组播在发送者和接收者之间实现点对多点网络连接。如果发送者同时给多个接收者传输相同数据，只需将相同的这份数据包发送到组播组地址，然后由组播地址再传输给接收者。

java中的组播socket用java.net.MulticastSocekt类来完成，该类继承了DataGramSocket类。三个构造函数如下：

public MulticastSocket() throws SocketException

public MulticastSocket(int port) throws SocketException

public MulticastSocket(SocketAddress bindAddress) throws SocketException

与组播通信需要几个操作：加入组播组、接收数据、发送数据、离开组播组

public void joinGroup（InetAddress address）throws IOException

public void joinGroup（SocketAddress address,NetworkInterface interface）throws IOException   //来自java1.4

public void leaveGroup（InetAddress address）throws IOException

public void leaveGroup（SocketAddress address,NetworkInterface interface）throws IOException   //来自java1.4

public void send(DatagramPacket packet，byte ttl) throws IOException

public void setLoopbackMode（boolean disable）throws SocketException   //java1.4