Java networking programs - theory

Java – Networking Theory Page 1

Java – Networking
Theory
Socket Basics
In client-server applications, the server provides some service, such as processing database queries or sending
out web pages. The client uses the service provided by the server, either displaying database query results to
the user or displaying the web page in a browser. The communication that occurs between the client and the
server must be reliable. That is, no data can be dropped and it must arrive on the client side in the same order
in which the server sent it.

TCP provides a reliable, point-to-point communication channel that client-server applications on the Internet
use to communicate with each other. To communicate over TCP, a client program and a server program
establish a connection to one another. Each program binds a socket to its end of the connection. To
communicate, the client and the server each reads from and writes to the socket bound to the connection.

What Is a Socket?
A socket is one end-point of a two-way communication link between two programs running on the network.

A socket is a connection between two hosts. It can perform seven basic operations:
1. Connect to a remote machine
2. Send data
3. Receive data
4. Close a connection
5. Bind to a port
6. Listen for incoming data
7. Accept connections from remote machines on the bound port

The java.net package provides two classes--Socket and ServerSocket--that implement the client
side of the connection and the server side of the connection, respectively.

The Socket Class
1. The java.net.Socket class is used for performing client-side TCP operations. Other client-
oriented classes that make TCP network connections, such as URL, and URLConnection all
ultimately end up invoking the methods of this class.
2. The methods of the Socket class set up and tear down connections and set as various socket
options.
3. Because TCP sockets are more or less reliable connections, the interface that the Socket class
provides to the programmer is streams. The actual reading and writing of data over the socket is
accomplished via stream classes.
4. Java's Socket class, which is used by both clients and servers, has methods that correspond to the
first four of the seven operations stated above. [The last three operations are needed only by
servers, which wait for clients to connect to them. They are implemented by the ServerSocket
class].
5. Constructors of the Socket class:
a) public Socket (String host, int port)
This connects a TCP socket to the given host and port.

mukeshtekwani@hotmail.com Prof. Mukesh N. Tekwani

2 Java – Networking Theory

Example of use of Socket constructor:
try
{
Socket s = new Socket("www.google.com", 80);
// send and receive data...
}
catch (UnknownHostException e)
{
System.err.println(e);
}
catch (IOException e)
{
}

b) public Socket(InetAddress host, int port) throws IOException
This constructor creates a TCP socket to the specified port on the specified host and tries to
connect. It differs from the previous constructor in that it uses an InetAddress object to
specify the host instead of a hostname. It throws an IOException if it can't connect, but does
not throw an UnknownHostException; if the host is unknown, you will find out when you
create the InetAddress object.

Example:

try
{
InetAddress iadd = InetAddress.getByName("www.google.com");

Socket s = new Socket(iadd, 80);
// send and receive data...
}
catch (UnknownHostException e)
{
}
{
}

6. Other methods of Socket class:
a) Socket() – creates a socket that has not yet been initialized.
b) void connect(SocketAddress add) - connects the socket to the given address
c) void connect(SocketAddress add, int timeoutInMillisecs) –
connects the socket to the given address or returns if the time interval expired.
d) void setTimeout(int timeoutMillisecs) - sets the blocking time for read
requests on the socket. When the time out is reached, the InterruptedIOException is raised.
e) boolean isConnected() – returns true if the socket is connected
f) boolean isClosed() – returns true if the socket is closed.

Prof. Mukesh N Tekwani mukeshtekwani@hotmail.com

Java - Networking Page 3

7. Java programs use client sockets in the following fashion:
a) The program creates a new socket with a Socket( ) constructor.
b) The socket attempts to connect to the remote host.
c) Once the connection is established, the local and remote hosts get input and output streams
from the socket and use those streams to send data to each other. This connection is full-
duplex; both hosts can send and receive data simultaneously. What the data means depends
on the protocol; different commands are sent to an FTP server than to an HTTP server. There
will normally be some agreed-upon hand-shaking followed by the transmission of data from
one to the other.
d) When the transmission of data is complete, one or both sides close the connection. Some
protocols, such as HTTP 1.0, require the connection to be closed after each request is
serviced. Others, such as FTP, allow multiple requests to be processed in a single connection.

The ServerSocket class:
1. The ServerSocket class contains everything you need to write servers in Java.
2. It has constructors that create new ServerSocket objects, methods that listen for connections on a
specified port, and methods that return a Socket object when a connection is made so that you
can send and receive data.

The basic life cycle of a server socket is:
1. A new ServerSocket is created on a particular port using a ServerSocket( ) constructor.
2. The ServerSocket listens for incoming connection on that port using its accept( ) method. When
the client connects to the server, accept( ) method returns a Socket object connecting the client
and the server.
3. Depending on the type of server, either the Socket's getInputStream() method,
getOutputStream( ) method, or both are called to get input and output streams that communicate
with the client.
4. The server and the client interact according to an agreed-upon protocol until it is time to close
the connection.
5. The server, the client, or both close the connection.
6. The server returns to step 2 and waits for the next connection.

If step 4 is likely to take a long or indefinite amount of time, certain servers create a new process to
handle each connection so that multiple clients can be serviced at the same time. Java programs
should spawn a thread to interact with the client so that the server can be ready to process the next
connection sooner.

Constructors of the ServerSocket class:
1. public ServerSocket(int port) throws IOException, BindException
This constructor creates a server socket on the port specified by the argument. If you don’t
pass the port number, the system selects an available port for you. A port chosen for you by
the system is sometimes called an anonymous port since you don't know its number.

2. public ServerSocket(int port, int queueLength) throws IOException,
BindException
This constructor creates a ServerSocket on the specified port with a queue length of your
choosing. The queueLength argument sets the length of the queue for incoming connection
requests — that is, howmany incoming connections can be stored at one time before the
host starts refusing connections. Some operating systems have a maximum queue length,


typically five. For example, to create a server socket on port 5776 that would hold up to 100
incoming connection requests in the queue, you would write:

try
{
ServerSocket httpd = new ServerSocket(5776, 100);
}
{
}
The constructor throws a BindException if the socket cannot be created and bound to the
requested port. An IOException means that the port is already in use.

3. public ServerSocket(int port, int queueLength, InetAddress
bindAddress) throws IOException
• This constructor creates a ServerSocket on the specified port with the specified queue length.
This ServerSocket binds only to the specified local IP address.
• This constructor is useful for servers that run on systems with several IP addresses because it
allows you to choose the address to which you'll listen. That is, this ServerSocket listens only
for incoming connections on the specified address; it won't listen for connections that come
in through the host's other addresses. The other constructors bind to all local IP addresses by
default.

Other methods of the ServerSocket class:
Socket accept() –
A ServerSocket generally operates in a loop that repeatedly accepts connections. Each pass
through the loop invokes the accept() method. This method blocks (stops the flow of
execution) the current thread until the connection is made. The method returns a Socket
object through which the program can communicate with the client.

void close() –
Closes the server socket an stop processing any further requests.

Internet Addresses:
1. Internet addresses are numerical addresses that consist of 4 bytes in IPv4 (or 16 bytes in IPv6).
2. These addresses are written as a dotted quad like this: 132.163.4.102. Each of these quads can
vary from 0 to 255.
3. When your browser requests a web page from another computer on the Internet, it automatically
sends your computer’s address where the server should send the web page. This is called your
computer’s IP address.
4. It is easier for human beings to deal with host names such as www.google.com instead of the
numeric IP addresses.
5. IP address can be static or dynamic. A static IP address is a fixed address and cannot be changed.
A dynamic IP address changes every time the user connects to the Internet.
6. The java.net package supports IPv6 addresses provided the host computer’s OS supports these
addresses.



7. The java.net package has the InetAddress class to convert between host names and Internet
addresses.
8. Methods of InetAddress class:
a) public static InetAddress getByName(String host) throws
UnknownHostException – this method determines the IP address of a host if the host
name is given.
b) static InetAddress [] getAllByName(String host) - constructs an array
of an Internet addresses for the given host name
c) static InetAddress getLocalHost() – constructs an InetAddress for the local
host
d) byte [] getAddress() – returns an array of bytes that contains numerical address.
e) String getHostAddress() – returns a string with decimal numbers separated by
periods, e.g., “132.163.4.102”
f) String getHostName() – returns the host name

The URL
1. URL is the abbreviation of Uniform Resource Locator.
2. It is a reference or an address to a resource on the Internet (e.g., java.sun.com)
3. The URL is the address of a web page; each web page has its own unique address.
4. A URL can be split into 5 parts.
Consider the URL http://www.somesite.com/dcn/chap1.html
The 5 parts of the URL are as follows:
(i) http:// - stands for the protocol. The most common protocol is the HyperText
Transfer Protocol. Others are FTP and HTTPS (Secure).
(ii) www – This is the sub domain. Some sites use the sub domain ftp, news, etc.
(iii) somesite.com – It is the host or domain name. www and the domain name combined
together give the IP address. Thus, www.somesite.com can be replaced by an IP
address.
(iv) dcn – It is a folder or directory and all files are stored in this directory.
(v) chap1.html - It is the file which is fetched from the server. If a filename is not
specified, the default file is index.html
5. The URL class
The URL class has the following constructors:
a) URL (String str) - Creates a URL object from the specified string
b) URL(URL baseurl, String relativeurl);
c) URL(String protocol, String host, int port, String file) – Creates a URL object from the
specified protocol, host, port number and file.
d) URL(String protocol, String host, String file) – Creates a URL from the specified
protocol, hostname and file name.
e) The URL object can be constructed like this:
URL add = new URL(http://www.dnaindia.com);
Such a URL is called an absolute URL. It contains all the information necessary to
reach the resource

We can also create a relative URL which contains just enough information to reach
the resource relative to another URL.



Example: Consider the base URL http://www.somesite.com/pages Suppose the pages
on this site are page1.html and page2.html. We can create URL objects for these
pages relative to their common base URL like this:

URL add = new URL(“http://www.somesite.com/pages”);
URL pg1url = new URL(add, “page1.html”);
URL pg2url = new URL(add, “page2.html”);

[Ref Program 2 in Java Networking Programs notes]

The URLConnection Class
1. The abstract class URLConnection is the superclass of all classes that represent a
communications link between the application and a URL.
2. Instances of this class can be used both to read from and to write to the resource referenced by
the URL.
3. Creating a connection to a URL requires these steps:
i. The connection object is created by invoking the openConnection method on a URL.
ii. The setup parameters and general request properties are manipulated.
iii. The actual connection to the remote object is made, using the connect method.
iv. The remote object becomes available. The header fields and the contents of the remote
object can be accessed.
4. The following methods are used to access the header fields and the contents after the connection
is made to the remote object:
• getContent() - Retrieves the contents of this URL connection.
• getHeaderField() - Returns the value of the named header field.
• getInputStream() - Returns an input stream that reads from this open connection.
• getOutputStream() - Returns an output stream that writes to this connection.
5. Certain header fields are accessed frequently. These are:
• getContentEncoding - Returns the value of the content-encoding header field.
• getContentLength - Returns the value of the content-length header field.
• getContentType - Returns the value of the content-type header field.
• getDate - Returns the value of the date header field.
• getExpiration - Returns the value of the expires header field.
• getLastModifed – Returns the last modified date

Serving Multiple Clients
1. A typical server runs on a server computer and clients from all over the Internet might want to
use the server at the same time. The server must be able to accept incoming requests from more
than one client at a time. This can be done by using threaded programs in Java.
2. Evert time a program has established a new socket connection we will launch a new thread to
take care of the connection between the server and that client.
3. The main program will go back and wait for the next connection.
4. Thus, we have to create a loop on the server as follows:



while(true)
{
Socket incoming = s.accept();
Runnable r = new ThreadedEchoHandler(incoming);

Thread t = new Thread(r);
t.start();
}

The ThreadedEchoHandler class implements the Runnable interface and contains the loop
with the client in its run method.

class ThreadedEchoHandler implements Runnable
{
…
public void run()
{
try
{
Inputstream inps = incoming.getInputStream();
OutputStream outs = incoming.getOutputStream();

//Process input and send response
incoming.close();
}
catch(Exception ex)
{
Handle exception
}
}
}

Since each connection starts a new thread, multiple clients can connect to the server at the
same time.

Sending E-Mail
1. A socket program can be used to send email.
2. To send an email, we have to make a socket connection to port 25 which is the SMTP
port (SMTP= Simple Mail Transfer Protocol). The SMTP describes the format for email
messages.
3. Steps are as follows:
i. Open a socket to the host system
Socket s = new Socket(“mail.yourserver.com”, 25); // 25 is for SMTP
PrintWriter out = new PrintWriter(s.getOutputStream());
[PrintWriter class is used to format representations of objects to a text-output
stream. Import the package java.io.Writer for this class.]
ii. Send the following information to the print stream:
HELO sending host
MAIL FROM: <sender e-mail address>



RCPT TO : <recipient e-mail address>
DATA
mail message
(any number of lines)

QUIT

The SMTP specification requires that all lines must be terminated with r followed
by n.

Understanding Ports
Generally speaking, a computer has a single physical connection to the network. All data destined for a
particular computer arrives through that connection. However, the data may be intended for different
applications running on the computer. So how does the computer know to which application to forward the
data? Through the use of ports.

Data transmitted over the Internet is accompanied by addressing information that identifies the computer and
the port for which it is destined. The computer is identified by its 32-bit IP address, which IP uses to deliver
data to the right computer on the network. Ports are identified by a 16-bit number, which TCP and UDP use
to deliver the data to the right application.

In connection-based communication such as TCP, a server application binds a socket to a specific port
number. This has the effect of registering the server with the system to receive all data destined for that port.
A client can then rendezvous with the server at the server's port, as illustrated here:

Definition: The TCP and UDP protocols use ports to map incoming data to a particular process
running on a computer.

In datagram-based communication such as UDP, the datagram packet contains the port number of its
destination and UDP routes the packet to the appropriate application, as illustrated in this figure:

Port numbers range from 0 to 65,535 because ports are represented by 16-bit numbers. The port numbers
ranging from 0 - 1023 are restricted; they are reserved for use by well-known services such as HTTP and FTP
and other system services. These ports are called well-known ports. Your applications should not attempt to
bind to them.



IMPORTANT QUESTIONS
1. Explain the terms socket and port.
2. How can you get the IP address of a machine from its host name?
3. How can you find out the current IP address of your computer?
4. What is ServerSocket and how is it used?
5. Network server programs are often multithreaded. Explain what this means and why it is true.
6. Using the URL class, write a program to retrieve the file
http://www.timesofindia.com/index.html and return the last modified date of this page.
7. Explain the URL class.
8. Explain the URLConnection class
9. Explain the following methods of the URL class: getHost(), getPort(), getProtocol()
10. How can DatagramSocket and DatagramPacket be used to send information across two
computers in a network?
11. Write a Java program to accept a URL from the command line. If it uses the HTTP protocol then
find the length of its contents.


Java networking programs - theory

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