java networking

Topic 13: Networking
Volume II,Chapter 3
Advanced Programming Techniques

Objective and Outline
• Objective:
– Introduction to Java networking features
• It is much easier to write networking programs inJava than in C++
• But less efficient.
• Outline
– Motivating example: ICQ Server and Client
– Networking basics
• IP addresses, ports, protocols, client-server interaction
– Socket-level programming
• Writing a client (Socket)
• Writing a server (ServerSocket)
• Example: writing your own icq
– Communicating with web servers
• Retrieving information (URL, URLConnection)
• Sending information

Networking Basics
• Internet protocol (IP) addresses
– 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
– One hostname can correspond to multiple internet addresses:
• www.yahoo.com:
66.218.70.49; 66.218.70.50; 66.218.71.80; 66.218.71.84; …
– Domain Naming Service (DNS) maps names to numbers

 java.net.InetAddress class converts between
hostnames and internet addresses
InetAddress tm = InetAddress.getByName(“www.yahoo.com");
InetAddress tm= InetAddress.getByName(“localhost");
//127.0.0.1
InetAddress tm = InetAddress.getLocalHost();
 Can get array of addresses (if more than one)
InetAddress[] addrs;
addrs=InetAddress.getAllByName(“www.yahoo.com");
for (int i = 0; i < addr.length; i++)
System.out.println(addrs[i].getHostAddress());
InetAddressTest.java
Networking Basics

 Ports
 Many different services can be running on the host
 A port identifies a service within a host
 Many standard port numbers are pre-assigned
time of day 13, ftp 21, telnet 23, smtp 25, 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
machines
 Examples:
 HTTP: HyperText Transfer Protocol
 FTP: File Transfer Protocol
 SMTP: Simple Message Transfer Protocol
 TCP: Transmission Control Protocol
 UDP: User Datagram Protocol, good for, e.g., video delivery)
 Protocols are standardized and documented
So machines can reliably work with one another
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
Get HTML page: telnet www.cs.ust.hk 80 and enter a GET command
 Many servers now refuse telnet connections due to security reasons.
Networking Basics

Outline
• Outline
– Networking basics
• IP addresses, ports, protocols, client-server interaction
– Socket-level programming
• Writing a client
• Writing a server
• Example: writing your own icq
– Communicating with web servers
• Retrieving information
• Sending information

Socket-Level Programming
 Socket is an abstraction of one type of bi-directional
communication channel between hosts
 Send and receive data using streams
 Next:
 How to write a client
 How to write a server
Client Server
OutputStream
InputStream
InputStream
OutputStream

• To write a client socket using java.net.Socket
– 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
Writing Clients

Writing Clients
 SocketTest:
Makes a socket connection to the atomic clock in Boulder, Colorado,
and prints the time that the server sends.
try
{ Socket s = new Socket("time-A.timefreq.bldrdoc.gov", 13);
BufferedReader in = new BufferedReader
(new InputStreamReader( s.getInputStream() ));
// read from in
}
catch (IOException e)
{ e.printStackTrace();
}

Writing Servers
 To write a server using java.net.ServerSocket
 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

Writing Servers
 Example: Echo server
ServerSocket s = new ServerSocket(8189);
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

A side note
• Many machines in CSD now refuse socket
connections due to security considerations.
• However, you can
– Run severs on any lab 4 machine and connect to
the server from any other lab 4 machines.
– Run severs on one of scpu1-14 and connect to the
server from others or from the PC network.

 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
Writing Servers

class 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; }
Writing Servers

• A more interesting example
– 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.
– ICQClient.java
• Allows user to connect to ICQServer and have one-to-
one conversation with partner
Servers & Client

• Reason for communicating with web servers
– To retrieve/send information
• Need to indicate location of resource
– URL stands for Uniform Resource Locator
• Neat scheme for uniquely identifying all kinds of network resources
– Basic form <protocol>:<sitename><pathname>
• http://www.cs.ust.hk/~lzhang/comp201/index.html
• ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/HttpTest/HttpTest.jav
a
• file:/MyDisk/Letters/ToMom2-11-98
• Protocols include files, http, ftp, gopher, news, mailto, etc.
Communicating with web servers

• Class java.net.URL represents a Uniform Resource Locator
– Create an java object that represents an URL
URL url = new
URL(“http://www.cs.ust.hk/~lzhang/comp201/index.html”);
• getHost(), getPath(), getPort(), getProtocol()
• java.net.URLConnection represents a communication link between the
application and a URL.
– Constructor:
• URLConnection cnn = new URLConnection( url)
– Obtainable also from URL:
• URLConnection cnn = url.openConnection();

• Steps for working with java.net.URLConnection
– Set properties of connection:
• setDoInPut(true) //default
• setDoOutPut(true) for sending information to the server
• …
– Make connection: cnn.connect();
– Query header information:
• getContentType, getContentLength,
getContentEncoding,
getDate, getExpiration, getLastModified
– getInputStream for reading and getOutputStream
for writing
• API of the class has a more detailed description.

• Can directly open a stream for reading in URL class:
– public final InputStream openStream() throws IOException
url.opentStream()
• Opens a connection to this URL and returns an InputStream for
reading from that connection.
• This method is a shorthand for:
openConnection().getInputStream()
URLTest.java

Retrieving Information
• URLConnectionTest.java
URL url = new URL(urlName);
URLConnection connection = url.openConnection();
connection.connect();
// print header fields
int n = 1;
String key;
while ((key = connection.getHeaderFieldKey(n)) != null)
{
String value = connection.getHeaderField(n);
System.out.println(key + ": " + value);
n++;
}

// print convenience functions
System.out.println("----------");
System.out.println("getContentType: "
+ connection.getContentType() );
System.out.println("getContentLength: "
+ connection.getContentLength() );
System.out.println("getContentEncoding: "
+ connection.getContentEncoding() );
….

// print first ten lines of contents
BufferedReader in = new BufferedReader(new
InputStreamReader( connection.getInputStream() ));
String line;
n = 1;
while ((line = in.readLine()) != null && n <= 10)
{
System.out.println(line);
n++;
}
if (line != null) System.out.println(". . .");

Sending Information
 Web servers receive information from clients using either GET or
POST
 GET requests are requests made by browsers when the user
 types in a URL on the address line,
 follows a link from a Web page, or
 makes an HTML form that does not specify a METHOD or specifically use
the GET method.
 POST requests are generated when someone creates an HTML
form that specifies METHOD="POST"
 Examples:
 http://maps.yahoo.com/py/maps.py: python,
<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">

Sending Information
 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

• Our task: Write java program to communicate
with CGI scripts
– The way we send parameters to a CGI script
depends on
• The parameters that CGI scripts expects
– What to send
• The way a CGI script receives parameters
– How to send
Sending Information

 Send information to CGI script 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)
 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
Sending Information

 Sending information to CGI script using POST:
Open URLConnection and send parameter using a stream
 Open a URLConnection:
URL url = new URL(“http:/host/script”);
URLConnection cnn = url.openConnection();
 Set up connection for output:
cnn.setDoOutput(true);
Sending Information

 Get a stream for sending data:
PrinterWriter out = new
PrintWriter(cnn.getOutputStream());
 Send parameters
Out.print(name1 + “=“ + URLEncoder.encode(value1, “UTF-8”) + “&” );
Out.print(name2 + “=“ + URLEncoder.encode(value2, “UTF-8”) ) + “n”);
Note: URLEncoder: Utility class for HTML form encoding.
This class contains static methods for converting a String to the application/x-www-
form-urlencoded MIME (Multipurpose Internet Mail Extensions ) format.
The World Wide Web Consortium Recommendation states that the UTF-8 encoding
scheme should be used.
PostTest.java
Sending Information

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