5S Methodology

Java Database Connectivity

• JDBC provides a standard library for accessing relational databases.
• JDBC API standardizes, the approach for connecting to databases, the syntax for sending SQL queries and committing transactions and provides the data structure representing the result.
• By using JDBC you can change database hosts, ports, and even database vendors with minimal changes to your code.

 7 basic steps in using JDBC

1. Load the JDBC driver
2. Define the connection URL
3. Establish the connection
4. Create a Statement object
5. Execute a query or update
6. Process the results
7. Close the connection

Note : Please note that below steps were demonstrated with MySQL Database.

1. Load the JDBC driver

• The driver is the software that knows the way to communicate with database server
• To load the driver, load the appropriate class; a static block in the driver class itself automatically makes a driver instance and registers it with the JDBC driver manager
• Class.forName method takes a string representing a fully qualified class name and loads the corresponding class.

public class SQLConnection {

   

    public void mySQLConnection() {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

2. Define the connection URL

• Specify the location of the database server
• URLs referring to databases use the jdbc: protocol and embed the server host, port, and database name
• The exact format is defined in the documentation that comes with the particular driver

public class SQLConnection {

    public void mySQLConnection() {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

3. Establish the connection

• To make the connection with the database, pass the URL, database username, and database password to the getConnection method of the DriverManager class

        

import java.sql.*;

public class SQLConnection {

    public void mySQLConnection() throws SQLException {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

            Connection con=DriverManager.getConnection(mySqlURL,"root","root");  

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

4. Create a Statement object

• Used to send queries and commands to the database.
• Most of the database drivers permit multiple concurrent Statement objects to be open on the same connection

import java.sql.*;

public class SQLConnection {

    public void mySQLConnection() throws SQLException {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

            Connection con=DriverManager.getConnection(mySqlURL,"root","root");

            Statement stmt=con.createStatement();

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

5. Execute a query or update

• You can use Statement object to send SQL queries by using the executeQuery method, which returns an object of type ResultSet
• executeUpdate method is used for UPDATE, INSERT, or DELETE commands, which will return the number of rows affected, which could be zero

import java.sql.*;

public class SQLConnection {

    public void mySQLConnection() throws SQLException {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

            Connection con=DriverManager.getConnection(mySqlURL,"root","root");

            Statement stmt=con.createStatement();

            String query = "select * from employee";

            ResultSet resultSet = stmt.executeQuery(query);

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

6. Process the results

• Use the next method of ResultSet to move through the table a row at a time
• Within a row, ResultSet provides various getXxx methods that take a column name or column index as an argument and return the result in a variety of different Java types
• The first column in a ResultSet row has index 1, not 0

import java.sql.*;

public class SQLConnection {

    public void mySQLConnection() throws SQLException {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

            Connection con=DriverManager.getConnection(mySqlURL,"root","root");

            Statement stmt=con.createStatement();

            String query = "select * from employee";

            ResultSet resultSet = stmt.executeQuery(query);

            while(rs.next())  

                System.out.println(rs.getInt(1)+"  "+rs.getString(2)+"  "+rs.getString(3));

 

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

7. Close the connection

• Closing the connection also closes the corresponding Statement and ResultSet objects
• Postpone closing the connection if there's to perform additional database operations
• Reusing existing connections is such an important optimization that the JDBC 2.0 API defines a ConnectionPoolDataSource interface for obtaining pooled connections

import java.sql.*;

public class SQLConnection {

    public void mySQLConnection() throws SQLException {

        try {

            Class.forName("com.mysql.cj.jdbc.Driver");

                           // "jdbc:mysql//" + host +":" + port + “/" + dbName;

            String mySqlURL = "jdbc:mysql://localhost:3306/cgj";

            Connection con=DriverManager.getConnection(mySqlURL,"root","root");

            Statement stmt=con.createStatement();

            String query = "select * from employee";

            ResultSet resultSet = stmt.executeQuery(query);

            while(rs.next())  

                System.out.println(rs.getInt(1)+"  "+rs.getString(2)+"  "+rs.getString(3));

            con.close();

 

        } catch (ClassNotFoundException e) {

            System.out.println(e);

        }

    }

}

Servlet

 Handling the Client Request: Form Data

• From data can be attached to the end of the URL for GET requests
• Read the URL
• Separate pairs at the ampersands, then separate the parameter names from the parameter values

• Form data can also be sent to the server on a separate line for POST request.

Previous Chapter : Introduction to J2EE

Next Chapter : JSP

Introduction to J2EE

 

 Web Application Model

Servlet

Servlets are Java programs that run on Web or application servers, acting as a middle layer between requests coming from Web browsers or other HTTP clients and databases or applications on the HTTP server

Tasks Performed

• Read the explicit data sent by the client
• Read the implicit HTTP request data sent by the browser
• Generate the results
• Send the explicit data (i.e., the document) to the client.
• Send the implicit HTTP response data

Advantages of Servlets

• Efficient
• If there are many requests to the same servlet, only a single copy of the servlet class would be loaded
• Convenient
• Have utilities like parsing, decoding HTML form data, cookie & session handling, etc.
• Powerful
• Servlets can communicate directly to the Web server
• Multiple servlets can also share data
• Portable
• Run on All operating systems and servers
• Inexpensive
• Secure
• Servlets are not executed by general-purpose operating system shells
• No buffer overflow attacks

What is a container? (Servlet Engine)

Servlets don’t have a main method. Because of that, they are under the control of another java application called container : Tomcat is an example of a container

Servlet Life Cycle

• init
• Executed once when the servlet is first loaded or at server start.
• Performs initializations such as setting up a database connection, loading a data file, etc.

• service
• Each time the server receives a request for a servlet, the server spawns a new thread and calls service(). 
• The service method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc., as appropriate.
• destroy
• Called when server deletes servlet instance.
• It will close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities

Basic Servlet Structure

Web Application Structure

Next Chapter : Servlet

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