Mad God video address: https://www.bilibili.com/video/BV1LJ411z7vY

1. General

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

Purpose of network programming

• Dissemination and exchange of information
• Data exchange and communication

What is needed to achieve this effect

• How to accurately locate a host on the network?
  192.168. 1.100: port to locate a resource on this computer.
• After finding the host, how to transmit data?
  JavaWeb: Web programming B/S architecture
  Network programming: TCP/IP C/S architecture

2. Elements of network communication

How to realize network communication?

• Address of communication parties: IP + port number (192.168.1.100:8080)
• Rules: protocols for network communication
  TCP/IP reference model
  
  Summary:

1. 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
3. Everything in Java is an object

3. IP

ip address: InetAddress

• Uniquely locate a computer on a network
  • 127.0. 0.1: local localhost
• Classification of ip addresses
  • IPV4/IPv6
    • IPV4 : 127.0. 0.1, composed of 4 bytes, 0-255, 4.2 billion, 3 billion, all in North America and 400 million in Asia. Exhausted in 2011
    • IPV6: 128 bit. 8 unsigned integers!
  • Public network (Internet) - private network (LAN)
    • 192.168.xx.xx: for internal use of the organization
• Domain name: memory ip problem

//Test ip
public class TestInetAddress {
    public static void main(String[] args) {
        try {
            //Query local address
            InetAddress address1 = InetAddress.getByName("127.0.0.1");
            System.out.println(address1); ///127.0.0.1
            InetAddress address3 = InetAddress.getByName("localhost");
            System.out.println(address3); //localhost/127.0.0.1
            InetAddress address4 = InetAddress.getLocalHost();
            System.out.println(address4); //Sunny/192.168.100.1

            //Query URL ip address
            InetAddress address2 = InetAddress.getByName("www.baidu.com");
            System.out.println(address2); //www.baidu.com/36.152.44.95
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }

    }
}

4. Port

A port represents the process of a program on a computer.

• Different processes have different port numbers to distinguish software
• Specified: 0-65535
• TCP/UDP: 65535 * 2. Port numbers cannot conflict under a single protocol
• Port classification:
  • Public port: 0 ~ 1023
    • HTTP : 80
    • HTTPS : 443
    • FTP : 21
    • Telet : 23
  • Program registration port: 1024-49151, assigned to users or programs
    • Tomcat: 8080
    • Mysql: 3306
    • Oracle: 1521
  • Dynamic, private: 49152-65535

netstat -ano #View all ports
netstat -ano  | findstr "5900" #View the specified port
tasklist | findstr "8696" #View processes on the specified port
Ctrl + Shift + ESC # Open Task Manager

public class TestInetSocketAddress {
    public static void main(String[] args) {
        InetSocketAddress socketAddress = new InetSocketAddress("127.0.0.1", 8080);
        InetSocketAddress socketAddress1 = new InetSocketAddress("localhost", 8080);

        System.out.println(socketAddress); ///127.0.0.1:8080
        System.out.println(socketAddress1); //localhost/127.0.0.1:8080

        System.out.println(socketAddress.getAddress()); //ip address / 127.0 zero point one
        System.out.println(socketAddress.getHostName()); //Host name 127.0 zero point one
        System.out.println(socketAddress.getHostString()); //127.0.0.1
        System.out.println(socketAddress.getPort());//Port 8080
    }
}

5. Communication protocol

Agreement: agreement, just as we speak Mandarin now.
Network communication protocol: rate, transmission code rate, code structure, transmission control

TCP/IP protocol cluster: it is actually a group of protocols

Important:

• TCP: user transport protocol
• UDP: User Datagram Protocol

Famous agreement:

• TCP
• IP

Comparison between TCP and UDP:

TCP: call

• Connection: stable
• Three handshakes

At least three times are required to ensure stable connection
A: What are you worried about?
B: What do you think?
A: Do it!

• Four waves

A: I'm disconnecting (I'm leaving)
B: I know you're disconnecting (are you really leaving?)
B: Are you really disconnected? Are you really leaving
A: I'm really disconnected (I'm really leaving)

• Client, server
• The transmission is completed, the connection is released, and the efficiency is low

UDP: send SMS

• Not connected, unstable
• Client and server: there is no clear boundary
• Ready or not, it can be sent to you
• Missile
• DDOS: flood attack! (saturation attack)

6. TCP

client

1. Connect to the server Socket
2. send message

//client
public class TcpClientDemo01 {
    public static void main(String[] args) {
        Socket socket = null;
        OutputStream os = null;
        try {
            // 1. Know the server address
            InetAddress serverIp = InetAddress.getByName("127.0.0.1");
            //Port number
            int port = 9999;
            // 2. Create a Socket connection
            socket = new Socket(serverIp,port);
            // 3. Send message IO stream
            os = socket.getOutputStream();
            os.write("Hello, welcome to study java".getBytes());
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

The server

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

//The server
public class TcpServerDemo01 {
    public static void main(String[] args) {
        ServerSocket serverSocket = null;
        Socket accept = null;
        InputStream is = null;
        ByteArrayOutputStream baos = null;
        try {
            // 1. I have to have an address
            serverSocket = new ServerSocket(9999);
            // 2. Wait for the client to connect
            accept = serverSocket.accept();
            // 3. Read the message from the client
            is = accept.getInputStream();

            //Pipe flow
            baos = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int len;
            while ((len=is.read(buffer)) != -1){
                baos.write(buffer,0,len);
            }
            System.out.println(baos.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            //close resource
            if (baos != null){
                try {
                    baos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (accept != null){
                try {
                    accept.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (serverSocket != null){
                try {
                    serverSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

File upload via TCP

//The server
public class TcpServerDemo02 {
    public static void main(String[] args) throws Exception {
        //1. Create service
        ServerSocket serverSocket = new ServerSocket(9000);
        //2. Listen for client connections
        Socket accept = serverSocket.accept(); //Blocking listening, waiting for the client to connect
        //3. Get input stream
        InputStream is = accept.getInputStream();

        //4. File output
        FileOutputStream fos = new FileOutputStream(new File("receive.png"));//The pipeline flow of the file is used to receive the file
        byte[] buff = new byte[1024];
        int len;
        while ((len = is.read(buff)) != -1){
            fos.write(buff,0,len);
        }

        //Notify the client that I have received it
        OutputStream os = accept.getOutputStream();
        os.write("I'm finished. You can disconnect".getBytes());

        fos.close();
        is.close();
        accept.close();
        serverSocket.close();
    }
}

//client
public class TcpClientDemo02 {
    public static void main(String[] args) throws Exception{
        //1. Establish connection
        Socket socket = new Socket(InetAddress.getByName("127.0.0.1"), 9000);
        //2. Create an output stream
        OutputStream os = socket.getOutputStream();
        //3. Read file
        FileInputStream is = new FileInputStream(new File("1.png"));
        byte[] buff = new byte[1024];
        int len;
        //4. Write out the document
        while ((len = is.read(buff)) != -1) {
            os.write(buff, 0, len);
        }

        //Notify the server that I'm finished
        socket.shutdownOutput();//I have transmitted the meaning

        //Make sure that the server has received it before you can disconnect
        InputStream inputStream = socket.getInputStream();//A pipe stream that receives characters and bytes
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        byte[] buff2 = new byte[1024];
        int len2;
        while ((len2 = inputStream.read(buff2)) != -1) {
            baos.write(buff2, 0, len2);
        }
        System.out.println(baos.toString());

        //5. Release resources
        baos.close();
        inputStream.close();
        is.close();
        os.close();
        socket.close();
    }
}

7. UDP

Send text messages without connecting. You need to know the other party's IP address.

• send message

Sender

//No connection to the server is required
public class UdpClientDemo1 {
    public static void main(String[] args) throws Exception{
        //1. Create a Socket
        DatagramSocket socket = new DatagramSocket();
        //2. Build a package
        String msg = "Hello, server";
        //3. To whom
        InetAddress address = InetAddress.getByName("localhost");
        int port = 9090;
        //Data, starting length of data, to whom
        DatagramPacket packet = new DatagramPacket(msg.getBytes(), 0, msg.getBytes().length, address, port);
        //4. Send packet
        socket.send(packet);
        //5. Close the flow
        socket.close();
    }
}

receiving end

//Or do you want to wait for the client to connect
public class UdpServerDemo1 {
    public static void main(String[] args) throws Exception{
        //Open port
        DatagramSocket socket = new DatagramSocket(9090);
        //Receive packet
        byte[] buff =new byte[1024];
        DatagramPacket packet = new DatagramPacket(buff, 0, buff.length);

        socket.receive(packet);//Blocking reception

        System.out.println(packet.getAddress());
        System.out.println(new String(packet.getData(),0,packet.getData().length));

        //Close connection
        socket.close();
    }
}

• UDP consultation
  Loop send message

send out:

public class UdpSenderDemo01 {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(8888);

        //Prepare data, read from console, system in
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        while (true) { //Cyclic input
            String data = reader.readLine();
            byte[] bytes = data.getBytes();
            DatagramPacket dgp = new DatagramPacket(bytes, 0, bytes.length, new InetSocketAddress("localhost", 6666));

            socket.send(dgp);
            if (data.equals("bye")){
                break;
            }
        }
        socket.close();
    }
}

receive

public class UdpReceiveDemo01 {
    public static void main(String[] args) throws Exception {
        DatagramSocket socket = new DatagramSocket(6666);

        while (true) { //Cyclic reception
            //Ready to receive package
            byte[] bytes = new byte[1024];
            DatagramPacket packet = new DatagramPacket(bytes,0,bytes.length);
            socket.receive(packet); //Blocking receiving package

            //Disconnect
            byte[] data = packet.getData();
            String receiceData = new String(data, 0, data.length);
            System.out.println(receiceData);
            if (receiceData.equals("bye")){
                break;
            }
        }
        socket.close();
    }
}

• Online consultation
  Both can be senders and receivers

send out:

public class TalkSend implements Runnable {
    DatagramSocket socket = null;
    BufferedReader reader = null;
    private int fromPort;
    private String toIp;
    private int toPort;

    public TalkSend(int fromPort, String toIp, int toPort) {
        this.fromPort = fromPort;
        this.toIp = toIp;
        this.toPort = toPort;

        try {
            socket = new DatagramSocket(fromPort);
            reader = new BufferedReader(new InputStreamReader(System.in));
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {

        while (true) { //Cyclic input
            String data = null;
            try {
                data = reader.readLine();
                byte[] bytes = data.getBytes();
                DatagramPacket dgp = new DatagramPacket(bytes, 0, bytes.length, new InetSocketAddress(this.toIp, this.toPort));

                socket.send(dgp);
                if (data.equals("bye")){
                    break;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
        socket.close();
    }
}

receive:

public class TalkReceive implements Runnable {
    DatagramSocket socket = null;
    private int port;
    private String msgFrom;

    public TalkReceive(int port, String msgFrom) {
        this.port = port;
        this.msgFrom = msgFrom;
        try {
            //Open connection
            socket = new DatagramSocket(port);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        while (true) { //Cyclic reception
            try {
                //Ready to receive package
                byte[] bytes = new byte[1024];
                DatagramPacket packet = new DatagramPacket(bytes,0,bytes.length);
                socket.receive(packet); //Blocking receiving package

                //Disconnect
                byte[] data = packet.getData();
                String receiceData = new String(data, 0, data.length);
                System.out.println(msgFrom + ":" + receiceData);
                if (receiceData.equals("bye")){
                    break;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        socket.close();
    }
}

student:

public class TalkStudent {
    public static void main(String[] args) {
        //Open two threads
        new Thread(new TalkSend(7777,"localhost",9999)).start();
        new Thread(new TalkReceive(8888,"teacher")).start();
    }
}

teacher:

public class TalkTeacher {
    public static void main(String[] args) {
        //Open two threads
        new Thread(new TalkSend(5555,"localhost",8888)).start();
        new Thread(new TalkReceive(9999,"student")).start();
    }
}

8. URL

Uniform resource locator: to locate a resource on the Internet.
https://www.baidu.com/

Protocol: / / ip address: port / project name / resource

public class TestUrlDemo {
    public static void main(String[] args) throws MalformedURLException {
        URL url = new URL("http://localhost:8080/hello.java/index.jsp?username=zs&password=123");
        System.out.println(url.getProtocol()); //Protocol http
        System.out.println(url.getHost()); //Host ip localhost
        System.out.println(url.getPort()); //Port 8080
        System.out.println(url.getPath()); //Full path / Hello java/index. jsp
        System.out.println(url.getFile()); //File / Hello java/index. jsp? username=zs&password=123
        System.out.println(url.getQuery()); //Parameter username = ZS & password = 123
    }
}

Download Network Resources

public class TestUrlDown {
    public static void main(String[] args) throws Exception {
        //Download address
        URL url = new URL("http://localhost:8080/urlTest/test.txt");

        //Connect to this resource http
        HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
        InputStream is = urlConnection.getInputStream();

        FileOutputStream fos = new FileOutputStream("test1.text");

        byte[] bytes = new byte[1024];
        int len;
        while ((len=is.read(bytes)) != -1){
            fos.write(bytes,0,len); //Write this data
        }
        fos.close();
        is.close();
        urlConnection.disconnect(); //Disconnect
    }
}