--需求2：客户端发送信息给服务端，服务端接收信息，然后服务端发送信息回给客户端 --需求3：循环执行客户端和服务端交流【Scanner】、当输入exit退出

实现需求2和需求3的代码如下：

客户端：

import java.io.IOException;
import java.io.PrintStream;
import java.net.Socket;
import java.util.Scanner;

public class Client {

    public static void main(String[] args) throws IOException {
        Socket client = new Socket("localhost", 8888);
        Scanner scanner = new Scanner(System.in);
        PrintStream out = new PrintStream(client.getOutputStream());
        Scanner in = new Scanner(client.getInputStream());
        boolean flag = true;
        while (flag) {
            System.out.print("请输入信息：");
            String input = scanner.nextLine();
            out.println(input);
            if ("exit".equals(input)) {
                flag = false;
                continue;
            }
            if (in.hasNext()) {
                String message = in.nextLine();
                System.out.println("服务端回复：" + message);
            }
        }
        scanner.close();
        if (client != null) {
            client.close();
        }
    }
}

服务端：

import java.io.IOException;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

public class Server {

    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(8888);
        System.out.println("等待客户端连接...");
        Socket client = server.accept();
        Scanner scanner = new Scanner(client.getInputStream());
        PrintStream out = new PrintStream(client.getOutputStream());
        boolean flag = true;
        while (flag) {
            if (scanner.hasNext()) {
                String message = scanner.nextLine();
                System.out.println("客户端发送：" + message);
                if ("exit".equals(message)) {
                    flag = false;
                    continue;
                }
                out.println("服务端收到信息：" + message);
            }
        }
        if (scanner != null) {
            scanner.close();
        }
        if (client != null) {
            client.close();
        }
        if (server != null) {
            server.close();
        }
    }
}

运行上述代码后，可以在控制台输入信息进行通信，当输入exit时，就会退出程序。