SSC - Communication and Networking Java Socket Programming (I)

Transcription

1 SSC - Communication and Networking Java Socket Programming (I) Shan He School for Computational Science University of Birmingham Module : SSC

2 Outline Outline of Topics Sockets and Socket-based Communication

3 Sockets and Socket-based Communication Client/Server Communication The simplest level of network communication Consists of a server, client, and a network for communication Client: a computer running a program that makes a request for services Server: a computer running a program that offers requested services, e.g., processing database queries from one or more clients Client Server Request Results Network (TCP/IP)

4 Sockets and Socket-based Communication TCP/IP Application layer: Sockets TCP/IP model Protocols FTP SMTP HTTP IMAP Application SSL/TLS ACSII Sockets Transport TCP UDP Internet IP Network Interface Physical

5 Sockets and Socket-based Communication TCP/IP model: Application layer - Sockets The lowest level before Transport layer Definition: endpoint of a two-way communication link. Provides an interface for programming network communication. Similar to performing file I/O, e.g., socket handle is treated like file handle. Independent of a programming language

6 Sockets and Socket-based Communication Sockets A socket consists of Local socket address: Local IP address and service port number Remote socket address: Only for established TCP sockets Protocol: A transport protocol, e.g., TCP or UDP. A socket address is the combination of an IP address (phone number) and service port number (extension). A socket API is an application programming interface (API), usually provided by the operating system.

7 Sockets and Socket-based Communication Service ports Computers often communicate and provide more than one type of service or to talk to multiple hosts/computers at a time Ports are used to distinguish these services Each service offered by a computer is identified by a port number Represented as a positive (16-bit) integer value Some ports have been reserved to support common services FTP: 21/TCP HTTP: 80/TCP,UDP IMAP: 143/TCP User-level process/services use port number value 1024

8 Sockets and Socket-based Communication Establish socket connection Step 1: The server listens to the socket of a specific port number for a client to make a connection request Step 2: the server accepts the connection if everything goes well Step 3: the server gets a new socket bound to a different (usually random) port for connection with the client Go to Step 1 Question: the service port in the TCP socket connection request is different from the service port for the connection. Why?

9 Sockets and Socket-based Communication Establish socket connection Server Client Service Port Connection request Server Client Service Port Service Port Connection

10 Sockets and Socket-based Communication Put them together Client is connected to a server at the physical address identified by a IP address. Connection binds a port number to a socket number. Both TCP and UDP protocols use ports to map incoming data to a particular process running on a computer. Apps FTP HTTP SMTP App Sockets Port Port Port Port TCP or UDP Data Packet Port# Data

11 Java.net class Java provides a set of classes, java.net, to enable the rapid development of network applications. Essentially provides classes, interfaces, and exceptions to simplify the complexity involved in creating client and server programs Two key classes for creating server and client programs ServerSocket Socket

12 Socket-based client and server Java programming Server: ServerSocket creates a new service socket (port) Clinet: Socket establishes connection with service socket Both: exchange data using input and output streams

13 Java.net class Server (IP: ) Client ServiceSocket 1254 Socket(( , 1254) Output/write stream Input/read stream

14 A simple Server Program in Java (I) Steps to create a simple server program: Step 1: Open the Server Socket: ServerSocket server = new ServerSocket( PORT ); Step 2: Wait for the Client Request: Socket client = server.accept(); Step 3: Create I/O streams for communicating to the client DataInputStream is = new DataInputStream(client.getInputStream()); DataOutputStream os = new DataOutputStream(client.getOutputStream());

15 A simple Server Program in Java (II) Step 4: Perform communication with client Receive from client: String line = is.readline(); Send to client: os.writebytes("hello!"); Step 5: Close the stream is.close(); os.close(); Step 6: Close the socket: client.close();

16 A simple Client Program in Java Step 1: Open a socket. Step 2: Open an input stream and output stream to the socket. Step 3: Read from and write to the stream according to the server s protocol. Step 4: Close the streams. Step 5: Close the socket.

17 A few explainations: PrintWriter vs DataOutputStream PrintWriter : Prints formatted representations of objects to a text-output stream. DataOutputStream : A data output stream lets an application write primitive Java data types to an output stream in a portable way PrintWriter inherited from Writer class, while DataOutputStream inherited from OutputStream. OutputStream (therefore DataOutputStream ) is designed for binary data, while Writer (therefore PrintWriter ) is designed for text data

18 A few explainations: InetAddress InetAddress class: represents an Internet Protocol (IP) address. getlocalhost() method: Returns the address of the local host.

19 A few explainations: Protocol A protocol is the language that the client and server have agreed to use to communicate. Using Java.net class, you can build you own protocol using socket One example: KnockKnockProtocol can be found at here

20 Socket programming for POP3, IMAP and SMTP are all protocols based on TCP socket communication Please take a look at my source code of send using only socket