连接到网络

1.重点

• a.要在新线程中发起HTTP请求
• b.注意先检查网络连接
• c.注意返回的结果,有的需要处理header
• d.返回的数据通常要转码

2.相关

• Transmitting Network Data Using Volley
• Optimizing Battery Life
• Transferring Data Without Draining the Battery
• Web Apps Overview
• Application Fundamentals

3.Connecting to the Network

　　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" />

4.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 usingHttpURLConnection for applications targeted at Gingerbread and higher. For more discussion of this topic, see the blog post Android's HTTP Clients.

5.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
    }
    ...
}

6.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 postMultithreading For Performance.

In the following snippet, the myClickHandler() method invokes new DownloadWebpageTask().execute(stringUrl). The DownloadWebpageTask class is a subclass ofAsyncTask. 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 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:

1. When users click the button that invokes myClickHandler(), the app passes the specified URL to theAsyncTask 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.

7.Connect and Download Data

In your thread that performs your network transactions, you can use HttpURLConnection to perform a GETand download your data. After you call connect(), you can get an InputStream of the data by callinggetInputStream().

In the following snippet, the doInBackground() method calls the method downloadUrl(). ThedownloadUrl() 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.

8.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);
}