模拟echo服务器：

package com.test;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class EchoServer {
    private static final int SERVER_SOCKET_PORT = 6378;

    public static void main(String[] args) throws Exception {
        ServerSocket serverSocket = new ServerSocket(SERVER_SOCKET_PORT);
        System.out.println("Echo server is started.");
        while (true) {
            Socket client = serverSocket.accept();
            new Thread(new ClientHandler(client)).start();
        }
    }

    private static class ClientHandler implements Runnable {
        private Socket client;

        public ClientHandler(Socket client) {
            this.client = client;
        }

        @Override
        public void run() {
            try (BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
                    PrintWriter pw = new PrintWriter(client.getOutputStream())) {
                String msg = br.readLine();
                pw.println("已收到 " + client.getInetAddress() + " 的信息：" + msg);
                pw.flush();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    client.close();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }

    }
}

模拟客户端:

package com.test;

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

public class EchoClient {
    public static void main(String[] args) throws Exception {
        Socket client = new Socket("192.168.0.104", 6378);
        try (BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
                PrintWriter pw = new PrintWriter(client.getOutputStream())) {
            System.out.println("Please enter your words:");
            Scanner scanner = new Scanner(System.in);
            String msg = scanner.nextLine();
            pw.println(msg);
            pw.flush();
            String echo = br.readLine();
            System.out.println("Echo message from server:  " + echo);
        }
    }
}