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How many Types of Statements in JDBC ?

>> Thursday, March 31, 2011

In JDBC there are three types of Statements:

1).Statement
2).PreparedStatement
3).CallableStatement
Once a connection is obtained we can interact with the database.These three are Interfaces.By using these three statements we can do the operations on Databases using the methods provided in Interfaces.

Java JDBC Prepared Statements

Java JDBC Prepared statements are pre-compiled SQL statements. Precompiled SQL is useful if the same SQL is to be executed repeatedly, for example, in a loop. Prepared statements in java only save you time if you expect to execute the same SQL over again. Every java sql prepared statement is compiled at some point. To use a java preparedstatements, you must first create a object by calling the Connection.prepareStatement() method. JDBC PreparedStatements are useful especially in situations where you can use a for loop or while loop to set a parameter to a succession of values. If you want to execute a Statement object many times, it normally reduces execution time to use a PreparedStatement object instead.

Struts logic tag library important tags

>> Tuesday, March 29, 2011

The "struts-logic" tag library contains tags that are useful in managing conditional generation of output text, looping over object collections for repetitive generation of output text, and application flow management.

Introduction

The Logic library supplies tags that are useful for manipulating presentation logic without the use of scriptlets.

Logic Tag Functionality

The functionality of the logic tags can be divided into four groups:

Value Comparisons - The purpose of these tags is to print out the body of the tag if the comparison evaluates to true.

equal,       notEqual
greaterEqual , lessEqual
greaterThan , lessThan

Substring Matching - The purpose of these tags is to match substrings inside of other Strings

match , notMatch

Presentation Location - The purpose of these tags is to change the location of the presentation page

forward
redirect

Collection Utilities -The purpose of these tags is to handle Collections

iterate

What is lazy loading in Hibernate?

>> Friday, March 25, 2011

What is lazy loading in Hibernate_JavabynataraJLazy setting decides whether to load child objects while loading the Parent Object.You need to do this setting respective hibernate mapping file of the parent class.lazy = true (means not to load child)By default the lazy loading of the child objects is true. This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent.In this case hibernate issues a fresh database call to load the child when getChild() is actully called on the Parent object.But in some cases you do need to load the child objects when parent is loaded. Just make the lazy=false and hibernate will load the child when parent is loaded from the database.

JSP Implicit Objects

>> Wednesday, March 23, 2011

Actually Servlet don't have these implicit Objects After introducing of JSP it contains by default and with readymade Objects to use in our web applications.

These are available for programmer through Container. The implicit objects are parsed by the container and inserted into the generated servlet code. They are available only within the jspService method and not in any declaration. Implicit objects are used for different purposes. Our own methods (user defined methods) can't access them as they are local to the service method and are created at the conversion time of a jsp into a servlet.


These are 9 implicit Objects 

request
response
page
pageContext
application
exception
out  
config
session

Difference between load( ) vs get( ) in Hibernate

The following Hibernate code snippet retrieves a User object from the database:

load()
--------------
1). Only use the load()method if you are sure that the object exists.
2). load() method will throw an exception if the unique id is not found in the database.
3). load() just returns a proxy by default and database won’t be hit until the proxy is first invoked.

What are the JSP page attributes

>> Tuesday, March 22, 2011

Syntax of the declaration of the page directive with it's attributes is <%@ page attributeName="values" %>. The space between the tag <%@ and %> before the page(directive name) and after values of the last attribute, is optional, you can leave the space or not.


Following are name of the attributes of the page directive used in JSP:

JSP Page Directive Attributes


  • language
  • extends
  • import
  • session
  • buffer
  • autoflush
  • info
  • errorPage
  • isErrorPage
  • isThreadSafe

What is the difference between doGet() and doPost() methods ?

>> Monday, March 21, 2011

The difference has given below......


 
doGet

doPost
In doGet Method the parameters are appended to the URL and sent along with header information
In doPost parameters are sent in separate line in the body
Maximum size of data that can be sent using doget is 240 bytes
There is no maximum size for data
Parameters are not encrypted
Parameters are encrypted
DoGet method generally is used to query or to get some information from the server
DoPost is slower compared to doGet since doPost does not write the content length
DoGet should be idempotent. i.e. doget should be able to be repeated safely many times
This method does not need to be idempotent. Operations requested through POST can have side effects for which the user can be held accountable for example updating stored data or buying items online.
DoGet should be safe without any side effects for which user is held responsible.
This method does not need to be either safe.





Disadvantages of Hibernate

>> Sunday, March 20, 2011

The main disadvantages of Hibernate is given in detail.

1) Steep learning curve.

2) Use of Hibernate is an overhead for the applications which are :

simple and use one database that never change
need to put data to database tables, no further SQL queries
there are no objects which are mapped to two different tables
Hibernate increases extra layers and complexity. So for these types of applications JDBC is the best choice.

3) Support for Hibernate on Internet is not sufficient.

JDBC Vs Hibernate

>> Saturday, March 19, 2011


Why is Hibernate better than JDBC

1)   Relational Persistence for JAVA

Working with both Object-Oriented software and Relational Database is complicated task with JDBC because there is mismatch between how data is represented in objects versus relational database. So with JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema. Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.

2)   Transparent Persistence

The automatic mapping of Java objects with database tables and vice versa is called Transparent Persistence. Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS. With JDBC this conversion is to be taken care of by the developer manually with lines of code.

Hibernate communication with RDBMS

General steps:

1. Load the Hibernate configuration file and create configuration object. It will automatically load all hbm           mapping files.
2. Create session factory from configuration object
3. Get one session from this session factory.
4. Create HQL query.
5. Execute query to get list containing Java objects.

Example: Retrieve list of employees from Employee table using Hibernate. /* Load the hibernate configuration file */

Configuration cfg = new Configuration(); cfg.configure(CONFIG_FILE_LOCATION);

/* Create the session factory */

SessionFactory sessionFactory = cfg.buildSessionFactory();

/* Retrieve the session */

Session session = sessionFactory.openSession();

/* create query */

Query query = session.createQuery("from  EmployeeBean”);

/* execute query and get result in form of Java objects */ List finalList = query.list();

EmployeeBean.hbm.xml File


"-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">




















4. Introduction to Hibernate

Hibernate is an Object-Relational Mapping (ORM) solution for JAVA. It is a powerful, high performance object/relational persistence and query service. It allows us to develop persistent classes following object-oriented idiom – including association, inheritance and polymorphism.

 Hibernate Architecture

Hibernate:

1) itself opens connection to database,
2) converts HQL (Hibernate Query Language) statements to database specific statement,
3) receives result set,
4) then performs mapping of these database specific data to Java objects which are directly used by Java application.

Hibernate uses the database specification from Hibernate Properties file. Automatic mapping is performed on the basis of the properties defined in hbm XML file defined for particular Java object.


3.Interaction with RDBMS

These are the General steps to follow  while usingn JDBC.

1) Load the RDBMS specific JDBC driver because this driver actually communicates with the database.
2) Open the connection to database which is then used to send SQL statements and get results back.
3) Create JDBC Statement object. This object contains SQL query.
4) Execute statement which returns resultset(s). ResultSet contains the tuples of database table as a result of SQL query.
5) Process the result set.
6) Close the connection.

Example: Retrieve list of employees from Employee table using JDBC.

String url = “jdbc:odbc:” + dbName;

List employeeList = new ArrayList();

//load the jdbc-odbc driver 

class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);

//Open a connection to database

Connection con = DriverManager.getConnection(url);

//create Statement object

Statement stmt = con.createStatement();

//execute statement
ResultSet rs = stmt.executeQuery("SELECT * FROM Sells"); while ( rs.next() ){
  EmployeeBean eb = new Employeebean(); 
  eb.setName(rs.getString("name")); 
  eb.setSalary(rs.getFloat("salary")); 
  employeeList.add(eb);
}
Reference Books:

1. Introduction to JDBC

The Brief introduction about JDBC .

1. Introduction to JDBC
2. JDBC Architecture
4. Introduction to Hibernate 
5. Hibernate Architecture 
6. Hibernate Communication with RDBMS 
7. Hibernate vs. JDBC 
7.1. Advantage of Hibernate over JDBC 
7.2. Disadvantages of Hibernate

What is the use of Constructor.?

>> Friday, March 18, 2011


A constructor is automatically called and executed at the time of creating an object.


  • It  is used to initialize the instance variables of a class.
  • It is used to create an Object of a class.


Example:



[code]

public class Con{
   Con(){
          ----
         -----
       }
}

[/code]

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