java network programming

1.1 general

Global village: you are in Xi'an, you are an American friend! How do you communicate with each other?

1.11 take a chestnut for sending letters

The address of the other party, and the other party's information is located to a specific individual. Own address, own information

With the above elements, regardless of the external environment, this letter can theoretically be sent to the other party.
What if we consider the external environment?

1.12 definition of computer network

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

1.13 purpose of network programming

Communication, information exchange, data exchange, communication (all the same meaning)

1.14 what is needed to achieve this effect?

1. How to accurately locate a host on the network
Address: port number is as follows
192.168.16.124: 8080
Locate a program on this computer

2. If you find a program on this host, how do you transfer data?
Network programming: TCP/IP protocol C/S architecture

1.2 elements of network communication

How to realize network communication?

Address of both parties:
iP
 end slogan
192.168.16.124:5900
Rules: protocols for network communication
TCP/IP

Summary:

1. There are two main problems in network programming
 how to accurately locate one or more hosts on the network
 how to communicate after finding the host

2. Elements in network programming
 IP and port number
 network communication protocol UDP,TCP

3. Everything is an object

1.3 IP address

 1.31 classes about ip addresses in Java

InetAddress

 1.32 a key address:

Local address: 127.0.0.1 and localhost

Query ip address practice code

package com.kuang.lession01;

import java.net.InetAddress;
import java.net.UnknownHostException;

//Remember that the class names are the same, otherwise an error will be reported
public class TestInetAddress {
public static void main(String[] args) {
	try {
		//Query local address
		InetAddress inetAddress1=InetAddress.getByName("127.0.0.1");
		System.out.println(inetAddress1);
		
		InetAddress inetAddress3=InetAddress.getByName("localhost");
		System.out.println(inetAddress3);	
		
		InetAddress inetAddress4=InetAddress.getLocalHost();
		System.out.println(inetAddress4);
		
		//Query network ip address
		InetAddress inetAddress2=InetAddress.getByName("www.baidu.com");//If you enter a domain name, it will return the ip address corresponding to the domain name
		System.out.println(inetAddress2);
		
		//common method
		System.out.println(inetAddress2.getAddress());//Freshman address
		System.out.println(inetAddress2.getCanonicalHostName());//Canonical name
		System.out.println(inetAddress2.getHostAddress());//ip
		System.out.println(inetAddress2.getHostName());//Domain name or computer name
		
	} catch (UnknownHostException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	}
}
}

1.33 ip address classification

Ipv4/ipv6
IPv4 consists of 127.0.0.1 and 4 bytes. 0 ~ 25.542 billion, 3 billion in North America and 400 million in Asia
IPV6:128 bit 8 unsigned integers! For example, 2001: obb2:aaaa:0015:0000:0000:1aaa:1312
 public network (Internet) – private network (LAN)
• ABCD class address
192.168.xx.xx for internal use of the organization

Little knowledge: domain names are for easy memory. Each domain name corresponds to an IP address. The computer recognizes the IP address mapped by the domain name and performs the operation.

1.4 ports

1.41 port indicates the process of a program on the computer

 different processes have different port numbers! Used to distinguish software!
 specified port number range: 0 ~ 65535
 the port numbers of different TCP/UDP protocols can be the same: TCP/UDP:65535*2 tcp:80 udp:80, which is OK
However, under a single protocol, port numbers cannot conflict

1.42 port classification

Public port 0 ~ 1024
 HTTP:80
 HTTPS:443
 FTP:21
 Telent:23
Program registration port: 1024 - 49151, which is assigned to users or programs
Tomcat:8080
MySQL:3306
Oracle:1521
Dynamic and private: 49152 ~ 65535

1.43 query port practice

 netstat -ano / / view all ports

 netstat -ano|findstr "5353" view the specified port
Note: make sure there are 5353 in all ports, or you may have retrieved a lonely port

 tasklist|findstr "20568" view the process of the specified port

Note: make sure that the port has a corresponding process
The confirmation method is as follows:
Ctrl+shift+ESC opens the task manager

 little knowledge: by modifying hosts C:\Windows\System32\drivers\etc\hosts, you can directly modify the mapping name of 127.0.0.1hostsname

Small practice of network address
Be slightly familiar with the usage of InetSocketAddress class in java

package com.kuang.lession01;

import java.net.InetSocketAddress;

public class TestInetSockeAddress {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		InetSocketAddress socketAddress=new InetSocketAddress("127.0.0.1",8080);
		InetSocketAddress socketAddress1=new InetSocketAddress("localhost",8080);
		System.out.println(socketAddress);
		System.out.println(socketAddress1);

			
		System.out.println(socketAddress.getAddress());
		System.out.println(socketAddress.getPort());//port
		System.out.println(socketAddress.getHostName());//address
	}

}

1.5 communication protocol

Agreement: the agreement is like we speak Mandarin and write Chinese characters now. You can understand it. If I draw in Spanish, you may not understand it, let alone communicate

Network communication protocol: rate, transmission code rate, code structure, transmission control

Two important agreements:
TCP: user transport protocol
UDP: User Datagram Protocol

TCP UDP comparison

TCP

• The destination connection is stable
• Connected triple handshake
• Four disconnected waves

Connect at least three times to ensure a stable connection! Disconnect at least four times to stably disconnect!
Three handshakes for connection: A: request connection, B agree connection and AB connection
Four waves of disconnection: A: request disconnection B: agree disconnection B: disconnect, a: disconnect

UDP

• No connection established and unstable
• Client, server: there is no clear boundary
• It can be sent to you whether it is ready or not
• DDOS saturation attack

TCP program practice

The following is a remote chat program implemented according to the network programming principle of java.

client

1. Connect to the server Socket
2. send message

 package lesson02;
//client
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;


//client
public class TcpClientDemo01 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc =new Scanner(System.in);
		System.out.println("Please enter the text you want to send to the server:");
		while(true)
		{
			
			Socket socket=null; //Expand the scope of Socket and OutpytStream to facilitate the final solution of exception throwing
			OutputStream os=null;
			
			String str =sc.next();
			
			try {//core
				//1. Know the address and port number of the server
				InetAddress serverIP=InetAddress.getByName("127.0.0.1");
				int port = 9921;
				
				//2. Create a socket connection
				socket=new Socket(serverIP, port);
				
				//3. Send message IO stream
				os=socket.getOutputStream();
				
				os.write(str.getBytes());
				
			} catch (Exception e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally {
				/*
				 * Close exception the following is the basic template for large companies to solve exceptions
				 */
				if(os!=null) {
					try {
						os.close();
					} catch (IOException e2) {
						// TODO: handle exception
					}
				}
				if(socket!=null)
				{
					try {
						socket.close();
					} catch (IOException e2) {
						// TODO: handle exception
					}
				}
			}
			
		}
	}

} 

The server

1. Establish service port ServerSocket
2. Wait for user's link accept
3. Accept user messages

package lesson02;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;


//Server
public class TcpServerDemo01 {
public static void main(String[] args) {
	 
	ServerSocket serverSocket=null;
	Socket socket=null;
	InputStream is =null;
	ByteArrayOutputStream baos=null;
	try {
		//1. Create a new address
		serverSocket=new ServerSocket(9921);
		while(true)
		{
		//2. Wait for the client to connect
		socket=serverSocket.accept();
		//3. Read the message from the client
		is =socket.getInputStream();
		
		
		
		//Connect the input stream through a pipe
		//Pipe flow
		baos=new ByteArrayOutputStream();//Output to console
		byte[]buffer=new byte[1024];
		int len;
		while((len=is.read(buffer))!=-1) {//When there is no end, the content is written to Bao
			baos.write(buffer,0,len);
		}
		System.out.println(baos.toString());	//output		
		}
	} catch (IOException e) {
		// TODO Auto-generated catch block
		//close
		e.printStackTrace();
	}finally {
		//close resource
		if(baos!=null) {
			try {
				baos.close();
			} catch (IOException e2) {
				// TODO: handle exception
			}
		}
		if(is!=null)
		{
			try {
				is.close();
			} catch (IOException e2) {
				// TODO: handle exception
			}
		}
		if(socket!=null)
		{
			try {
				socket.close();
			} catch (IOException e2) {
				// TODO: handle exception
			}
		}
		if(serverSocket!=null)
		{
			try {
				serverSocket.close();
			} catch (IOException e2) {
				// TODO: handle exception
			}
		}
	}
}
}

Note: 1. How does eclipse run on multiple consoles?
You can see this blog:
2. bind: address already in use encountered
What should I do?
Solution: you can read this blog