| 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/volley/index.html">Transmitting Network Data Using Volley</a></li> |
| <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><uses-permission android:name="android.permission.INTERNET" /> |
| <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /></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 && 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; |
| |
| @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); |
| } |
| } |
| ... |
| }</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> |
| |
| |
| |