2．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。

下面是一个示例代码，实现了多线程的UDP网络程序：

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

public class ReceiveThread implements Runnable {
    DatagramSocket socket;

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

    public void run() {
        try {
            byte[] buf = new byte[1024];
            DatagramPacket packet = new DatagramPacket(buf, buf.length);

            while (true) {
                socket.receive(packet);
                String received = new String(packet.getData(), 0, packet.getLength());
                System.out.println("Received: " + received);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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

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

    public void run() {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

            while (true) {
                String message = reader.readLine();
                byte[] buf = message.getBytes();
                DatagramPacket packet = new DatagramPacket(buf, buf.length, address, port);
                socket.send(packet);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class MultiThreadedUDP {
    public static void main(String[] args) {
        try {
            DatagramSocket socket = new DatagramSocket(12345);
            InetAddress address = InetAddress.getByName("localhost");

            Thread receiveThread = new Thread(new ReceiveThread(socket));
            Thread sendThread = new Thread(new SendThread(socket, address, 12346));

            receiveThread.start();
            sendThread.start();

            receiveThread.join();
            sendThread.join();
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在这个示例程序中，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。接收数据的线程实现了Runnable接口，重写了run()方法，通过不断调用socket.receive()方法来接收数据，并将接收到的数据打印出来。发送数据的线程同样实现了Runnable接口，重写了run()方法，通过从键盘读入数据并将其发送给指定的地址和端口。

在主函数中，我们创建了一个DatagramSocket对象，绑定到本地的12345端口，并将要发送数据的地址设置为localhost，端口设置为12346。然后创建两个线程，并分别启动它们。最后调用join()方法等待两个线程执行完成。