How many Types of Statements in JDBC ?

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.

Statement: Use for general-purpose access to your database. Useful when you are using static SQL statements at runtime. The Statement interface cannot accept parameters.

PreparedStatement   : Use when you plan to use the SQL statements many times. The PreparedStatement interface accepts input parameters at runtime.

CallableStatement   : Use when you want to access database stored procedures. The CallableStatement interface can also accept runtime input parameters.

Read more...

Java 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.

The syntax is straightforward: just insert question marks for any parameters that you'll be substituting before you send the SQL to the database. As with CallableStatements, you need to call close() to make sure database resources are freed as soon as possible. Below is a JDBC Program showing the use of jdbc prepared statements to insert data into tables using jdbc programming.

You need to supply values to be used in place of the question mark placeholders (if there are any) before you can execute a PreparedStatement object. You do this by calling one of the setXXX methods defined in the PreparedStatement class. There is a setXXX method for each primitive type declared in the Java programming language.

PreparedStatement pstmt = con.prepareStatement("update Orders set pname = ? where Prod_Id = ?");
pstmt.setInt(2, 100);
pstmt.setString(1, "Bob");
pstmt.executeUpdate();

An important feature of a PreparedStatement object is that, unlike a Statement object, it is given an SQL statement when it is created. This SQL statement is sent to the DBMS right away, where it is compiled. As a result, the PreparedStatement object contains not just an SQL statement, but an SQL statement that has been precompiled. This means that when the PreparedStatement is executed, the DBMS can just run the PreparedStatement SQL statement without having to compile it first.

Using Prepared Statements in jdbc, objects can be used for SQL statements with no parameters, you probably use them most often for SQL statements that take parameters. The advantage of using SQL statements that take parameters is that you can use the same statement and supply it with different values each time you execute it.

 source: http://www.jdbc-tutorial.com/

Read more...

Struts logic tag library important tags

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

Read more...

What is lazy loading in Hibernate?

Lazy 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.
                     
                                Example: lazy=true (default)Address child of User class can be made lazy if it is not required frequently. lazy=false but you may need to load the Author object for Book parent whenever you deal with the book for online bookshop.

Lazy fetching means for example in hibernate if we use load() method then load() is lazy fetching i.e it is not going to touch the database until we write empobject.getString( eno );So when we write above statement in that instance it touch the database and get all the data from database. It is called as lazy loading.If we see another example i.e session.get(...) method is used then at that instance it is going to touch the database and get the data and place the data in session object it is called as eager loading.

Read more...

JSP Implicit Objects

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

Read more...

Hibernate load( ) vs get( )

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.

Session session = << Get session from SessionFactory >>
Long itemId = << Get the item id from request >>

try{
   Item item = session.load(Item.class, itemId);
   Bid bid = new Bid();
   bid.setItem(item);
   session.saveOrUpdate(bid);
} catch(ObjectNotFoundException e) {
   log.error("Bid placed for an unavailable item");
   // Handle the error condition appropriately
}

get( )
---------
1). If you are not sure that the object exists, then use one of the get() methods.
2). get() method will return null if the unique id is not found in the database.
3). get() will hit the database immediately. 

Session session = << Get session from SessionFactory >>
Long itemId = << Get the item id from request >>

Item item = (Item) session.get(Item.class, itemId);

if(item != null) {
   Bid bid = new Bid();
   bid.setItem(item);
   session.saveOrUpdate(bid);
} else {
   log.error("Bid placed for an unavailable item");
   // Handle the error condition appropriately
}

For References:

   

Read more...

What are the JSP page attributes

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

Read more...

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

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.

Read more...

Disadvantages of Hibernate

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.

4) Anybody wanting to maintain application using Hibernate will need to know Hibernate.

5) For complex data, mapping from Object-to-tables and vise versa reduces performance and increases time of conversion.

6) Hibernate does not allow some type of queries which are supported by JDBC. For example It does not allow to insert multiple objects (persistent data) to same table using single query. Developer has to write separate query to insert each object.

Read more...

JDBC Vs Hibernate

7.1 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.

3)   Support for Query Language

JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e to select effective query from a number of queries to perform same task. Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.

4)   Database Dependent Code

Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table. Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.

5)   Maintenance Cost

With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually. Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.

6)   Optimize Performance

Caching is retention of data, usually in application to reduce disk access. Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many


times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code. With JDBC, caching is maintained by hand-coding.

7)   Automatic Versioning and Time Stamping

By database versioning one can be assured that the changes done by one person is not being roll backed by another one unintentionally. Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow to save it because this user does not has updated data. In JDBC there is no check that always every user has updated data. This check has to be added by the developer.

8)   Open-Source, Zero-Cost Product License

Hibernate is an open source and free to use for both development and production deployments.

9)   Enterprise-Class Reliability and Scalability

Hibernate scales well in any environment, no matter if use it in-house Intranet that serves hundreds of users or for mission-critical applications that serve hundreds of thousands. JDBC can not be scaled easily.

Read more...

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">

Read more...

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.

Read more...

3. Interaction with RDBMS

General steps:

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);

}

Read more...

1. Introduction to JDBC

JDBC stands for Java Database Connectivity allows developers to connect, query and update a database using the Structured Query Language. JDBC API standard provides Java developers to interact with different RDBMS and access table data through Java application without learning RDBMS details and using Database Specific JDBC Drivers.

2.  JDBC Architecture

JDBC makes the interaction with RDBMS simple and intuitive. When a Java application needs to access database:

• open connection to database,

• use JDBC driver to send SQL queries to database,

• process the results that are returned, and

• close the connection.

JDBC uses two architectures to communicate with database:

1) The driver connects to database and executes SQL statements. Results are sent back from driver to driver manager and finally to the application.
2) The JDBC driver communicates with ODBC driver. ODBC driver executes SQL query and then results are sent back to JDBC driver to driver manager and then to application.

1. Introduction to JDBC

2. JDBC Architecture 

3. Interaction of JDBC with Database 

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 

Read more...

What is the use of Constructor.?

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]

Read more...