这是在一本java教材上看到的，做了点修改。具体本教材记不清楚了，Sorry.放在这里，只是让自己需要的时候能够容易找到。

     程序分两部分，服务端和客户端。先把服务端运行起来，在运行客户端。整个过程就是客户端想服务端发送String,服务端再返回。客户端的String由键盘输入得到。

服务器端。

package net.tcp;

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

public class Server extends Thread
{

    ServerSocket server = null;
    Socket sk = null;
    BufferedReader rdr = null;
    PrintWriter wtr = null;

    public Server()
    {
        try
        {
            server = new ServerSocket(1987);
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }

    public void run()
    {

        while (true)
            {
                System.out.println("Listenning...");
                try
                {
//                  每个请求交给一个线程去处理
                    sk = server.accept();
                    ServerThread th = new ServerThread(sk);
                    th.start();
                    sleep(1000);
                }
                catch (Exception e)
                {
                    e.printStackTrace();
                }

            }
    }

    public static void main(String [] args)
    {
        new Server().start();
    }






    class ServerThread extends Thread
    {

        Socket sk = null;
        public ServerThread(Socket sk)
        {
            this.sk = sk;
        }
        public void run()
        {
            try
            {
                wtr = new PrintWriter(sk.getOutputStream());
                rdr = new BufferedReader(new InputStreamReader(sk
                        .getInputStream()));
                String line = rdr.readLine();
                System.out.println("从客户端来的信息：" + line);
//              特别，下面这句得加上     “\n”,
                wtr.println("你好，服务器已经收到您的信息！'" + line + "'\n");
                wtr.flush();
                System.out.println("已经返回给客户端！");
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }

        }

    }

}

客户端：

package net.tcp;

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

public class Client extends Thread
{

    Socket sk = null;
    BufferedReader reader = null;
    PrintWriter wtr = null;
    BufferedReader keyin = null;

    public Client()
    {
        keyin = new BufferedReader(new InputStreamReader(System.in));
        try
        {
            sk = new Socket("127.0.0.1", 1987);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

    }

    public void run()
    {
        try
        {
            reader = new BufferedReader(new InputStreamReader(sk
                    .getInputStream()));
            wtr = new PrintWriter(sk.getOutputStream());
            String get = keyin.readLine();

            while (true)
            {
                if (get != null & get.length() > 0)
                {
                    wtr.println(get);
                    wtr.flush();
//                  wtr.close();
//                  System.out.println(get + "发送完毕");
                }
                if (reader != null)
                {
                    String line = reader.readLine();
                    System.out.println("从服务器来的信息：" + line);

                }

            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    public static void main(String [] args)
    {
        new Client().start();
    }
}

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