java,net提供了两个TCP通信的Socet类，分别代表客户端和服务端

构造方法

Socket(InetAddress address, int port);
Socket(InetAddress address, int port, boolean stream);
Socket(String host, int prot);
Socket(String host, int prot, boolean stream);
Socket(SocketImpl impl)
Socket(String host, int port, InetAddress localAddr, int localPort)
Socket(InetAddress address, int port, InetAddress localAddr, int localPort)
ServerSocket(int port);
ServerSocket(int port, int backlog);
ServerSocket(int port, int backlog, InetAddress bindAddr)

Client

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;
 
public class Client {
    public static final int port = 8080;   
    public static final String host = "localhost";
    public static void main(String[] args) {    
        System.out.println("Client Start...");    
        while (true) {    
            Socket socket = null;  
            try {  
                //创建一个流套接字并将其连接到指定主机上的指定端口号  
                socket = new Socket(host,port);    
 
                //读取服务器端数据    
                BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));    
                //向服务器端发送数据    
                PrintStream out = new PrintStream(socket.getOutputStream());    
                System.out.print("请输入: \t");    
                String str = new BufferedReader(new InputStreamReader(System.in)).readLine();    
                out.println(str);    
 
                String ret = input.readLine();     
                System.out.println("服务器端返回过来的是: " + ret);    
                // 如接收到 "OK" 则断开连接    
                if ("OK".equals(ret)) {    
                    System.out.println("客户端将关闭连接");    
                    Thread.sleep(500);    
                    break;    
                }    
 
                out.close();  
                input.close();  
            } catch (Exception e) {  
                System.out.println("客户端异常:" + e.getMessage());   
            } finally {  
                if (socket != null) {  
                    try {  
                        socket.close();  
                    } catch (IOException e) {  
                        socket = null;   
                        System.out.println("客户端 finally 异常:" + e.getMessage());   
                    }  
                }  
            }  
        }    
    }    
}

Server

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
 
public class Server {
    public static final int port = 8080;//监听的端口号     
 
    public static void main(String[] args) {    
        System.out.println("Server...\n");    
        Server server = new Server();    
        server.init();    
    }    
 
    public void init() {    
        try {    
            //创建一个ServerSocket，这里可以指定连接请求的队列长度  
            //new ServerSocket(port,3);意味着当队列中有3个连接请求是，如果Client再请求连接，就会被Server拒绝 
            ServerSocket serverSocket = new ServerSocket(port);    
            while (true) {    
                //从请求队列中取出一个连接
                Socket client = serverSocket.accept();    
                // 处理这次连接    
                new HandlerThread(client);    
            }    
        } catch (Exception e) {    
            System.out.println("服务器异常: " + e.getMessage());    
        }    
    }    
 
    private class HandlerThread implements Runnable {    
        private Socket socket;    
        public HandlerThread(Socket client) {    
            socket = client;    
            new Thread(this).start();    
        }    
 
        public void run() {    
            try {    
                // 读取客户端数据    
                BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));    
                String clientInputStr = input.readLine();//这里要注意和客户端输出流的写方法对应,否则会抛 EOFException  
                // 处理客户端数据    
                System.out.println("客户端发过来的内容:" + clientInputStr);    
 
                // 向客户端回复信息    
                PrintStream out = new PrintStream(socket.getOutputStream());    
                System.out.print("请输入:\t");    
                // 发送键盘输入的一行    
                String s = new BufferedReader(new InputStreamReader(System.in)).readLine();    
                out.println(s);    
 
                out.close();    
                input.close();    
            } catch (Exception e) {    
                System.out.println("服务器 run 异常: " + e.getMessage());    
            } finally {    
                if (socket != null) {    
                    try {    
                        socket.close();    
                    } catch (Exception e) {    
                        socket = null;    
                        System.out.println("服务端 finally 异常:" + e.getMessage());    
                    }    
                }    
            }   
        }    
    }    
}