这篇文章主要介绍了UDP协议格式及在java中的使用,本文给大家介绍得非常详细，对大家的学习或工作具有一定的参考借鉴价值，需要的朋友可以参考下
UDP是面向无连接的通讯协议，由于通讯不需要连接，所以可以实现广播发送。UDP通讯时不需要接收方确认，属于不可靠的传输，可能会出现丢包现象，实际应用中要求程序员编程验证。
UDP适用于DNS、视频音频等多媒体通信、广播通信（广播、多播）。例如我们常用的QQ，就是一个以UDP为主，TCP为辅的通讯协议。
UDP报文格式如下：
UDP首部有8个字节，由4个字段构成，每个字段都是两个字节，
源端口：数据发送方的端口号.目的端口：数据接收方的端口号。长度：UDP数据报的整个长度（包括首部和数据），其最小值为8（只有首部）。校验和：检测UDP数据报在传输中是否有错，有错则丢弃。
可以使用nc发送UDP数据包：
echo hello | nc -uv 127.0.0.1 9999。
用tcpdump抓取到的数据包如下（注意先运行tcpdump，然后再执行nc命令）：
# tcpdump -i lo -X udp port 9999tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes11:19:39.267912 IP localhost.45666 > localhost.distinct: UDP, length 6 0x0000: 4500 0022 5914 4000 4011 e3b4 7f00 0001 E.."Y.@.@....... 0x0010: 7f00 0001 b262 270f 000e fe21 6865 6c6c .....b'....!hell 0x0020: 6f0a o.... ...
说明：
源端口：0xb262，十进制的45666。目的端口：0x270f，十进制的9999。长度：0x000e，14个字节的报文长度。校验和：0xfe21。bio之单播
单播就是一对一通信.
服务器端代码如下：
package com.morris.udp.bio.single; import java.io.IOException;import java.net.DatagramPacket;import java.net.DatagramSocket; public class Server { public static void main(String[] args) throws IOException { DatagramSocket datagramSocket = new DatagramSocket(9999); byte[] bytes = new byte[1024]; DatagramPacket datagramPacket = new DatagramPacket(bytes, bytes.length); datagramSocket.receive(datagramPacket); System.out.println("receive from client: " + new String(bytes)); byte[] req = "hello client".getBytes(); DatagramPacket resp = new DatagramPacket(req, req.length, datagramPacket.getSocketAddress()); datagramSocket.send(resp); }}
客户端代码如下：
package com.morris.udp.bio.single; import java.io.IOException;import java.net.DatagramPacket;import java.net.DatagramSocket;import java.net.InetSocketAddress; public class Client { public static void main(String[] args) throws IOException { DatagramSocket datagramSocket = new DatagramSocket(); byte[] req = "hello server".getBytes(); DatagramPacket datagramPacket = new DatagramPacket(req, req.length, new InetSocketAddress("127.0.0.1", 9999)); datagramSocket.send(datagramPacket); datagramSocket.receive(datagramPacket); System.out.println("receive from server: " + new String(datagramPacket.getData())); }}
客户端和服务端的代码几乎一致，只不过接收和发送数据的顺序不一致，receive和send都欧式阻塞方法。
bio之广播
广播：同一网段所有主机都能接收，前提是端口要开启监听。
只需要将单播的例子中客户端发送数据的IP修改为255.255.255.255即可，具体修改如下：
`DatagramPacket datagramPacket =` `new` `DatagramPacket(req, req.length,` `new` `InetSocketAddress(``"255.255.255.255"``,` `9999``));`[](javascript:; "全选")[](javascript:; "")
bio之多播（组播）
多播数据报套接字类用于发送和接收IP多播包。MulticastSocket是一种DatagramSocket，它具有加入Internet上其他多播主机的“组”的附加功能。
多播组通过D类IP地址和标准UDP端口号指定。D类IP地址在224.0.0.0和239.255.255.255的范围内。地址224.0.0.0被保留，不应使用。
可以通过首先使用所需端口创建MulticastSocket，然后调用joinGroup(InetAddress groupAddr)方法来加入多播组。
服务器端代码如下：
package com.morris.udp.bio.multicast; import java.io.IOException;import java.net.DatagramPacket;import java.net.DatagramSocket;import java.net.InetAddress;import java.net.MulticastSocket; public class Server { public static void main(String[] args) throws IOException { InetAddress group = InetAddress.getByName("228.5.6.7"); MulticastSocket s = new MulticastSocket(6789); s.joinGroup(group); byte[] buf = new byte[1000]; DatagramPacket recv = new DatagramPacket(buf, buf.length); s.receive(recv); System.out.println("receive : " + new String(buf)); s.leaveGroup(group); }}
客户端代码如下：
package com.morris.udp.bio.multicast; import java.io.IOException;import java.net.*; public class Client { public static void main(String[] args) throws IOException { String msg = "Hello"; InetAddress group = InetAddress.getByName("228.5.6.7"); MulticastSocket s = new MulticastSocket(); s.joinGroup(group); DatagramPacket hi = new DatagramPacket(msg.getBytes(), msg.length(), group, 6789); s.send(hi); s.leaveGroup(group); }}
NIO实现单播
服务器端代码如下：
package com.morris.udp.nio; import java.io.IOException;import java.net.InetSocketAddress;import java.net.SocketAddress;import java.nio.ByteBuffer;import java.nio.channels.DatagramChannel; public class Server { public static void main(String[] args) throws IOException { DatagramChannel datagramChannel = DatagramChannel.open(); datagramChannel.bind(new InetSocketAddress(9999)); // datagramChannel.configureBlocking(false); ByteBuffer byteBuffer = ByteBuffer.allocate(128); SocketAddress receive = datagramChannel.receive(byteBuffer); byteBuffer.flip(); byte[] bytes = new byte[byteBuffer.remaining()]; byteBuffer.get(bytes); System.out.println("receive from client: " + new String(bytes)); byteBuffer.clear(); byteBuffer.put("hello client".getBytes()); datagramChannel.send(byteBuffer, receive); }}
客户端代码如下：
package com.morris.udp.nio; import java.io.IOException;import java.net.InetSocketAddress;import java.net.SocketAddress;import java.nio.ByteBuffer;import java.nio.channels.DatagramChannel; public class Client { public static void main(String[] args) throws IOException { DatagramChannel datagramChannel = DatagramChannel.open(); // datagramChannel.configureBlocking(false); String req = "hello server"; ByteBuffer byteBuffer = ByteBuffer.allocate(req.length()); byteBuffer.put(req.getBytes()); byteBuffer.flip(); datagramChannel.send(byteBuffer, new InetSocketAddress("127.0.0.1", 9999)); datagramChannel.receive(byteBuffer); byteBuffer.flip(); byte[] bytes = new byte[byteBuffer.remaining()]; byteBuffer.get(bytes); System.out.println("receive from server: " + new String(bytes)); }}
Netty实现单播
服务器端代码如下：
package com.morris.udp.netty.single; import io.netty.bootstrap.Bootstrap;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOption;import io.netty.channel.SimpleChannelInboundHandler;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.DatagramPacket;import io.netty.channel.socket.nio.NioDatagramChannel;import io.netty.util.CharsetUtil; public class Server { private static final int port = 8899; public static void main(String[] args) throws InterruptedException { NioEventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap bootstrap = new Bootstrap(); bootstrap.group(group).channel(NioDatagramChannel.class) .handler(new SimpleChannelInboundHandler<DatagramPacket>() { @Override protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception { // 接收数据 System.out.println(msg.content().toString(CharsetUtil.UTF_8)); // 发送数据 ctx.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello client", CharsetUtil.UTF_8), msg.sender())); ctx.close(); } }); bootstrap.bind(port).sync().channel().closeFuture().await(); } finally { group.shutdownGracefully(); } }}
客户端代码如下：
package com.morris.udp.netty.single; import io.netty.bootstrap.Bootstrap;import io.netty.buffer.Unpooled;import io.netty.channel.Channel;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOption;import io.netty.channel.SimpleChannelInboundHandler;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.DatagramPacket;import io.netty.channel.socket.nio.NioDatagramChannel;import io.netty.util.CharsetUtil; import java.net.InetSocketAddress; public class Client { public static void main(String[] args) throws InterruptedException { NioEventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap bootstrap = new Bootstrap(); bootstrap.group(group).channel(NioDatagramChannel.class) .handler(new SimpleChannelInboundHandler<DatagramPacket>() { @Override protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception { // 接收数据 System.out.println(msg.content().toString(CharsetUtil.UTF_8)); ctx.close(); } }); Channel channel = bootstrap.bind(0).sync().channel(); // 发送数据 channel.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello server", CharsetUtil.UTF_8), new InetSocketAddress("127.0.0.1", 8899))); if (!channel.closeFuture().await(30 * 1000)) { System.err.println("查询超时"); } } finally { group.shutdownGracefully(); } }}
Netty实现广播
只需要将netty实现的单播的客户端代码做如下修改：
1.增加option：
.option(ChannelOption.SO_BROADCAST, true)
2.将IP地址修改为广播地址255.255.255.255：
channel.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello server"
然后使用nc命令充当客户端进行连接：echo hello | nc -uv 127.0.0.1 9999。
产生的系统调用中关键信息如下：
ocket(AF_INET6, SOCK_DGRAM, IPPROTO_IP) = 4bind(4, {sa_family=AF_INET6, sin6_port=htons(9999), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = 0recvfrom(4, "hello\n", 1024, 0, {sa_family=AF_INET6, sin6_port=htons(7361), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, [28]) = 6write(1, "receive from client: hello\n\0\0\0\0\0"..., 1045) = 1045write(1, "\n", 1) sendto(4, "hello client", 12, 0, {sa_family=AF_INET6, sin6_port=htons(7361), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = 12
可见发送和接收数据确实使用了上面的系统调用，另外上面的系统调用中并没有listen函数，不需要监听端口，再次验证UDP是面向无连接的。
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