use the Socket API to create connections to remote computers

send data(bytes)

receive data(bytes)

on a remote machine

receive/send data

computer programs need to interact with the world

- bring in information from an external source

- send out information to an external destination

Input/Output:

Input(Read): to bring in information

Output(Write): to send out information

Information

特点

stream can carry data

– byte streams, for machine-formatted data

• InputStream, OutputStream

• Writing and reading are very efficient.

– character streams (textual), for human-readable data • Reader / Writer

• Require translation

Opening

• When you open a stream, you are making a connection to that external place.

• Once the connection is made, you forget about the external place and just use the stream

Closing

• A stream is an expensive resource.

There is a limit on the number of streams that you can have open at one time.

• You should not have more than one stream open on the same file.

• You must close a stream before you can open it again. 

• Some streams can be used only for input, others only for output, others for both.

• Using a stream means doing input from it or output to it.

– involves creating multiple streams;

– the streams are connected to provide the required functionality;

– read from/write to text files require declaring and using the correct I/O streams.

•  The Socket object presents a stream to you (the programmer)

– You don’t need to worry about how the network sends bytes

– You just interact with the stream

• Java’s built-in multithreading:

– Handles multiple connections at once.

– E.g. a web server will create 1 thread for each request it receives

– So it can handle multiple clients in parallel

作用

identifier：Identifying computers on a network

Domain names： DNS (Domain Name System) form (www.qmul.ac.uk）

IP (Internet Protocol) address：

- “dotted quad” format

-139.255.27.125, a 32-bit number

- java.net package：static InetAddress.getByName()

- An object of type InetAddress that you can use to build a socket.

- 127.0.0.1 is for local machine.

www.eecs.qmul.ac.uk -> 138.37.95.147

步骤：

- The Domain Name System (DNS) performs this mapping

- DNS servers handle lookups

- return the IP address that maps to a domain name

- send DNS queries to a DNS server

- nslookup DN

//Find out your IP address 
import java.net.*;
public class IPFinder { 
    public static void main(String[] args) throws Exception { 
        String domainName = “www.qmul.ac.uk”;
        InetAddress a = InetAddress.getByName(domainName); 
        System.out.println(a); 
    } 
}

allows two machines to connect and talk to each other.

• One machine has to stay and listen: server.

• The other machine makes requests: client.

• The client is trying to connect to the server. Once connected, there is a two way communication.

• If your code is not working, check if both computers are online

• Open your terminal window: Type – ping www.eecs.qmul.ac.uk

• A special address called localhost（ 127.0.0.1. ）

• Run both client and server on one machine (localhost)

• Producing a localhost:

InetAddress addr = InetAddress.getByName(null);  InetAddress.getByName("localhost");

InetAddress.getByName("127.0.0.1");

An IP address isn’t enough to identify a unique server:

Many services can exist on one machine

• When setting up a client or a server:

– Must choose a port.

– Both client and server agree to connect.

• The port is not a physical location in a machine, but a software abstraction.

There are several standard ports that always get used for the same applications

80 for web servers (HTTP)

443 for encrypted web servers (HTTPS)

22 for secure shell (SSH)

20 and 21 for File Transfer Protocol (FTP)

25 for Simple Mail Transfer Protocol (SMTP)

Door is the IP address Letter boxes are the ports;

Allows us to talk to multiple people (services) in one house (computer)

原因

• When a client wants a service, it attempts the connection with the server by supplying the port number associated with the service.

• There are likely to be multiple clients waiting for the same service at the same time (e.g. web browsers wanting a web page).

• The server needs a way to distinguish between clients and keeping their communication separate.

– This is achieved by the use of sockets.

• The client creates a socket at its end of the communication link.

• The server, upon receiving the client’s initial request (on a particular port number), creates a new socket at its end, dedicated to the communication with that specific client.

定义

• Server socket: a server is used to listen for incoming connections.

• Socket: a client is used to initiate a connection.

• Making a socket connection:

– ServerSocket returns a corresponding Socket

– via the accept() method

– through which communications will take place on the server side.

• Then it is a true Socket to Socket connection:

– May treat both ends the same way. 

• Once you’ve created a socket, you can get access to its input and output streams

• To produce the corresponding InputStream and OutputStream objects from each Socket, use the methods:

– mySocket.getInputStream()

– mySocket.getOutputStream()

• Create a ServerSocket:

– Only need to give the port number.

– IP address is not necessary.

• Create a Socket:

– Give both the IP address and port number.

– Indicate where to connect.