Android Wifi获取组播

Android的Wifi，默认情况下是不接受组播的，见：http://developer.android.com/reference/android/net/wifi/WifiManager.MulticastLock.html

默认情况下，应用是不接收组播信息的，这样要接收处理的报文太多，很快就会把电池用尽。要知道移动设备（特指电话一类的，平板要好得多）目前最重要的因素是电量。

要想打开组播功能，有以下几个步骤：

在Manifest文件中加入：android.permission.CHANGE_WIFI_MULTICAST_STATE，这个权限
获取到MulticastLock对象，这个对象不能直接实例化，要通过WifiManager间接得到，工厂模式
调用MulticastLock对象的acquire方法，获取到组播锁
相应的，用完组播，为了不浪费电力，要调用MulticastLock的release方法释放锁
下面写了个简单示例，通过组播发现服务器。


Activity写的比较简单：

public class MulticastDemoActivity extends Activity {   
      
    MulticastLock multicastLock;   
      
    /** Called when the activity is first created. */   
    @Override   
    public void onCreate(Bundle savedInstanceState) {   
        super.onCreate(savedInstanceState);   
        setContentView(R.layout.main);   
          
        allowMulticast();   
          
        try {   
            NetUtil.findServerIpAddress();   
        } catch (IOException e) {   
            throw new RuntimeException(e);   
        }   
          
        Log.d("multicast.demo", "find ip ok.");   
          
        multicastLock.release();   
    }   
      
    private void allowMulticast(){   
        WifiManager wifiManager=(WifiManager)getSystemService(Context.WIFI_SERVICE);   
        multicastLock=wifiManager.createMulticastLock("multicast.test");   
        multicastLock.acquire();   
    }   
}  

在Activity中打开和释放组播锁。

使用组播发送报文和接收ip地址信息的工具类代码

public class NetUtil {   
      
    private static final String TAG="Net.Utils";   
    private static final int MULTICAST_PORT=5111;   
    private static final String GROUP_IP="224.5.0.7";   
    private static byte[] sendData;   
      
    static{   
        sendData=new byte[4];   
        // 0xEE78F1FB   
        sendData[3] = (byte) 0xEE;   
        sendData[2] = (byte) 0×78;   
        sendData[1] = (byte) 0xF1;   
        sendData[0] = (byte) 0xFB;   
    }   
      
    public static String findServerIpAddress() throws IOException{   
        String ip=null;  
  
        MulticastSocket multicastSocket=new MulticastSocket(MULTICAST_PORT);   
        multicastSocket.setLoopbackMode(true);   
        InetAddress group = InetAddress.getByName(GROUP_IP);   
        multicastSocket.joinGroup(group);   
          
        DatagramPacket packet=new DatagramPacket(sendData, sendData.length,group,MULTICAST_PORT);   
          
        for(;;){   
            multicastSocket.send(packet);   
            Log.d(TAG,">>>send packet ok");   
              
            byte[] receiveData=new byte[256];   
            packet=new DatagramPacket(receiveData, receiveData.length);   
            multicastSocket.receive(packet);   
              
            String packetIpAddress=packet.getAddress().toString();   
            packetIpAddress=packetIpAddress.substring(1, packetIpAddress.length());   
            Log.d(TAG,"packet ip address: "+packetIpAddress);   
              
            StringBuilder packetContent=new StringBuilder();   
            for(int i=0;i<receiveData.length;i++){   
                if(receiveData[i]==0){   
                    break;   
                }   
                packetContent.append((char)receiveData[i]);   
            }   
            ip=packetContent.toString();   
            Log.d(TAG,"packet content ip is: "+ip);   
              
            if(ip.equals(packetIpAddress)){   
                Log.d(TAG,"find server ip address: "+ip);   
                break;   
            }else{   
                Log.d(TAG,"not find server ip address, continue …");   
                try {   
                    Thread.sleep(1000);   
                } catch (InterruptedException e) {   
                }   
            }   
        }   
          
        return ip;   
    }   
}