1.通訊過程

網路分為應用層，http、ssh、telnet就是屬於這一類，建立在傳輸層的基礎上、其實就是定義了各自的編碼解碼格式，分層如下：

2.Socket連線

上述通訊都要先在傳輸層有建立連線的基礎上才能完成，TCP通過三次握手建立連線：

Socket與http的區別可以看下這個：
https://www.cnblogs.com/meier1205/p/5971313.html
還有搜尋：http下午茶

3.JAVA socket 客戶端與服務端互發訊息

實現結構圖：

客戶端和服務端分別通過倆個執行緒來完成傳送訊息和接受訊息，此前需要等待客戶端與伺服器端建立連線，這個java程式都幫你寫好了，接收後就可以發訊息了：

3.1 服務端程式碼

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;


public  class  Server_Test extends Thread{
    ServerSocket server = null;
    Socket socket = null;
    public Server_Test(int
 
 port) {
        try {
            server = new ServerSocket(port);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    @Override
    public void run(){

        super.run();
        try{
            System.out.println("wait client connect...");
            socket = server
 
.accept();
            new sendMessThread().start();//連線並返回socket後，再啟用傳送訊息執行緒
            System.out.println(socket.getInetAddress().getHostAddress()+"SUCCESS TO CONNECT...");
            InputStream in = socket.getInputStream();
            int len = 0;
            byte[] buf = new byte[1024];
            while ((len=in.read(buf))!=-1){
                System.out.println("client saying: "+new String(buf,0,len));
            }

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


    class sendMessThread extends Thread{
        @Override
        public void run(){
            super.run();
            Scanner scanner=null;
            OutputStream out = null;
            try{
                if(socket != null){
                    scanner = new Scanner(System.in);
                    out = socket.getOutputStream();
                    String in = "";
                    do {
                        in = scanner.next();
                        out.write(("server saying: "+in).getBytes());
                        out.flush();//清空快取區的內容
                    }while (!in.equals("q"));
                    scanner.close();
                    try{
                        out.close();
                    }catch (IOException e){
                        e.printStackTrace();
                    }
                }
            }catch (IOException e) {
                e.printStackTrace();
            }

        }

    }

    //函式入口
    public static void main(String[] args) {
        Server_Test server = new Server_Test(1234);
        server.start();
    }
}

3.2 客戶端程式碼

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;

public class Client extends Thread {

    //定義一個Socket物件
    Socket socket = null;

    public Client(String host, int port) {
        try {
            //需要伺服器的IP地址和埠號，才能獲得正確的Socket物件
            socket = new Socket(host, port);
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void run() {
        //客戶端一連線就可以寫資料個伺服器了
        new sendMessThread().start();
        super.run();
        try {
            // 讀Sock裡面的資料
            InputStream s = socket.getInputStream();
            byte[] buf = new byte[1024];
            int len = 0;
            while ((len = s.read(buf)) != -1) {
                System.out.println(new String(buf, 0, len));
            }

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

    //往Socket裡面寫資料，需要新開一個執行緒
    class sendMessThread extends Thread{
        @Override
        public void run() {
            super.run();
            //寫操作
            Scanner scanner=null;
            OutputStream os= null;
            try {
                scanner=new Scanner(System.in);
                os= socket.getOutputStream();
                String in="";
                do {
                    in=scanner.next();
                    os.write((""+in).getBytes());
                    os.flush();
                } while (!in.equals("bye"));
            } catch (IOException e) {
                e.printStackTrace();
            }
            scanner.close();
            try {
                os.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
    //函式入口
    public static void main(String[] args) {
        //需要伺服器的正確的IP地址和埠號
        Client clientTest=new Client("127.0.0.1", 1234);
        clientTest.start();
    }
}

輸出：
伺服器端：

wait client connect...
127.0.0.1SUCCESS TO CONNECT...
client saying: hi
i am server #服務端說得話
hi,client #服務端說得話
client saying: hi,server

客戶端：

hi #客戶端說得話
server saying: i
server saying: amserver saying: server
server saying: hi,client
hi,server #客戶端說得話