Hibernate : Core Interfaces

Following are Core interfaces provided by Hibernate API

org.hibernate.Session
org.hibernate.SessionFactory
org.hibernate.Criteria
org.hibernate.Query

1. The Session is main runtime interface between a Java application and Hibernate. This is a persistence manager that manages operation like storing and retrieving objects. Instances of Session are inexpensive to create and destroy. They are not thread safe.
2. The main contract here is the creation of Session instances. Usually an application has a single SessionFactory instance and threads servicing client requests obtain Session instances from this factory. SessionFactory instances are not lightweight and typically only one instance is created for the whole application. If the application accesses multiple databases, then it needs to be one per database.
3. Criteria is a simplified API for retrieving entities by composing Criterion objects. The Session is a factory for Criteria. Criterion instances are usually obtained via the factory methods on Restrictions. This provides a provision for conditional search over the resultset. 
4. Query represents object oriented representation of a Hibernate query. A Query instance is obtained by calling Session.createQuery().

Use of persistence.xml file

The EntityManager API is great, but how does the server/ application know which database it is supposed to save / update / query the entity objects?

How do we configure the underlying object-relational-mapping engine and cache for better performance and trouble shooting? The persistence.xml file gives you complete flexibility to configure the EntityManager.

• The persistence.xml file is a standard configuration file in JPA.
• It has to be included in the META-INF directory inside the JAR file that contains the entity beans.
• The persistence.xml file must define a persistence-unit with a unique name in the current scoped classloader.
• The provider attribute specifies the underlying implementation of the JPA EntityManager. 
• In JBoss AS, the default and only supported / recommended JPA provider is Hibernate.
• The jta-data-source points to the JNDI name of the database this persistence unit maps to e.g. java:/DefaultDS.

Disadvantages of Hibernate

1. Steep learning curve.
2. Use of Hibernate is an overhead for the applications which are :(1)simple and use one database that never change. (2)need to put data to database tables, no further SQL queries(3)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.

Difference JDBC and Hibernate

Open-Source, Zero-Cost Product License
Hibernate is an open source and free to use for both development and production deployments

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.

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.

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.

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.

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.

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.

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.

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.