执行网络操作（一）：连接到网络

- 执行网络操作需要的权限：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

选择HTTP客户端

-  HttpURLConnection：2.3+推荐使用

- Apache HttpClient：太大

- 都支持HTTPS，流上传和下载，配置超时，IPv6，连接池

- 参看：   Android's HTTP Clients

检查网络连接

- 设备可能在网络覆盖范围之外

- Wifi和移动网络连接可能都被关闭

- 关键接口：

>  getActiveNetworkInfo ()

>  isConnected ()

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

    }

    ...
}

- ConnectivityManager：用来查询网络连接状态的组件。当网络连接变动时，它也通知应用程序。主要职责：

  * 监视网络连接（Wi-Fi, GPRS, UMTS等）

  * 网络连接变化时发送广播意图

  * 当连接丢失时尝试转向另一个连接

  * 提供API以允许应用程序插叙可用网络的粗粒度或细粒度状态

单开一个线程执行网络操作

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);

       }

    }

    ...
}

连接并下载数据

- 用HttpURLConnection执行GET操作以下载数据

- 调用connect()方法之后，通过getInputStream()可以获得一个输入流

// 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 );

        conn.setConnectTimeout(15000 );

        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();

        } 

    }
}

- 注意，调用 getResponseCode()返回可以返回连接的状态码   status code，以此获取连接附加信息

- 200表示成功

将输入流转化为字符串

- 输入流由字节构成

- 拿到输入流之后，通常进行解码，或转化为目标数据类型

- 位图例子：

InputStream is = null;
...
Bitmap bitmap = BitmapFactory.decodeStream(is);
ImageView imageView = (ImageView) findViewById(R.id.image_view);

imageView.setImageBitmap(bitmap);

- 转换一个网页文本：

// 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);
}