android MulticastLock

转载出处：http://blog.csdn.net/hknock/article/details/44244031

MulticastSocket API说明http://www.apihome.cn/api/java/MulticastSocket.html

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;
     }
 } 
      转自： http://marshal.easymorse.com/archives/4461