1 Overview

Computer network:

• It refers to a computer system that connects multiple computers and their external devices with independent functions in different geographical locations through communication lines and realizes resource sharing and information transmission under the management and coordination of network operating system, network management software and network communication protocol

Network programming

• Under the network communication protocol, data can be exchanged between programs running on different computers to realize network interconnection

Three elements of network programming

1. IP address
   In order for computers in the network to communicate with each other, each computer must be assigned an identification number, which specifies the computer to receive data and identifies the computer to send data, and the lP address is this identification number. That is, the identification of the equipment

2. port
   If the IP address can uniquely identify the device in the network, the port number can uniquely identify the application in the device. That is, the identity of the application

3. agreement
   The rules of connection and communication are called network communication protocol. It makes unified provisions on data transmission format, transmission rate and transmission steps. Both sides of communication must abide by them at the same time to complete data exchange. The common protocols are UDP and TCP

2 InetAddress (ip)

This class represents an ip address

port

Port: the unique identification of the application on the device

Port number: an integer represented by two bytes. Its value range is 0 ~ 65535. Among them, the port number between 0 and 1023 is used for some well-known network services and applications. Ordinary applications need to use a port number of more than 1024. If the port number is occupied by another service or application, the current program will fail to start

3 agreement

Protocol: in computer networks, the rules of connection and communication are called network communication protocol

UDP

No connection, unreliable transmission

TCP

Connection oriented, reliable and error free
Three handshakes

4 UDP communication

UDP protocol is an unreliable network protocol. It establishes a Socket object at both ends of the communication, but the two sockets are only objects for sending and receiving data

Both sides of communication have no client and server

Java provides DatagramSocket class as a Socket based on UDP protocol

To send data using UDP

1. Create the Socket object (DatagranSocket) of the sender

Datagram socket() constructs a datagram socket and binds it to any available port on the local host

2 create data and package the data

Create data and package it
DatagramPacket (byte[ ] buf， int length，InetAddress oddress， int port)

3. Call the method of DatagramSocket object to send data

Call the method of DatagramSocket object to send data
Void send (datagram packet P) sends datagram packets from this socket

4 close the sender

Close sender
void close close this datagram socket

package com.yy;

import org.junit.Test;

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPTest {

    @Test
    public void SendTest() throws Exception {
        //Datagram socket() constructs a datagram socket and binds it to any available port on the local host
        DatagramSocket ds = new DatagramSocket();
        //Create data and package it
        //DatagramPacket (byte[ ] buf， int length，InetAddress oddress， int port)
        //Construct a packet and send the packet with length to the specified port number on the specified host.
        byte[] bytes = "upd，I'm coming.".getBytes();
        DatagramPacket dp = new DatagramPacket(bytes,bytes.length, InetAddress.getByName("192.168.20.220"),10086);

        //Call the method of DatagramSocket object to send data
        //Void send (datagram packet P) sends datagram packets from this socket
        ds.send(dp);
        //Close sender
        //void close close this datagram socket
        ds.close();
    }
}

UDP receive data

To receive data

1. Create a Socket object (datagram Socket) at the receiving end
   Datagram socket (int port) constructs a datagram socket and binds it to the specified port on the local host
2. Create a packet to receive data
   Datagram packet (byte [J buf, int length) constructs a datagram packet for receiving data packets with length
3. Call the method of DatagramSocket object to receive data
   Call the method of DatagramSocket object to receive data
4. Parse the data package and display the data on the console
   byte[] getData() return data buffer
   int getLength() returns the length of data to be sent or received
5. Close the receiver

@Test
    public void sdTest() throws Exception {
        //1. Create the Socket object (DatagramSocket) of the receiving end
        //Datagram socket (int port) constructs a datagram socket and binds it to the specified port on the local host
        DatagramSocket ds = new DatagramSocket(10086);
        //2. Create a packet for receiving data
        //Datagram packet (byte [J buf, int length) constructs a datagram packet for receiving data packets with length
        byte[] bytes = new byte[1024];
        DatagramPacket dp = new DatagramPacket(bytes,bytes.length);
        //3. Call the method of DatagramSocket object to receive data
        ds.receive(dp);
        //4. Parse the data package and display the data on the console
        // byte[] getData() return data buffer
        byte[] datas  =dp.getData();
        //int getLength() returns the length of data to be sent or received
        int length = dp.getLength();
        String dataString = new String(datas,0,length);
        System.out.println("date is : "+dataString);
        //5. Close the receiver
        ds.close();
    }

When running this method, execute the above sending method and how to obtain the above results, as shown in the following figure

5 TCP communication

TCP communication protocol is a reliable network protocol. It establishes a Socket object at both ends of the communication, so as to form a network virtual chain at both ends of the communication. Once a virtual network link is established, the programs at both ends can communicate through the virtual link

Java provides a good package for the network based on TCP protocol, uses the Socket object to represent the communication ports at both ends, and generates IO flow through the Socket for network communication

Java provides a Socket class for the client and a ServerSocket class for the server

TCP send data

To send data via TCP

1. Create the client's Socket object (Socket)
   Socket (string host, int port) creates a stream socket and connects it to the specified port number on the specified host
2. Get output stream and write data
   outputStream getoutputStream() returns the output stream of this socket
3. Release resources

@Test
    public void SendTest() throws Exception {
        //1. Create the client's Socket object (Socket)
        //Socket (InetAddress, int port) creates a stream socket and connects it to the specified port number of the specified IP address
//        InetAddress byName1 = InetAddress.getByName("www.baidu.com");
//        InetAddress byName2 = InetAddress.getByName("192.168.20.220");
//        InetAddress byName3 = InetAddress.getByName("localhost");
//        System.out.println(byName1+"-----"+byName2+"-----------"+byName3);//www.baidu.com/110.242.68.3-----/192.168.20.220-----------localhost/127.0.0.1

        //Socket (String host,int port) creates a stream socket and connects it to the specified port number on the specified host
        Socket s = new Socket("192.168.20.220",10000);

        //2. Obtain the output stream and write data
        //outputStream getoutputStream() returns the output stream of this socket
        OutputStream os = s.getOutputStream();
        os.write("tcp,I am coming".getBytes());

        //3. Release resources
        os.close();

    }

TCP receive data

Steps for TCP to receive data

1. Create a server-side Socket object (ServerSocket)
   ServerSocket (int port) creates a server socket bound to a specified port

2. Listen to the client connection and return a Socket object
   Socket accept() listens for the socket to connect to and accept it

3. Get the input stream, read the data, and display the data on the console
   InputStream getInputStream()

4. Release resources
   void close()

@Test
    public void sdTest() throws Exception {
        //1. Create a server-side Socket object (ServerSocket)
        //ServerSocket (int port) creates a server socket bound to a specified port
        ServerSocket ss = new ServerSocket(10086);
        //Socket accept() listens for the socket to connect to and accept it
        Socket accept = ss.accept();

        //2. Obtain the input stream, read the data, and display the data on the console
        InputStream is = accept.getInputStream();
        byte[] bytes = new byte[1024];
        int len = is.read();
        String data = new String(bytes,0,len);
        System.out.println("data is : "+ data);
        //3. Release resources
        ss.close();
        accept.close();
    }