单线程版本：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

//服务端
public class Server {
    public static void main(String[] args) {
        try {
            //Socket套接字(TCP/IP)
            //设置端口号
            ServerSocket server = new ServerSocket(6666);
            Socket s = server.accept();
            PrintWriter out = new PrintWriter(s.getOutputStream());
            BufferedReader br = new BufferedReader(new
                    InputStreamReader(s.getInputStream()));
            System.out.println("已连接");
            while (true) {

                String sss = br.readLine();
                System.out.println(sss);
                System.out.print("请回复消息：");
                out.write((new Scanner(System.in)).nextLine()+"\r\n");
                out.flush();
            }
        } catch (Exception e) {
            System.out.println(e.toString());
        }
    }
}

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
//客户端
public class Client {
    public static void main(String[] args) {
        try {
            //输入服务端的ip地址和端口号
            //ip地址可以用控制台ipconfig命令查询
            Socket s = new Socket("192.168.21.28", 6666);
            BufferedReader br = new BufferedReader(new
                    InputStreamReader(s.getInputStream()));
            PrintWriter out = new PrintWriter(s.getOutputStream());
            while (true) {
                System.out.print("请输入：");
                String sss = (new Scanner(System.in)).nextLine();
                out.write(sss+"\r\n");
                out.flush();
                System.out.println(br.readLine());
            }
        } catch (Exception e) {
            System.out.println(e.toString());
        }
    }
}

A发完1条消息后，B回复1条后，A才能继续发送1条消息，B也同理

 多线程版本：Server、Client、Read接收类、Send发送类

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.StandardCharsets;
import java.util.List;
import java.util.Scanner;

/**
 * Server.java 是服务端类，还有Read线程类，Send线程类
 */
public class Server {
    public static void main(String[] args) {
        try {
            ServerSocket server = new ServerSocket(6688);
            //accept()将会有一个中断等待
            Socket a = server.accept();
            System.out.println("有人连线");
            new Thread(new Read(a)).start();
            new Thread(new Send(a, "服务器")).start();
        } catch (IOException e) {
            System.out.println("服务器启动失败，端口6688被占用");
        }
    }
}

class Read implements Runnable {
    Socket ss = null;

    public Read(Socket ss) {
        this.ss = ss;
    }

    @Override
    public void run() {
        BufferedReader br = null;
        try {
            br = new BufferedReader(new
                    InputStreamReader(ss.getInputStream()));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        while (true) {
            String sss = null;
            try {
                sss = br.readLine();
                System.out.println(sss);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

class Send implements Runnable {
    String name = "";
    Socket ss = null;

    public Send(Socket ss, String name) {
        this.ss = ss;
        this.name = name;
    }

    @Override
    public void run() {
        PrintWriter out = null;
        try {
            out = new PrintWriter(ss.getOutputStream());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        while (true) {
            out.printf("%s:%s\r\n", name, new Scanner(System.in).nextLine());
            out.flush();
        }
    }
}

import java.net.Socket;

public class Client {
    public static void main(String[] args) {
        try {
            Socket c = new Socket("192.168.21.28", 6688);
            new Thread(new Read(c)).start();
            new Thread(new Send(c, "客户")).start();
        } catch (Exception e) {
        }
    }
}

连接后可以随时发送和接收