使用Socket演示三次握手

? ? ? ? 非常重要的知识点：三次握手发生在内核态，操作系统内核去，执行握手操作。Socket连接成功后，已经完成了三次握手。以下演示的程序，只是模拟演示一下。

????????TcpClient客户端代码如下：

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketAddress;

/**
 * 〈一句话功能简述〉<br>
 * 〈使用Socket演示Tcp三次握手〉
 * <p>
 *
 * @author hanxiaozhang
 * @create 2023/5/31
 * @since 1.0.0
 */
public class TcpClient {

    /**
     * 服务器监听的端口号
     */
    private static final int PORT = 56002;
    private static final int TIMEOUT = 15000;

    public static void main(String[] args) {

        Socket client = null;
        try {

            // 调用无参构造方法
            client = new Socket();
            // 构造服务器地址结构
            SocketAddress serverAddr = new InetSocketAddress("127.0.0.1", PORT);
            // 连接服务器，超时时间是 15 毫秒
            client.connect(serverAddr, TIMEOUT);
            System.out.println("Client start: " + client.getLocalSocketAddress().toString());

            // 向服务器发送数据  client SYN
            String req = "SYN seq 543067661";
            TcpUtil.sendMsg(client, req);
            System.out.println("Client Send to server: " + req);

            // 接收服务器的数据 server SYN + ACK
            String msg = TcpUtil.receiveMsg(client);
            System.out.println("Client Recv from server: " + msg);


            // 向服务器发送数据  client ACK
            req = "ACK ack 3824468341";
            TcpUtil.sendMsg(client, req);
            System.out.println("Client Send to server: " + req);


            // ---- 通信  ----
            req = "Hello,Nice to meet you";
            TcpUtil.sendMsg(client, req);
            System.out.println("Client Send to server: " + req);

            msg = TcpUtil.receiveMsg(client);
            System.out.println("Client Recv from server: " + msg);

            req = "Bye";
            TcpUtil.sendMsg(client, req);
            System.out.println("Client Send to server: " + req);

            msg = TcpUtil.receiveMsg(client);
            System.out.println("Client Recv from server: " + msg);

        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (client != null) {
                try {
                    client.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

}

?????????TcpServer服务端代码如下：

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.TimeUnit;

/**
 * 〈一句话功能简述〉<br>
 * 〈使用Socket演示Tcp三次握手〉

 *
 * @author hanxiaozhang
 * @create 2023/5/31
 * @since 1.0.0
 */
public class TcpServer {

    private static final int PORT = 56002;

    public static void main(String[] args) {
        ServerSocket ss = null;
        try {
            // 创建一个服务器 Socket
            ss = new ServerSocket(PORT);
            // 监听新连接请求，阻塞获取
            Socket conn = ss.accept();
            System.out.println("Accept a new connection: " + conn.getRemoteSocketAddress().toString());

            // 读取客户端数据 client ACK
            String msg = TcpUtil.receiveMsg(conn);
            System.out.println("Server Recv from client: " + msg);

            // 向客户端发送数据  server ACK + SYN
            String req = "SYN seq 3824468340 ACK ack 543067662";
            TcpUtil.sendMsg(conn, req);
            System.out.println("Server Send to client: " + req);

            // 读取客户端数据 client ACK
            msg = TcpUtil.receiveMsg(conn);
            System.out.println("Server Recv from client: " + msg);

            // ---- 通信  ----
            msg = TcpUtil.receiveMsg(conn);
            System.out.println("Server Recv from client: " + msg);

            req = "Hello,Nice to meet you too";
            TcpUtil.sendMsg(conn, req);
            System.out.println("Server Send to client: " + req);

            // ---- 关闭链接 ----
            req = "see you";
            TcpUtil.sendMsg(conn, req);
            System.out.println("Server Send to client: " + req);
            TimeUnit.SECONDS.sleep(1);
            conn.close();

        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        } finally {
            if (ss != null) {
                try {
                    ss.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        System.out.println("Server exit!");
    }

}

?????????TcpUtil工具类

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.Socket;

/**
 * 〈一句话功能简述〉<br>
 * 〈〉
 *
 * @author hanxiaozhang
 * @create 2023/6/9
 * @since 1.0.0
 */
public class TcpUtil {

    /**
     * 接受消息
     *
     * @param socket
     * @return
     * @throws IOException
     */
    public static String receiveMsg(Socket socket) throws IOException {
        StringBuilder inMessage = new StringBuilder();
        BufferedInputStream in = new BufferedInputStream(socket.getInputStream());
        while (true) {
            int c = in.read();
            if (c == -1 || c == '\n') {
                break;
            }
            inMessage.append((char) c);
        }
        return inMessage.toString();
    }

    /**
     * 发送消息
     *
     * @param socket
     * @param req
     * @throws IOException
     */
    public static void sendMsg(Socket socket, String req) throws IOException {
        req = req + "\n";
        OutputStream out = new BufferedOutputStream(socket.getOutputStream());
        out.write(req.getBytes());
        // 不能忘记 flush 方法的调用
        out.flush();
    }
}