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Connecting to the Network

青乌 2023-02-16 阅读 99


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 method ​​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.
  • ​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 DownloadWebpageText().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 DownloadWebpageText extends AsyncTask { @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:

  1. When users click the button that invokes ​​myClickHandler()​​​, the app passes the specified URL to the ​​AsyncTask​​​ subclass ​​DownloadWebpageTask​​.
  2. The ​​AsyncTask​​​ method ​​doInBackground()​​​ calls the ​​downloadUrl()​​ method.
  3. The ​​downloadUrl()​​​ method takes a URL string as a parameter and uses it to create a ​​URL​​ object.
  4. The ​​URL​​​ object is used to establish an ​​HttpURLConnection​​.
  5. Once the connection has been established, the ​​HttpURLConnection​​​ object fetches the web page content as an ​​InputStream​​.
  6. The ​​InputStream​​​ is passed to the ​​readIt()​​ method, which converts the stream to a string.
  7. Finally, the ​​AsyncTask​​​'s ​​onPostExecute()​​ 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); }

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