使用Java制作多点发送程序 2001-03-23· 林建刚 ·yesky
IP协议是Internet上所有信息的传播手段，UDP（User Datagram Protocol，用户数据报协议)数据报包装在IP包中，发送到网络上的适当机器。众所周知，大多数IP使用单点发送，即从一个主机发送一个包到另一个主机。然而，IP协议也有多点发送的能力。如果要使用多点发送，一个报纸标记有一组目标主机地址。当报纸发布时，整个小组都可以收到它。为了支持多点发送，将一定范围的IP地址分开划分。这类IP地址为D类地址，范围为224.0.0.0至239.255.255.2555.255。
IP多点发送(或多点广播)是一项相当新的技术，是对简单广播的改进。多点广播功能类似于向多个接收者发送单一信息的广播，但只发送给等待它的接收者。其思想是设置一组网络地址作为多点广播地址，其范围为225.0.0.0到239.255.255.2555.255。每个多点广播地址都被视为一个组。当用户需要从地址信息中添加到该组时。例如，225.23.32.36作为多点广播地址，可以为其股票报价系统设置地址。如果要接收股票查询报价程序，必须加入225.23.32.36组。
一、制作多点发送的基础知识
它的MulticastSocket类在使用Java进行多点发送时，是实现此功能的关键。Multicastsocket允许用户发送和接收多点发送IP的数据报告。如果要发送或接收多点广播数据，首先要创建多点广播套接字(多点广播套接字类似于数据报套接字，其实Multicastsocket是Datagramsocket的子类)。如果要发送数据报告，可以使用缺失的端口号创建多点广播套接字，也可以在构造器指定的端口号创建多点广播套接字：
public MulticastSocket（）throws IOException
public MulticastSocket（int portNumber）throws IOException
为了添加一个多点广播地址，可以使用jionGroup方法；如果你想离开一个组，使用leaveGroup方法：
public void jionGroup（InetAddress multicastAddr）throws IOException
public void leaveGroup（InetAddress multicastAddr）throws IOException
在某些系统中，可能有多个网络接口。这可能会给多点广播带来问题，因为用户需要在指定的接口上进行调查。通过调用setinterface，可以选择多点广播套接字使用的接口:
public void setInterface（InetAddress interface）throws SocketException
多点广播套接字的接口可以通过调用getinterface来查询：
public InetAddress getInterface（）throws SocketException
用户使用send方法发送多点发送的数据：
public synchronized void send（DatagramPacket packet, byte timeTolive）throws IOException
TTL值指定数据包应跨越多少个网络。当TTL为0时，指定数据包应停留在本地主机上；当TTL值为1时，指定数据包应发送到本地网络；当TTL值为32时，仅发送到本网站的网络；当TTL为64时，数据包应保留在本地区；当TTL值为128时，这意味着数据应该保留在本大洲；当TTL为255时，这意味着数据包应该发送到所有地方；如果使用缺失的send方法，TTL值为1。
二、制作多点发送应用程序
使用Java制作多点发送应用程序。其中程序1（MulticastCastSender.java）它是一个将数据发送到指定的多点发送IP地址的程序。该程序运行时使用两个参数：一个是指定的多点发送IP地址，另一个是指定侦听应用程序的UDP端口。其中，Main()方法保证了这些参数收到后，实例化为Multicastsender对象。此外，构造器使用多点发送IP地址的String对象，创建Inetaddress实例，然后在动态分配端口上创建MulticastSocket，然后构造器进入while循环，从标准输入逐步输入。该程序将每行前512个字节包装到标有地址的DatagramPacket中，并通过MulticastSocket发送数据报告。
程序Multicastsender.java的源代码如下：
import java.net.*; // Import package names used.
import java.io.*;
class MultiCastSender {
private static final byte TTL = 1;
private static final int DATAGRAM_BYTES = 1024;
private int mulcastPort;
private InetAddress mulcastIP;
private BufferedReader input;
private MulticastSocket mulcastSocket;
public static void main(String[] args) {
// This must be the same port and IP address used by the receivers.
if (args.length != 2) {
System.out.print("Usage: MultiCastSender " + " \n\t can be one of 224.x.x.x " + "- 239.x.x.x\n");
System.exit(1);
}
MultiCastSender send = new MultiCastSender(args);
System.exit(0);
}
public MultiCastSender(String[] args) {
DatagramPacket mulcastPacket; // UDP datagram.
String nextLine; // Line from STDIN.
byte[] mulcastBuffer; file:// Buffer for datagram.
byte[] lineData; // The data typed in.
int sendLength; file:// Length of line.
input = new BufferedReader(new InputStreamReader(System.in));
try {
// Create a multicasting socket.
mulcastIP = InetAddress.getByName(args[0]);
mulcastPort = Integer.parseInt(args[1]);
mulcastSocket = new MulticastSocket();
} catch(UnknownHostException excpt) {
System.err.println("Unknown address: " + excpt);
System.exit(1);
} catch(IOException excpt) {
System.err.println("Unable to obtain socket: " + excpt);
System.exit(1);
}
try {
file:// Loop and read lines from standard input.
while ((nextLine = input.readLine()) != null) {
mulcastBuffer = new byte[DATAGRAM_BYTES];
file:// If line is longer than your buffer, use the length of the buffer available.
if (nextLine.length() > mulcastBuffer.length) {
endLength = mulcastBuffer.length;
// Otherwise, use the line's length.
} else {
sendLength = nextLine.length();
}
// Convert the line of input to bytes.
lineData = nextLine.getBytes();
// Copy the data into the blank byte array file://which you will use to create the DatagramPacket.
for (int i = 0; i < sendLength; i++) {
mulcastBuffer[i] = lineData[i];
}
mulcastPacket=new DatagramPacket (mulcastBuffer, mulcastBuffer.length, mulcastIP, mulcastPort);
// Send the datagram.
try {
System.out.println("Sending:\t" + nextLine);
mulcastSocket.send(mulcastPacket,TTL);
} catch(IOException excpt) {
System.err.println("Unable to send packet: " + excpt);
}
}
} catch(IOException excpt) {
System.err.println("Failed I/O: " + excpt);
}
mulcastSocket.close(); file:// Close the socket.
}
} 程序2（MultiCastReceiver.java）发送者通过接收多点发送的数据报告来实现。该应用程序有两个参数，必须对应IP地址和激活Multicastsender的端口。Main()方法首先检查命令的参数，然后创建Multicastreceiver对象。该对象的构造器用于激活应用程序的端口，创建Inetadress和Multicastsocket。将多点发送者添加到包含在Inetaddress中的地址中，然后进入循环。该对象的构造器从一套接字中接收数据报告，并打印包含在数据报告中的数据，包括发送数据包的计算机和端口。
程序2（MultiCastReceiver）源代码如下：
import java.net.*; // Import package names used.
import java.io.*;
class MultiCastReceiver {
private static final int DATAGRAM_BYTES = 1024;
private int mulcastPort;
private InetAddress mulcastIP;
private MulticastSocket mulcastSocket;
private boolean keepReceiving = true;
public static void main(String[] args) {
// This must be the same port and IP address used by the sender.
if (args.length != 2) {
System.out.print("Usage: MultiCastReceiver \n\t can be one of " + "224.x.x.x - 239.x.x.x\n");
System.exit(1);
}
multiCastReceiver send = new MultiCastReceiver(args);
System.exit(0);
}
public MultiCastReceiver(String[] args) {
DatagramPacket mulcastPacket; // Packet to receive.
byte[] mulcastBuffer; // byte[] array buffer
InetAddress fromIP; file:// Sender address.
int fromPort; // Sender port.
String mulcastMsg; // String of message.
try {
// First, set up your receiving socket.
mulcastIP = InetAddress.getByName(args[0]);
mulcastPort = Integer.parseInt(args[1]);
mulcastSocket = new MulticastSocket(mulcastPort);
// Join the multicast group.
mulcastSocket.joinGroup(mulcastIP);
} catch(UnknownHostException excpt) {
System.err.println("Unknown address: " + excpt);
System.exit(1);
} catch(IOException excpt) {
System.err.println("Unable to obtain socket: " + excpt);
System.exit(1);
}
while (keepReceiving) {
try {
// Create a new datagram.
mulcastBuffer = new byte[DATAGRAM_BYTES];
mulcastPacket = new DatagramPacket(mulcastBuffer, mulcastBuffer.length);
// Receive the datagram.
mulcastSocket.receive(mulcastPacket);
fromIP = mulcastPacket.getAddress();
fromPort = mulcastPacket.getPort();
mulcastMsg = new String(mulcastPacket.getData());
// Print out the data.
System.out.println("Received from " + fromIP + " on port " + fromPort + ": " + mulcastMsg);
} catch(IOException excpt) {
system.err.println("Failed I/O: " + excpt);
}
}
try {
mulcastSocket.leaveGroup(mulcastIP); file:// Leave the group.
} catch(IOException excpt) {
System.err.println("Socket problem leaving group: " + excpt);
}
mulcastSocket.close(); // Close the socket.
}
public void stop() {
if (keepReceiving) {
keepReceiving = false;
}
}
}
如果要使用该应用程序，首先编译Multicastsender和Multicastreceiver，然后将Multicastreceiver传输到其他机器，以示多个参与者接收报告。最后，Java解释器操作应用程序。例如，向225.22.32.6组在端口111上发送多点发送报文，应按以下方法进行：
-/class ->java 225.2.32.6 1111
Here is just to test multicast message.
Sending: Here is just to test multicast message.
Do you received it?
Sending: Do you received it?
为了接收这些报纸，Multicastreceiver应用程序应该在一个或多个系统上运行，这些系统应该添加相同的多点发送组225.2.32.6，并在同一端口1111上侦听：
-/class ->java MultiCastReceiver 225.2.32.6 1111
Received from 225.106.36.32 on port 32911: Here is just to test multicast message.
Received from 225.106.36.32 on port 32911: Do you received it?
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