COMP201 Java Programming Part III: Advanced Features Topic 14: Networking Volume II,Chapter 3.

COMP201 Java Programming

Part III: Advanced Features

Topic 14: Networking

Volume II,Chapter 3

COMP201 Topic 14 / Slide 2

Outline

Networking basics IP addresses, ports, protocols, client-server interaction

Socket-level programming Writing a client Writing a server

URL connections and web servers Retrieving information Sending information


Networking Basics Internet protocol (IP) address

Every host on Internet has a unique IP address

143.89.40.46, 203.184.197.198

203.184.197.196, 203.184.197.197, 127.0.0.1

More convenient to refer to using hostname string

cs.ust.hk, tom.com, localhost

Domain Naming Service (DNS) maps names to numbers

One hostname can correspond to multiple internet addresses:

tom.com: 203.184.197.196, 203.184.197.196, 203.184.197.197 (down to one now)


java.net.InetAddress class converts between hostnames and internet addresses InetAddress tm = InetAddress.getByName(“tom.com");InetAddress tm= InetAddress.getByName(“localhost"); //127.0.0.1InetAddress tm = InetAddress.getLocalHost();

Can get array of addresses (if more than one)

InetAddress[] addrs;

addrs=InetAddress.getAllByName(“tom.com");

for (int i = 0; i < addr.length; i++)

System.out.println(addrs[i].getHostAddress());InetAddressTest.java

Networking Basics


Ports

A port identifies a service within a host

Many different services can be running on the host

Many standard port numbers are pre-assigned

time of day 13, ftp 21, telnet 23, smtp 25, finger 79, http 80

see /etc/services on workstation for list of all assigned ports

IP address + port number = "phone number“ for service

Networking Basics


protocols : rules that facilitate communications between machinesTime of day just reports time, ftp has put/get commands, etc.

Protocols must be standardized and documentedSo machines can reliably work with one another

TCP (Transmission Control Protocol) vs UDP (User Datagram Protocol, good for, e.g., video delivery)

Networking Basics


Client-Server interaction

Communication between hosts is two-way, but usually the two hosts take different roles

Server waits for client to make request

Server registered on a known port with the host ("public phone number")

Usually running in endless loop

Listens for incoming client connections

Networking Basics


Client "calls" server to start a conversation

Client making calls uses hostname/IP address and port number

Sends request and waits for response

Standard services always running

ftp, http, smtp, etc. server running on host using expected port

Server offers shared resource (information,database, files, printer, compute power) to clients

Networking Basics


Using telnet to try out some services of servers:

Telnet assumes you want to connect to port 23 on the receiving host (port 23 is where the telnet server is listening)

However there is an optional argument after the hostname that allows you to connect to a different port

Try the following

Get time: telnet time-A.timefreq.bldrdoc.gov 13

Finger some one: telnet cssu100.cs.ust.hk 79 & type a username

Get HTML page: telnet tom.com 80 and enter a GET command

Networking Basics


Socket-Level Programming

A socket is a bi-directional communication channel between hosts

Send and receive data using streams

Client Server

OutputStream

InputStream

InputStream

OutputStream

Next: How to write a client How to write a server


To write a client Create a new Socket with hostname and port number of the connection

Socket s = New Socket(String hostName,int portNumber);

Call s.getOutputStream() and s.getInputStream() to get streams for sending and receiving infomation

Need to learn protocol used to communicate

– Know how to properly form requests to send to server

– Know how to interpret the server’s responses

Socket-Level Programming/Client


Socket-Level Programming/Client Example: A client using http to request page

Socket s = new Socket(“tom.com", 80);

PrintWriter send = new PrintWriter(s.getOutputStream());

send.print("GET \n");

send.flush();

BufferedReader recv = new BufferedReader(

new InputStreamReader(s.getInputStream()));

String line;

while ((line = recv.readLine()) != null)

System.out.println(line);

s.close()//HttpTest.java. This is basically what web browsers do.


Socket-Level Programming/Client

Use setSoTimeout method to limit the amount of time waiting for data

Socket s = new Socket(“tom.com", 80);

s.setSoTimeout( 10000 ); // time out after 10 seconds

Can still wait indefinitely for initial connection

(construction of socket object).


Socket-Level Programming/Client Solution: Construct socket in a separate thread and use the join

method to time out.class SocketOpener implements Runnable

{ public SocketOpener(String aHost, int aPort)

{ socket = null; host = aHost; port = aPort;

}

public void run()

{ try

{ socket = new Socket(host, port);}

catch (IOException exception) {}

}

public Socket getSocket()

{ return socket; }

…

private String host; private int port;

private Socket socket;


Socket-Level Programming/Clientclass SocketOpener implements Runnable

{ // method to be called by main thread

public static Socket openSocket(String aHost, int aPort,

int timeout)

{ SocketOpener opener = new SocketOpener(aHost, aPort);

Thread t = new Thread(opener);

t.start();

try

{

t.join(timeout); // wait at most timeout milliseconds

} // for the thread to dead

catch (InterruptedException exception){}

return opener.getSocket();

} SocketOpenerTest.java


Socket-Level Programming/Server

To write a server Create a new ServerSocket with a port number to listen on the port

ServerSocket s = New ServerSocket( portNumber);

Use accept() to listen on the port.

accept() returns a socket incoming when a client calls

Socket incoming = s.accept();

Call incoming.getOutputStream() and incoming.getInputStream() to get streams for sending and receiving information


Socket-Level Programming/Server Example: Echo server

ServerSocket s = new ServerSocket(8189);

while (true)

{ Socket incoming = s.accept( );

BufferedReader in = new BufferedReader

(new InputStreamReader(incoming.getInputStream()));

PrintWriter out = new PrintWriter

(incoming.getOutputStream(), true /* autoFlush */ );

out.println( "Hello! Enter BYE to exit." );

…

}

EchoServer.java


Socket-Level Programming/Server Multithread server: starts a separate thread for each connection.

public class ThreadedEchoServer

{ public static void main(String[] args )

{ int i = 1;

try{ServerSocket s = new ServerSocket(8190);

while (true)

{ Socket incoming = s.accept( );

System.out.println("Spawning " + i);

new ThreadedEchoHandler(incoming, i).start();

i++;

}

} catch (Exception e) …. //ThreadedEchoServer.java


Socket-Level Programming/Serverclass ThreadedEchoHandler extends Thread

{ public ThreadedEchoHandler(Socket i, int c)

{ incoming = i; counter = c; }

public void run()

{ try

{ BufferedReader in = new BufferedReader

(new InputStreamReader(incoming.getInputStream()));

PrintWriter out = new PrintWriter

(incoming.getOutputStream(), true /* autoFlush */);

out.println( "Hello! Enter BYE to exit." );

…

private Socket incoming;

private int counter; }


ICQServer.java A simple server that listens on port 7777.

Connect two clients so that they can talk to each other.

Can handle more than one pairs.

See code for details.

Socket-Level Programming/Server


URL Connections

URL stands for Uniform Resource LocatorNeat scheme for uniquely identifying all kinds of network resources

Basic form <protocol>:<sitename><pathname>http://www.cs.ust.hk/~liao/comp201/index.html

ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/HttpTest/HttpTest.java

file:/MyDisk/Letters/ToMom2-11-98

Protocols include files, http, ftp, gopher, news, mailto, etc.

Connecting to a URL, the hard wayManually parse out the host, protocol, path from URL

Create a socket to host

Use protocol to send the right request & interpret response

But Java makes this much easier than that...


URL Connections/Retrieve Info Open connection with java.net.URL

URL url1 = new URL(“http://www.cs.ust.hk/~liao/comp201/index.html”);

URL url2 = new URL (“ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/net/HttpTest/HttpTest.java”);

To fetch contents of resource, use openStream method of URL, which returns a InputStream

InputStream in1 = url1.openStream();

InputStream in2 = url2.openStream();

Now we can nest the InputStreams with other Java streams to retrieve contents

Advantages

1. No need to worry about protocol details & port.

2. Connection to particular file, not easy to do at Socket level


URL Connections/Retrieve Info Use java.net.URLConnection for additional info about resource

1. Create URLConnection object from URL,

URLConnection cnn = url.openConnection();

2. Set properties of connections:

E.g. setDoOutPut(true) for sending information to the server

3. Make connection: cnn.connect();

4. Query header information:

getContentType, getContentLength, getContentEncoding,

getDate, getExpiration, getLastModified

5. getInputStream for reading and getOutputStream for writing

URLConnectionTest.java


URL Connections/Sending Info Web servers receive information from clients using either GET or

POST

http://maps.yahoo.com/py/maps.py:

<form action="/py/maps.py?Pyt=Tmap&YY=28457" method=GET> … </form>

http://www.census.gov/ipc/www/idbprint.html:

<form method=post action="/cgi-bin/ipc/idbsprd">

Appropriate CGI (common gateway interface) script is called to process info received and produce an HTML page to send back to client

CGI scripts usually written in C, Perl, shell script. (Out of the scope of this course.)

Will discuss servlets, Java alternative to CGI scripts


URL Connections/Sending Info Send information to server using GET

Attach parameters to the end of URL

http://host/script?parameters

Separate parameters using “&” and encode parameters as follows to avoid misinterpretation (URL encoding)

Static String encode (String s)

Replace space with “+”

Replace each non-alphanumeric character with “%” followed by the hexadecimal code of the character

“Mastering C++” “Mastering+C%2b%2b”

Disadvantage: long parameter string, might exceed limits of browsers.GetTest.java


URL Connections/Sending Info Sending information to server using POST:

Open URLConnection and send parameter using a stream

1. Open a URLConnection:

URL url = new URL(“http:/host/script”);

URLConnection cnn = url.openConnection();

2. Set up connection for output:

cnn.setDoOutput(true);

3. Get a stream for sending data:

PrinterWriter out = new PrintWriter(cnn.getOutputStream());

4. Send parameters

Out.print(name1 + “=“ + URLEncoder.encode(value1) + “&” );

Out.print(name2 + “=“ + URLEncoder.encode(value2) + “\n”);PostTest.java

COMP201 Java Programming Part III: Advanced Features Topic 14: Networking Volume II,Chapter 3.

Documents

Transcript of COMP201 Java Programming Part III: Advanced Features Topic 14: Networking Volume II,Chapter 3.