编写多线程的UDP网络程序： 编写数据接收类ReceiveThread，该类实现了Runnable接口，重写run()方法实现不断接受客户端发送数据的功能；编写数据发送类SendThread，该类同样实现了SendThread接口，重写run()方法实现通过键盘录入数据，将数据向接收端发送的功能。

下面是一个简单的多线程UDP网络程序实现：

import java.io.IOException;
import java.net.*;

public class ReceiveThread implements Runnable {
    DatagramSocket ds;

    public ReceiveThread(DatagramSocket ds) {
        this.ds = ds;
    }

    @Override
    public void run() {
        try {
            while (true) {
                byte[] buf = new byte[1024];
                DatagramPacket dp = new DatagramPacket(buf, buf.length);
                ds.receive(dp);
                String str = new String(dp.getData(), 0, dp.getLength());
                System.out.println("接收到数据：" + str);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class SendThread implements Runnable {
    DatagramSocket ds;
    InetAddress address;
    int port;

    public SendThread(DatagramSocket ds, InetAddress address, int port) {
        this.ds = ds;
        this.address = address;
        this.port = port;
    }

    @Override
    public void run() {
        try {
            while (true) {
                byte[] buf = new byte[1024];
                System.in.read(buf);
                DatagramPacket dp = new DatagramPacket(buf, buf.length, address, port);
                ds.send(dp);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class UDPMultiThread {
    public static void main(String[] args) throws Exception {
        DatagramSocket ds = new DatagramSocket(8888);
        InetAddress address = InetAddress.getByName("127.0.0.1");
        int port = 9999;

        Thread t1 = new Thread(new ReceiveThread(ds));
        Thread t2 = new Thread(new SendThread(ds, address, port));

        t1.start();
        t2.start();
    }
}

在这个例子中，我们创建了两个线程，一个用于接收数据，一个用于发送数据。ReceiveThread类实现了Runnable接口，并覆盖了run()方法，该方法通过DatagramSocket对象接收数据包并将其打印到控制台。SendThread类同样实现了Runnable接口，并覆盖了run()方法，该方法从键盘读取数据并将其发送到指定的地址和端口。UDPMultiThread类是程序的入口，创建了DatagramSocket对象、接收方地址和端口，并启动了两个线程来接收和发送数据。