TCP Socket programming

Introduction to computer networking

Introduction to Socket and ServerSocket

Introduction to relevant knowledge

The process of using Socket for network communication

• The server program binds a socket to a specific port, and waits for and listens to the client's connection request through this socket
• The client program sends a connection request according to the host name and port name where the server program is located
• If everything is OK, the server accepts the client connection request. And get a new socket bound to different port addresses
• The client and server communicate through write and read sockets

No matter how complete and complex a Socket communication program is, its basic structure is the same, including the following four basic steps:

1. Create Socket/ServerSocket instances on the client and server sides
2. Open the input / output stream linked to the Socket
3. Use the input / output stream to read / write to the Socket according to a certain protocol
4. Close the I / O stream and socket

The programmer's main job is to program the functions to be completed in the third step

Introduction to main Socket methods

Construction method

• public Socket(String host,int port) / / remote server IP and response port
• public Socket(InetAddress address,int port)
• public Socket(String host,int port,InetAddress localAddr,int localPort)
• Public socket (InetAddress, address, int port, InetAddress, localaddr, int localport) / / run on the specified port on the specified machine

These methods will throw an exception IOException, and the program summary needs to be captured and processed

Socket input / output stream management

• public InputStream getInputStream()
• public void shutdownInput()
• public OutputStream getOutputStream()
• public void shutdownOutput()

These methods will throw an exception IOException, which needs to be caught in the program

Close Socket

public void close() throws IOException

Set / get Socket data

• public InetAddress getInetAddress()
• public int getPort()
• public void setSoTimeout(int timeout)

These methods throw an exception SocketException, which needs to be caught in the program

Socket demo

Establish Socket – on the client side

try{
	Socket client = new Socket(host,4444);
}catch(IOException e){}

Establish Socket - on the server side

try{
	ServerSocket server = new ServerSocket(4444);
}catch(IOException e){}
Socket socket = null;
try{
	socket = server.accept();//Waiting for client connection
}catch(IOException e){}

Open input / output streams on both the client and server sides

• Class Socket provides methods getInputStream() and getOutputStream() to get the input / output stream corresponding to the Socket for data reading and writing operations. They return InputStream object and OutputStream object respectively
• In order to facilitate reading and writing data, filter streams should be established on the returned input / output stream objects, such as BufferedReader and PrintStream
• BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
• PrintStream out = new PrintStream(socket.getOutputStream()));

Close the I / O stream and Socket

Close the I / O stream and Socket on the client and server respectively: close all relevant I / O streams first, and then close the Socket

Code example

Port scan

import java.net.*;
import java.io.*;
public class ScanPort {
    public static void main(String[] args) {
        for(int i = 1;i <= 1024;i++){
            try{
                Socket s = new Socket(args[0],i);
                System.out.println(" "+i);
                s.close();
            }catch (IOException e){}
        }
    }
}

client

import java.net.*;
import java.io.*;
public class MyClientA {
    public static void main(String[] args) {
        try{
            Socket s = new Socket("127.0.0.1",5432);
            BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String x = in.readLine();
            System.out.println(x);
            in.close();
        }catch(IOException e){}
    }
}

Server side

import java.io.IOException;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.*;

public class MyServerA {
    public static void main(String[] args) {
        try{
            ServerSocket ss = new ServerSocket(5432);
            while(true){
                Socket s = ss.accept();
                PrintStream out = new PrintStream(s.getOutputStream());
                out.println("Hi");
                out.close();
                s.close();
            }
        }catch(IOException e){}
    }
}