the method <code>downloadUrl()</code>. It passes the  web page URL as a
parameter. The method <code>downloadUrl()</code> fetches and processes the web
page content. When it finishes, it passes back a result string.</li>

      <li>{@link android.os.AsyncTask#onPostExecute onPostExecute()} takes the
returned string and displays it in the UI.</li>


    </ul>

<pre>
public class HttpExampleActivity extends Activity {
    private static final String DEBUG_TAG = "HttpExample";
    private EditText urlText;
    private TextView textView;

    &#64;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 &amp;&amp; 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&lt;String, Void, String&gt; {
        &#64;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.
        &#64;Override
        protected void onPostExecute(String result) {
            textView.setText(result);
       }
    }
    ...
}</pre>

<p>The sequence of events in this snippet is as follows:</p>
<ol>

  <li>When users click the button that invokes {@code myClickHandler()},
  the app passes
the specified URL to the {@link android.os.AsyncTask} subclass
<code>DownloadWebpageTask</code>.</li>

 <li>The {@link android.os.AsyncTask} method {@link
android.os.AsyncTask#doInBackground doInBackground()} calls the
<code>downloadUrl()</code> method. </li>

  <li>The <code>downloadUrl()</code> method takes a URL string as a parameter
and uses it to create a {@link java.net.URL} object.</li>

  <li>The {@link java.net.URL} object is used to establish an {@link
java.net.HttpURLConnection}.</li>

  <li>Once the connection has been established, the {@link
java.net.HttpURLConnection} object fetches the web page content as an {@link
java.io.InputStream}.</li>

  <li>The {@link java.io.InputStream} is passed to the <code>readIt()</code>
method, which converts the stream to a string.</li>

  <li>Finally, the {@link android.os.AsyncTask}'s {@link
android.os.AsyncTask#onPostExecute onPostExecute()} method displays the string
in the main activity's UI.</li>

</ol>

 <h2 id="download">Connect and Download Data</h2>

 <p>In your thread that performs your network transactions, you can use
 {@link java.net.HttpURLConnection} to perform a {@code GET} and download your data.
 After you call {@code connect()}, you can get an {@link java.io.InputStream} of the data
 by calling {@code getInputStream()}.

 <p>In the following snippet, the {@link android.os.AsyncTask#doInBackground
doInBackground()} method calls the method <code>downloadUrl()</code>. The
<code>downloadUrl()</code> method takes the given URL and uses it to connect to
the network via {@link java.net.HttpURLConnection}. Once a connection has been
established, the app uses the method <code>getInputStream()</code> to retrieve
the data as an {@link java.io.InputStream}.</p>

<pre>
// 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();
        }
    }
}</pre>

<p>Note that the method <code>getResponseCode()</code> returns the connection's
<a href="http://www.w3.org/Protocols/HTTP/HTRESP.html">status code</a>. This is
a useful way of getting additional information about the connection. A status
code of 200 indicates success.</p>

<h2 id="stream">Convert the InputStream to a String</h2>

<p>An {@link java.io.InputStream} is a readable source of bytes. Once you get an {@link java.io.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:</p>

<pre>InputStream is = null;
...
Bitmap bitmap = BitmapFactory.decodeStream(is);
ImageView imageView = (ImageView) findViewById(R.id.image_view);
imageView.setImageBitmap(bitmap);
</pre>

<p>In the example shown above, the {@link java.io.InputStream} represents the text of a
web page. This is how the example converts the {@link java.io.InputStream} to
a string so that the activity can display it in the UI:</p>

<pre>// 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);
}</pre>