DatagramSocket只允许数据报发送给指定的目标地址，而MulticastSocket可以将数据报以广播的方式发送至多个客户端。其主要思想是设置一组特殊网络地址作为多点广播地址，每个多点广播地址都被看做一个组，当客户端需要发送，接收广播消息时，加入到该组即可。
IP协议为多点广播提供了这些特殊的IP地址，这些IP地址的范围是224.0.0.0至239.255.255.255。当MulticastSocket把一个DatagramPacket发送到多点广播IP地址时，该数据将被自动广播到加入该地址的所有MulticastSocket，同时也可以设置该MulticastSocket接收自身发送的数据。
如果仅仅是用于发送数据报的MulticastSocket对象，使用默认地址，随机端口即可。但如果创建接收用的MulticastSocket对象，则该MulticastSocket对象必须指定端口，否则发送方无法确定发送数据报的目标端口。
下面通过一个简单的例子实现多点广播图片：
多点广播的工具类：
public class ComUtil
{
public static final String BROADCAST_IP = "224.2.2.2";
public static final int BOADCAST_PORT = 30000;
private static final int DATA_LEN = 100 * 1024;
//定义本程序的MulticastSocket实例
private MulticastSocket socket = null;
//定义广播的IP地址
private InetAddress broadcastAddress = null;
//定义接收网络数据的字符数组
byte[] inBuff = new byte[DATA_LEN];
//以指定字节数组创建准备接受的DatagramPacket对象
private DatagramPacket inPacket = new DatagramPacket(inBuff , inBuff.length);
//定义一个用于发送的DatagramPacket对象
private DatagramPacket outPacket = null;
private Handler handler;
//构造器，初始化资源
public ComUtil(Handler handler) throws Exception
{
this.handler = handler;
//因为该MultcastSocket对象需要接受数据，所以有指定端口
socket = new MulticastSocket(BOADCAST_PORT);
broadcastAddress = InetAddress.getByName(BROADCAST_IP);
//将该socket加入指定的多点广播地址
socket.joinGroup(broadcastAddress);
//设置本MultcastSocket发送的数据报将被送到本身
socket.setLoopbackMode(false);
//初始化发送用的DatagramSocket，它包含一个长度为0的字节数组
outPacket = new DatagramPacket(new byte[0] , 0 , broadcastAddress , BOADCAST_PORT);
new ReadBroad().start();
}
//广播消息的工具方法
public void broadCast(byte[] msg)
{
try
{
//将msg字符串转换为字节数组
byte[] buff = msg;
//设置发送用的DatagramPacket里的字节数组
outPacket.setData(buff);
//发送数据
socket.send(outPacket);
}
catch (IOException e)
{
e.printStackTrace();
}
}
//持续读取MulticastSocket的线程
class ReadBroad extends Thread
{
public void run()
{
while (true)
{
try
{
//读取Socket中的数据
socket.receive(inPacket);
Message msg = new Message();
msg.what = 0x123;
msg.obj = inBuff;
handler.sendMessage(msg);
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
}
}
MainActivity类：
public class MainActivity extends Activity
{
private Button button;
private ImageView img;
private ComUtil comUitl;
Handler handler = new Handler()
{
@Override
public void handleMessage(Message msg)
{
if (msg.what == 0x123)
{
byte[] result = (byte[]) msg.obj;
img.setImageBitmap(BitmapFactory.decodeByteArray(result , 0 , result.length));
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main_activity);
try
{
comUitl = new ComUtil(handler);
}
catch (Exception e)
{
e.printStackTrace();
}
button = (Button) findViewById(R.id.send_img_all);
img = (ImageView) findViewById(R.id.receiver_img);
button.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View view)
{
sendData();
}
});
}
private void sendData()
{
Bitmap bitmap = BitmapFactory.decodeResource(getResources() , R.drawable.wenqing2);
ByteArrayOutputStream byteArray = new ByteArrayOutputStream();
bitmap.compress(Bitmap.CompressFormat.PNG , 100 , byteArray);
bitmap.recycle();
final byte[] msg = byteArray.toByteArray();
new Thread()
{
@Override
public void run()
{
comUitl.broadCast(msg);
}
}.start();
try
{
byteArray.close();
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。
