1. Network programming knowledge summary
Java is a language on the Internet. It provides support for network applications from the language level, and programmers can easily develop common network applications.
The network class library provided by Java can realize painless network connection. The underlying details of the network are hidden in Java's native installation system and controlled by the JVM. And Java implements a cross-platform network library, and programmers are faced with a unified network programming environment.
computer network:
Connect computers distributed in different geographical areas with specialized external equipment using communication lines to form a large-scale, powerful network system, so that numerous computers can easily transfer information to each other, share hardware, software, data information and other resources .
The purpose of network programming:
directly or indirectly realize data exchange and communication with other computers through network protocols.
There are two main problems in network programming:
how to accurately locate one or more hosts on the network; locate specific applications on the
host how to reliably and efficiently transmit data after finding the host
Telecommunication
Addresses of the communication parties
a) IP
b) Port number
Certain rules (ie: network communication protocol. There are two sets of reference models)
c) OSI reference model: the model is too ideal to be widely promoted on the Internet
d) TCP/IP Reference model (or TCP/IP protocol): the de facto international standard.
IP
IP address
IP address: InetAddress
uniquely identifies the computer (communication entity) on the Internet.
Local loopback address (hostAddress): 127.0.0.1 Host name (hostName): localhost
IP address classification method 1: IPV4 and IPV6
IPV4: 4 bytes, 4 0-255. About 4.2 billion, 3 billion are in North America, 400 million in Asia. It was exhausted in early 2011. Expressed in dotted decimal notation, such as 192.168.0.1
IPV6: 128 bits (16 bytes), written as 8 unsigned integers, each integer is represented by four hexadecimal digits, and the numbers are separated by colons (:), such as: 3ffe:3201:1401:1280 :c8ff:fe4d:db39:1984
IP address classification method 2: public network address (used by the World Wide Web) and private address (used by the local area network). The private address starts with 192.168.0.0–192.168.255.255, which is specifically for the internal use of the organization.
Features: not easy to remember
Port number The
port number identifies the process (program) that is running on the computer
Different processes have different port numbers, which
are defined as a 16-bit integer 0~65535.
Port
classification:
Recognized port: 0~1023. Occupied by pre-defined service communication (for example: HTTP occupies port 80, FTP occupies port 21, Telnet occupies port 23)
Registered port: 1024~49151. Assigned to a user process or application. (For example: Tomcat occupies port 8080, MySQL occupies port 3306, Oracle occupies port 1521, etc.).
Dynamic/private port: 49152~65535.
The combination of port number and IP address results in a network socket: Socket.
OSI seven-layer reference model
The application layer provides a variety of application services: email (MHS), file transfer (FTAM), virtual terminal (VT), electronic data interchange (EDI) and other
major protocols: FTP (21), SMTP (25), DNS .HTTP(80)
Presentation layer: Provide a common language for communication to facilitate interaction, because different computer system structures use different data representations, for example: IBM hosts use EBCDIC encoding, and most PCs use ASCII encoding.
Other functions such as Data encryption, data compression
Session layer: provides services that enable applications to establish and maintain sessions, and enable sessions to be synchronized
Transport layer: When two computers communicate data through the network, it is an end-to-end level and has a buffering
effect.Protocol: TCP/UDP
Network layer: IP, data transfer in the form of IP messages
Data link layer: can be understood as a data channel,
MAC address represents uniqueness
Physical layer: Provides a path for data segment equipment to transmit data. The data path can be a physical medium or a connection of multiple physical media.
TCP/IP
There are two very important protocols in the transport layer protocol:
Transmission Control Protocol (TCP) and
User Datagram Protocol (UDP).
TCP/IP is named after its two main protocols: Transmission Control Protocol (TCP) and Internet Interconnection Protocol (IP). In fact, it is a set of protocols, including multiple protocols with different functions and related to each other.
The IP (Internet Protocol) protocol is the main protocol of the network layer, supporting data communication between networks.
The TCP/IP protocol model forms an efficient four-layer system structure from a more practical perspective, namely, the physical link layer, the IP layer, the transport layer and the application layer.
The classes under java/net under rt.jar are for network programming
Network architecture: B/S and C/S
B/S: Browser/Server based on browser, such as web version Taobao
C/S: Client/Server based on client, such as mobile phone or APP on Taobao
TCP :
It can be retransmitted without packet loss. If it is lost, it will be recorded and re-sent, but it will not be repeated.
Reliable, orderly, and the order will not be wrong. It is 132, but what you see must be 123
-oriented link. If the connection fails, the data will not be sent. The
three-way handshake can ensure the security of the data. It can ensure that the interaction is
equivalent to a phone call. If the connection fails , the data is not communicated. Past
Socket
The use of sockets to develop network applications has long been widely adopted, so that it has become a de facto standard.
Only the uniquely identified IP address and port number on the network can be combined to form a uniquely identifiable identifier socket.
Both ends of the communication must have a Socket, which is the endpoint of communication between the two machines.
Network communication is actually the communication between Sockets.
Socket allows the program to treat the network connection as a stream, and data is transmitted between the two Sockets via IO.
Generally, the application that initiates communication is the client, and the one waiting for the communication request is the server.
Socket classification:
stream socket: use TCP to provide reliable byte stream services. Datagram socket: use UDP to provide "best effort" datagram service
Common method
Commonly used constructor of the
Socket class: public Socket(InetAddress address,int port) creates a stream socket and connects it to the specified port number of the specified IP address.
public Socket(String host,int port) creates a stream socket and connects it to the specified port number on the specified host.
Common methods of the Socket class:
public InputStream getInputStream() returns the input stream of this socket. Can be used to receive network messages
public OutputStream getOutputStream() returns the output stream of this socket. Can be used to send network messages
public InetAddress getInetAddress() The remote IP address that this socket is connected to; if the socket is not connected, it returns null.
public InetAddress getLocalAddress() gets the local address bound to the socket. That is, the local IP address
public int getPort() The remote port number that this socket is connected to; if the socket is not connected yet, it returns 0.
public int getLocalPort() returns the local port to which this socket is bound. If the socket has not been bound, -1 is returned. That is, the port number of the local end.
public void close() close this socket. After the socket is closed, it cannot be used in future network connections (that is, it cannot be reconnected or re-bound). Need to create a new socket object. Closing this socket will also close the InputStream and OutputStream of the socket.
public void shutdownInput() If you call shutdownInput() on the socket and then read content from the socket input stream, the stream will return EOF (end of file). That is, no data can be received in the input stream from this socket.
public void shutdownOutput() disables the output stream of this socket. For TCP sockets, any previously written data will be sent, followed by TCP's normal connection termination sequence. If you call shutdownOutput() on the socket and then write to the socket output stream, the stream will throw an IOException. That is, no data can be sent through the output stream of this socket.
Server
The working process of the server program includes the following four basic steps:
Call ServerSocket(int port): Create a server-side socket and bind it to the specified port. Used to monitor client requests.
Call accept(): monitor the connection request, if the client requests a connection, then accept the connection and return the communication socket object.
Call the getOutputStream() and getInputStream() of the Socket class object: Obtain the output stream and input stream, and start sending and receiving network data.
Close the ServerSocket and Socket objects: the client's access is over, and the communication socket is closed.
The ServerSocket object is responsible for waiting for the client's request to establish a socket connection, similar to a salesperson in a window of a post office. In other words, the server must establish in advance a ServerSocket object waiting for the client's request to establish a socket connection.
The so-called "receiving" the client's socket request means that the accept() method will return a Socket object
Client
The working process of the client Socket includes the following four basic steps:
Create Socket: Construct a Socket class object according to the IP address or port number of the specified server. If the server responds, a communication line from the client to the server is established. If the connection fails, an exception will occur.
Open the input/output stream connected to the Socket: Use the getInputStream() method to obtain the input stream, use the getOutputStream() method to obtain the output stream, and perform data transmission
Read/write the Socket according to a certain protocol: read through the input stream The server puts the information into the line (but cannot read the information put into the line by itself), and writes the information to the thread through the output stream.
Close Socket: disconnect the connection from the client to the server and release the line
The client program can use the Socket class to create an object, and it will automatically initiate a connection to the server when it is created. The Socket constructor is:
Socket(String host,int port) throws UnknownHostException, IOException: initiate a TCP connection to the server (domain name is host. Port number is port), if successful, create a Socket object, otherwise throw an exception.
Socket (InetAddress address, int port) throws IOException: Initiate a connection based on the IP address and port number port represented by the InetAddress object.
The process of establishing a socketAtClient object by the client is to send a socket connection request to the server
UDP/IP
UDP:
fast speed,
not guaranteed reliability,
possible packet loss,
no connection,
equivalent to sending a text message, no matter whether you can receive it or not, it will be sent to you anyway
The classes DatagramSocket and DatagramPacket implement network programs based on the UDP protocol.
UDP datagrams are sent and received through the datagram socket DatagramSocket. The system does not guarantee that UDP datagrams can be safely delivered to the destination, nor can it be determined when they will arrive.
The
DatagramPacket object encapsulates the UDP datagram. The datagram contains the IP address and port number of the sender and the IP address and port number of the receiver.
Each datagram in the UDP protocol gives complete address information, so there is no need to establish a connection between the sender and the receiver. It's like sending a parcel.
Common method
Common methods of the DatagramSocket class
public DatagramSocket(int port) creates a datagram socket and binds it to the specified port on the local host. The socket will be bound to a wildcard address, and the IP address will be chosen by the kernel.
public DatagramSocket (int port, InetAddress laddr) creates a datagram socket and binds it to the specified local address. The local port must be between 0 and 65535 (including both). If the IP address is 0.0.0.0, the socket will be bound to a wildcard address, and the IP address is chosen by the kernel.
public void close() closes this datagram socket.
public void send(DatagramPacket p) sends datagram packets from this socket. The information contained in DatagramPacket indicates: the data to be sent, its length, the IP address of the remote host and the port number of the remote host.
public void receive(DatagramPacket p) receives datagram packets from this socket. When this method returns, the buffer of DatagramPacket is filled with the received data. The datagram packet also contains the IP address of the sender and the port number on the sender's machine. This method blocks until the datagram is received. The length field of the datagram packet object contains the length of the received information. If the information is longer than the length of the packet, the information will be truncated.
public InetAddress getLocalAddress() gets the local address bound to the socket.
public int getLocalPort() returns the port number on the local host to which this socket is bound.
public InetAddress getInetAddress() returns the address of this socket connection. If the socket is not connected, null is returned.
public int getPort() returns the port of this socket. If the socket is not connected, -1 is returned.
Common methods of DatagramPacket class
public DatagramPacket(byte[] buf,int length) constructs DatagramPacket to receive
data packets of length length. The length parameter must be less than or equal to buf.length.
public DatagramPacket(byte[] buf, int length, InetAddress address, int port) constructs a datagram packet, which is used to send a packet of length length to the specified port number on the specified host. The length parameter must be less than or equal to buf.length.
public InetAddress getAddress() returns the IP address of a certain machine, this datagram will be sent to
or received from this machine.
public int getPort() returns the port number of a remote host. This datagram will be sent to or received from the host.
public byte[] getData() returns the data buffer. The received or to-be-sent data starts
at the offset offset in the buffer and continues for length length.
public int getLength() returns the length of the data to be sent or received.
Server
Process:
1. DatagramSocket and DatagramPacket
2. Establish the sender and receiver
3. Build the data package
4. Call the sending and receiving methods of Socket
5. Close the Socket
The sending end and the receiving end are two independent running programs