Java UDP协议

UDP（User Datagram Protocol）是无连接、不可靠的传输层协议，适用于实时通信场景。

UDP特点

UDP vs TCP

核心类

发送数据

public class UdpSender {
    public static void main(String[] args) throws IOException {
        // 创建UDP套接字（不需要指定目标）
        DatagramSocket socket = new DatagramSocket();

        // 准备数据
        String message = "Hello UDP";
        byte[] data = message.getBytes();

        // 创建数据包，指定目标地址和端口
        InetAddress address = InetAddress.getByName("localhost");
        int port = 8888;
        DatagramPacket packet = new DatagramPacket(data, data.length, address, port);

        // 发送
        socket.send(packet);
        System.out.println("已发送: " + message);

        socket.close();
    }
}

接收数据

public class UdpReceiver {
    public static void main(String[] args) throws IOException {
        // 创建UDP套接字，绑定端口
        DatagramSocket socket = new DatagramSocket(8888);
        System.out.println("UDP接收端启动，监听8888端口");

        // 准备接收缓冲区
        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

        // 接收数据（阻塞）
        socket.receive(packet);

        // 解析数据
        String message = new String(packet.getData(), 0, packet.getLength());
        InetAddress senderAddress = packet.getAddress();
        int senderPort = packet.getPort();

        System.out.println("收到: " + message);
        System.out.println("来源: " + senderAddress + ":" + senderPort);

        socket.close();
    }
}

双向通信

public class UdpChat {
    public static void main(String[] args) throws IOException {
        DatagramSocket socket = new DatagramSocket(8888);
        byte[] buffer = new byte[1024];

        // 接收线程
        new Thread(() -> {
            while (true) {
                try {
                    DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                    socket.receive(packet);
                    String msg = new String(packet.getData(), 0, packet.getLength());
                    System.out.println("收到: " + msg);
                } catch (IOException e) {
                    break;
                }
            }
        }).start();

        // 发送线程
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        InetAddress targetAddress = InetAddress.getByName("localhost");
        int targetPort = 9999;

        while (true) {
            String line = reader.readLine();
            if ("quit".equals(line)) break;

            byte[] data = line.getBytes();
            DatagramPacket packet = new DatagramPacket(data, data.length, targetAddress, targetPort);
            socket.send(packet);
        }

        socket.close();
    }
}

DatagramPacket常用方法

// 创建发送包
DatagramPacket packet = new DatagramPacket(data, data.length, address, port);

// 创建接收包
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

// 获取数据
byte[] data = packet.getData();
int length = packet.getLength();      // 实际数据长度
int offset = packet.getOffset();      // 数据偏移

// 获取来源信息
InetAddress address = packet.getAddress();
int port = packet.getPort();

// 设置目标
packet.setAddress(address);
packet.setPort(port);
packet.setData(data);
packet.setLength(length);

DatagramSocket常用方法

DatagramSocket socket = new DatagramSocket(8888);

// 基本信息
int localPort = socket.getLocalPort();
InetAddress localAddress = socket.getLocalAddress();

// 超时设置
socket.setSoTimeout(3000);  // 读超时3秒

// 缓冲区大小
socket.setSendBufferSize(64 * 1024);
socket.setReceiveBufferSize(64 * 1024);

// 广播支持
socket.setBroadcast(true);

// 状态
socket.isConnected();  // 是否已关联目标地址
socket.isClosed();     // 是否已关闭

// 绑定特定地址
socket.bind(new InetSocketAddress("192.168.1.1", 8888));

// 关闭
socket.close();

广播发送

// 广播地址：255.255.255.255 或 子网广播地址
DatagramSocket socket = new DatagramSocket();
socket.setBroadcast(true);

String message = "Broadcast Message";
byte[] data = message.getBytes();

// 广播地址
InetAddress broadcastAddress = InetAddress.getByName("255.255.255.255");
DatagramPacket packet = new DatagramPacket(data, data.length, broadcastAddress, 8888);

socket.send(packet);
socket.close();

适用场景

注意事项

UDP数据包大小有限制，超过MTU会被分片或丢弃

receive() 是阻塞方法，可设置超时

UDP不保证送达，应用层需自行实现重传（如DNS）

广播需要 setBroadcast(true)

DatagramPacket.getData() 返回整个缓冲区，需用 getLength() 获取实际长度

要点总结

1. UDP无连接、不可靠、速度快
2. 使用DatagramSocket发送/接收，DatagramPacket封装数据
3. 发送需指定目标地址和端口，接收需绑定本地端口
4. 适用于实时通信、DNS、游戏等场景
5. 数据包有边界，每次发送/接收完整消息