Android Wifi獲取組播

Android的Wifi，默認狀況下是不接受組播的，見：http://developer.android.com/reference/android/net/wifi/WifiManager.MulticastLock.html

默認狀況下，應用是不接收組播信息的，這樣要接收處理的報文太多，很快就會把電池用盡。要知道移動設備（特指電話一類的，平板要好得多）目前最重要的因素是電量。

要想打開組播功能，有如下幾個步驟：

• 在Manifest文件中加入：android.permission.CHANGE_WIFI_MULTICAST_STATE，這個權限
• 獲取到MulticastLock對象，這個對象不能直接實例化，要經過WifiManager間接獲得，工廠模式
• 調用MulticastLock對象的acquire方法，獲取到組播鎖
• 相應的，用完組播，爲了避免浪費電力，要調用MulticastLock的release方法釋放鎖

下面寫了個簡單示例，經過組播發現服務器。

Activity寫的比較簡單：

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中打開和釋放組播鎖。

使用組播發送報文和接收ip地址信息的工具類代碼：

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; 
    } 
}