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网络程序的示例代码：

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

public class ReceiveThread implements Runnable {
    private DatagramSocket socket;

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

    @Override
    public void run() {
        byte[] buffer = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
        System.out.println("接收线程已经启动，等待接收数据...");
        try {
            while (true) {
                socket.receive(packet);
                String message = new String(packet.getData(), 0, packet.getLength());
                System.out.println("收到来自" + packet.getAddress().getHostAddress() + "的数据：" + message);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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

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

    @Override
    public void run() {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
            while (true) {
                String message = reader.readLine();
                byte[] buffer = message.getBytes();
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length, address, port);
                socket.send(packet);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class Main {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(8000);
        InetAddress address = InetAddress.getByName("localhost");
        int port = 9000;

        Thread receiveThread = new Thread(new ReceiveThread(socket));
        receiveThread.start();

        Thread sendThread = new Thread(new SendThread(socket, address, port));
        sendThread.start();
    }
}

在该程序中，ReceiveThread和SendThread都实现了Runnable接口，并重写了run()方法。ReceiveThread用于接收UDP数据报文，每收到一个数据报文，就在控制台输出收到的消息。SendThread用于发送UDP数据报文，通过键盘输入消息，并将消息发送给接收方。

Main类中创建了一个DatagramSocket对象，该对象绑定在本地端口8000上，并将接收方的IP地址和端口号设置为localhost和9000。然后创建了两个线程，一个用于接收数据，另一个用于发送数据。最后，启动两个线程并等待它们完成。