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Java Network Applications
24.1 Java Socket Programming
A socket is a software endpoint that establishes bidirectional communication between
a server program and one or more client programs.
A server program typically provides resources to a network of client programs. Client
programs send requests to the server program, and the server program responds to
the request.
Using threads, a multi-threaded server program can accept a connection from a client,
start a thread for that communication, and continue listening for requests from other
The package provides support for the two common network protocols:
TCP: TCP stands for Transmission Control Protocol, which allows for reliable
communication between two applications. TCP is typically used over the
Internet Protocol, which is referred to as TCP/IP.
UDP: UDP stands for User Datagram Protocol, a connection-less protocol that
allows for packets of data to be transmitted between applications.
When the connection is made, the server creates a socket object on its end of the
communication. The client and server can now communicate by writing to and reading
from the socket.
The following steps occur when establishing a TCP connection between two
computers using sockets:
1. The server instantiates a ServerSocket object, denoting which port number
communication is to occur on.
2. The server invokes the accept() method of the ServerSocket class. This
method waits until a client connects to the server on the given port.
3. A client instantiates a Socket object, specifying the server name and port
number to connect to.
4. The constructor of the Socket class attempts to connect the client to the
specified server and port number. If communication is established, the client
now has a Socket object capable of communicating with the server.
After the connections are established, communication can occur using I/O streams.
Each socket has both an OutputStream and an InputStream. The client's
OutputStream is connected to the server's InputStream, and the client's InputStream
is connected to the server's OutputStream. TCP is a two way communication protocol,
so data can be sent across both streams at the same time.
public class SimpleServer extends Thread
private ServerSocket serverSocket;
public SimpleServer(int port) throws IOException
serverSocket = new ServerSocket(port);
public void run()
System.out.println("Waiting for client on port "+ serverSocket.getLocalPort());
Socket server = serverSocket.accept();
System.out.println("Just connected to " + server.getRemoteSocketAddress());
DataInputStream in = new DataInputStream(server.getInputStream());
DataOutputStream out = new DataOutputStream(server.getOutputStream());
out.writeUTF("Thank you for connecting to " + server.getLocalSocketAddress()
+ "\nGoodbye!");
}catch(SocketTimeoutException s)
System.out.println("Socket timed out!");
}catch(IOException e)
public static void main(String [] args)
// use TCP 6666 port
Thread t = new SimpleServer(6666);
}catch(IOException e)
public class SimpleClient
public static void main(String [] args)
String serverName = "localhost";
int port = 6666;
System.out.println("Connecting to " + serverName + " on port " + port);
Socket client = new Socket(serverName, port);
System.out.println("Just connected to " + client.getRemoteSocketAddress());
OutputStream outToServer = client.getOutputStream();
DataOutputStream out = new DataOutputStream(outToServer);
out.writeUTF("Hello from " + client.getLocalSocketAddress());
InputStream inFromServer = client.getInputStream();
DataInputStream in = new DataInputStream(inFromServer);
System.out.println("Server says " + in.readUTF());
}catch(IOException e)
24.2 Sending E-mail
To send an email using your Java Application is simple enough but to start with you
should have JavaMail API installed on your machine.
You can download latest version of JavaMail from Java's standard website.
Download and unzip files, you will find a number of jar files for both the applications.
You need to add mail.jar files in your CLASSPATH (In Eclipse drag and drop into src
folder, right click mail.jar and select Build Path-> Add to Build Path)
Here is an example to send a simple email via GOOGLE GMAIL SMTP SERVER.
Here it is assumed that your localhost is connected to the internet and capable
enough to send an email.
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;
public class SendEmail {
public static void main(String[] args) {
// Sender's email ID needs to be mentioned
String from = "[email protected]";
String pass = "123456";
// Recipient's email ID needs to be mentioned.
String to = "[email protected]";
String host = "";
// Get system properties
Properties properties = System.getProperties();
// Setup mail server
properties.put("mail.smtp.starttls.enable", "true");
properties.put("", host);
properties.put("mail.smtp.user", from);
properties.put("mail.smtp.password", pass);
properties.put("mail.smtp.port", "587");
properties.put("mail.smtp.auth", "true");
// Get the default Session object.
Session session = Session.getDefaultInstance(properties);
try {
// Create a default MimeMessage object.
MimeMessage message = new MimeMessage(session);
// Set From: header field of the header.
message.setFrom(new InternetAddress(from));
// Set To: header field of the header.
message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
// Set Subject: header field
message.setSubject("This is the Subject Line!");
// Now set the actual message
message.setText("This is actual message");
// Send message
Transport transport = session.getTransport("smtp");
transport.connect(host, from, pass);
transport.sendMessage(message, message.getAllRecipients());
System.out.println("Sent message successfully....");
} catch (MessagingException mex) {
24.3 Java Applets
An applet is a Java program that runs in a Web browser. An applet can be a fully
functional Java application because it has the entire Java API. There are some
important differences between an applet and a standalone Java application, including
the following:
An applet is a Java class that extends the java.applet.Applet class.
A main() method is not invoked on an applet, and an applet class will not
define main().
Applets are designed to be embedded within an HTML page.
When a user views an HTML page that contains an applet, the code for the
applet is downloaded to the user's machine.
A JVM is required to view an applet. The JVM can be either a plug-in of the
Web browser or a separate runtime environment.
The JVM on the user's machine creates an instance of the applet class and
invokes various methods during the applet's lifetime.
Life Cycle of an Applet:
Four methods in the Applet class give you the framework on which you build any
serious applet:
init: This method is intended for whatever initialization is needed for your
start: This method is automatically called after the browser calls the init
method. It is also called whenever the user returns to the page containing the
applet after having gone off to other pages.
paint: Invoked immediately after the start() method, and also any time the
applet needs to repaint itself in the browser.
stop: This method is automatically called when the user moves off the page on
which the applet sits. It can, therefore, be called repeatedly in the same applet.
destroy: This method is only called when the browser shuts down normally.
"Hello, World" Applet:
The following is a simple applet named
import java.applet.*;
import java.awt.*;
public class HelloWorldApplet extends Applet
public void paint (Graphics g)
g.drawString ("Hello World", 25, 50);
Applet CLASS:
Every applet is an extension of the java.applet.Applet class. The base Applet
class provides methods that a derived Applet class may call to obtain information and
services from the browser context.
Invoking an Applet:
An applet may be invoked by embedding directives in an HTML file and viewing the
file through an applet viewer or Java-enabled browser.
The <applet> tag is the basis for embedding an applet in an HTML file. Below is an
example that invokes the "Hello, World" applet:
<title> Hello World Applet</title>
<applet code="HelloWorldApplet.class" width="320" height="120">
The code attribute of the <applet> tag is required. It specifies the Applet class to run.
Width and height are also required to specify the initial size of the panel in which an
applet runs. The applet directive must be closed with a </applet> tag.
Specifying Applet Parameters:
The following example demonstrates how to make an applet respond to setup
parameters specified in the document.
The Applet.getParameter() method fetches a parameter given the parameter's name
(the value of a parameter is always a string).
import java.applet.*;
import java.awt.*;
public class appletParameter extends Applet {
private String strDefault = "Hello! Java Applet.";
public void paint(Graphics g) {
String strParameter = getParameter("Message");
if (strParameter == null)
strParameter = strDefault;
g.drawString(strParameter, 50, 25);
The following is an example of an HTML file with an appletParameter.class
embedded in it. The HTML file specifies both parameters to the applet by means of
the <param> tag.
<title>Applet Test</title>
<h1> This is the applet: </h1>
<applet code="appletParameter.class" width="800" height="100">
<param name="message" value="This is HTML parameter Text.">
Note: Parameter names are not case sensitive.
Java Database Connectivity (JDBC)
25.1 What is JDBC ?
JDBC stands for Java Database Connectivity, which is a standard Java API for
database-independent connectivity between the Java programming language and a
wide range of databases.
The JDBC library includes APIs for each of the tasks commonly associated with
database usage:
Making a connection to a database
Creating SQL or MySQL statements
Executing that SQL or MySQL queries in the database
Viewing & Modifying the resulting records
25.2 JDBC Architecture
The JDBC Architecture consists of two layers:
JDBC API: This provides the application-to-JDBC Manager connection.
JDBC Driver API: This supports the JDBC Manager-to-Driver Connection.
Following is the architectural diagram, which shows the location of the driver manager
with respect to the JDBC drivers and the Java application:
25.3 Common JDBC Components
The JDBC API provides the following interfaces and classes:
DriverManager: This class manages a list of database drivers.
Driver: This interface handles the communications with the database server.
You will interact directly with Driver objects very rarely. Instead, you use
DriverManager objects, which manages objects of this type.
Connection : This interface with all methods for contacting a database.
Statement : You use objects created from this interface to submit the SQL
statements to the database.
ResultSet: These objects hold data retrieved from a database after you
execute an SQL query using Statement objects. It acts as an iterator to allow
you to move through its data.
SQLException: This class handles any errors that occur in a database
25.6 Install Database
Install a database that is most suitable for you. You can have plenty of choices and
most common are:
MySQL DB: MySQL is an open source database. You can download it
from MySQL Official Site. We recommend downloading the full Windows
installation. In addition, download and install MySQL Administrator as well as MySQL
Query Browser. These
are GUI based tools that will make your development much
Finally, download and unzip MySQL Connector/J (the MySQL JDBC driver) in a
convenient directory.
PostgreSQL DB: PostgreSQL is an open source database. You can download
it fromPostgreSQL
Official Site.
The Postgres installation contains a GUI based
administrative tool called pgAdmin III. JDBC drivers are also included as part
of the installation.
Oracle DB: Oracle DB is an commercial database. Oracle installation includes
a GUI based administrative tool called Enterprise Manager. JDBC drivers are
also included as part of the installation.
25.7 Set up MySQL Database
For this tutorial we are going to use MySQL database. When you install any of the
above database, its administrator ID is set to root and gives provision to set a
password of your choice.
Using root ID and password you can either create another users ID and password or
you can use root ID and password for your JDBC application.
There are various database operations like database creation and deletion, which
would need administrator ID and password.
25.7.1 Create Database
To create the EMP database, use the following steps:
Step 1:
Open a Command Prompt and change to the installation directory as follows:
C:\>cd Program Files\MySQL\bin
C:\Program Files\MySQL\bin>
Note: The path to mysqld.exe may vary depending on the install location of MySQL
on your system.
Step 2:
Start the database server by executing the following command, if it is already not
C:\Program Files\MySQL\bin>mysqld
C:\Program Files\MySQL\bin>
Step 3:
Create the EMP database by executing the following command
C:\Program Files\MySQL\bin> mysqladmin create EMP -u root -p
Enter password: ********
C:\Program Files\MySQL\bin>
25.7.2 Create Table
To create the Employees table in EMP database, use the following steps:
Step 1:
Open a Command Prompt and change to the installation directory as follows:
C:\>cd Program Files\MySQL\bin
C:\Program Files\MySQL\bin>
Step 2:
Login to database as follows
C:\Program Files\MySQL\bin>mysql -u root -p
Enter password: ********
Step 3:
Create the table Employee as follows:
mysql> use EMP;
mysql> create table Employees
-> (
-> id int not null,
-> age int not null,
-> first varchar (255),
-> last varchar (255)
-> );
Query OK, 0 rows affected (0.08 sec)
25.7.3 Create Data Records
Finally you create few records in Employee table as follows:
mysql> INSERT INTO Employees VALUES (100, 18, 'Zara', 'Ali');
Query OK, 1 row affected (0.05 sec)
mysql> INSERT INTO Employees VALUES (101, 25, 'Mahnaz', 'Fatma');
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO Employees VALUES (102, 30, 'Zaid', 'Khan');
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO Employees VALUES (103, 28, 'Sumit', 'Mittal');
Query OK, 1 row affected (0.00 sec)
Now you are ready to start experimenting with JDBC. Next tutorial would give your a
sample example on JDBC Programming.
25.8 Creating JDBC Application
There are following six steps involved in building a JDBC application:
Import the packages . Requires that you include the packages containing the
JDBC classes needed for database programming. Most often, using import
java.sql.* will suffice.
Register the JDBC driver: Requires that you initialize a driver so you can
open a communications channel with the database.
Open a connection: Requires the DriverManager.getConnection() method to
create a Connection object, which represents a physical connection with the
Execute a query: Requires using an object of type Statement for building and
submitting an SQL statement to the database.
appropriate ResultSet.getXXX() method to retrieve the data from the result set.
Clean up the environment:
Requires explicitly closing all database
This sample example can serve as a template when you need to create your own
JDBC application in the future.
//STEP 1. Import required packages
import java.sql.*;
public class FirstExample {
// JDBC driver name and database URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/EMP";
// Database credentials
static final String USER = "username";
static final String PASS = "password";
public static void main(String[] args) {
Connection conn = null;
Statement stmt = null;
//STEP 2: Register JDBC driver
//STEP 3: Open a connection
System.out.println("Connecting to database...");
conn = DriverManager.getConnection(DB_URL,USER,PASS);
//STEP 4: Execute a query
System.out.println("Creating statement...");
stmt = conn.createStatement();
String sql;
sql = "SELECT id, first, last, age FROM Employees";
ResultSet rs = stmt.executeQuery(sql);
//STEP 5: Extract data from result set
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
//STEP 6: Clean-up environment
}catch(SQLException se){
//Handle errors for JDBC
}catch(Exception e){
//Handle errors for Class.forName
//finally block used to close resources
}catch(SQLException se2){
}// nothing we can do
}catch(SQLException se){
}//end finally try
}//end try
}//end main
}//end FirstExample
Now let us compile above example as follows:
When you run FirstExample, it produces following result:
C:\>java FirstExample
Connecting to database...
Creating statement...
ID: 100, Age: 18, First: Zara, Last: Ali
ID: 101, Age: 25, First: Mahnaz, Last: Fatma
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal