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" />
Most network-connected Android apps use HTTP to send and receive data. The Android platform includes the {@link java.net.HttpURLConnection} client, which supports HTTPS, streaming uploads and downloads, configurable timeouts, IPv6, and connection pooling.
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 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
}
...
}
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 Multithreading For Performance.
In the following snippet, the myClickHandler() method invokes new
DownloadWebpageTask().execute(stringUrl). The
DownloadWebpageTask class is a subclass of {@link
android.os.AsyncTask}. DownloadWebpageTask implements the following
{@link android.os.AsyncTask} methods:
downloadUrl(). It passes the web page URL as a
parameter. The method downloadUrl() fetches and processes the web
page content. When it finishes, it passes back a result string.
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:
DownloadWebpageTask.downloadUrl() method. downloadUrl() method takes a URL string as a parameter
and uses it to create a {@link java.net.URL} object.readIt()
method, which converts the stream to a string.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()}.
In the following snippet, the {@link android.os.AsyncTask#doInBackground
doInBackground()} method calls the method downloadUrl(). The
downloadUrl() 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 getInputStream() to retrieve
the data as an {@link java.io.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.
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:
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 {@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:
// 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);
}