Hibernate is an object-relational mapping (ORM) library for the Java language, providing a framework for mapping an object-oriented domain model to a traditional relational database. Hibernate solves object-relational impedance mismatch problems by replacing direct persistence-related database accesses with high-level object handling functions. Hibernate is free as open source software that is distributed under the GNU Lesser General Public License (formerly the GNU Library General Public License) or LGPL.
Hibernate’s primary feature is mapping from Java classes to database tables (and from Java data types to SQL data types). Hibernate also provides data query and retrieval facilities. Hibernate generates the SQL calls and relieves the developer from manual result set handling and object conversion, keeping the application portable to all supported SQL databases, with database portability delivered at very little performance overhead.
Hibernate was developed by a team of Java software developers around the world led by Gavin King. JBoss, Inc. (now part of Red Hat). The current version of Hibernate is Version 4.x.
Features of Hibernate
1) Mapping: Mapping Java classes to database tables are accomplished through the configuration of an XML file or by using Java Annotation. When using an XML file, Hibernate can generate skeletal source code for the persistence classes. This is unnecessary when annotation is used. Hibernate can use the XML file or the annotation to maintain the database schema. Facilities to arrange one-to-many and many-to-many relationships between classes are provided.
In addition to managing association between objects, Hibernate can also manage reflexive associations where an object has a one-to-many relationship with other instances of its own type. Hibernate supports the mapping of custom value types. This makes the following scenarios possible:
- Overriding the default SQL type that Hibernate chooses when mapping a column to a property
- Mapping Java Enum to columns as if they were regular properties
- Mapping a single property to multiple columns
2) Persistence: Hibernate provides transparent persistence for Plain Old Java Objects (POJOs). The only strict requirement for a persistent class is a no-argument constructor, not necessarily public. Proper behavior in some applications also requires special attention to the equals() and hashCode() methods. Collections of data objects are typically stored in Java collection objects such as Set and List. Java generics, introduced in Java 5, are supported. Hibernate can be configured to lazy load associated collections. Lazy loading is the default as of Hibernate 3.
Related objects can be configured to cascade operations from one to the other. For example, a parent such as an Album object can be configured to cascade its save and/or delete operation to its child Track objects. This can reduce development time and ensure referential integrity. A dirty checking feature avoids unnecessary database write actions by performing SQL updates only on the modified fields of persistent objects.
3) Hibernate Query Language (HQL): Hibernate provides a SQL inspired language called HQL which allows SQL-like queries to be written against Hibernate’s data objects. Criteria Queries are provided as an object-oriented alternative to HQL.
4) Integration: Hibernate can be used both in standalone Java applications and in Java EE applications using Servlets or EJB session beans. It can also be included as a feature in other programming languages. For example, Adobe integrated Hibernate into version 9 of ColdFusion (Which runs on J2EE app servers) with an abstraction layer of new functions and syntax added into ColdFusion Markup Language (CFML).
5) Entities and components: In Hibernate jargon, an entity is a stand-alone object in Hibernate’s persistent mechanism which can be manipulated independently of other objects. In contrast, a component is subordinate to other entities and can be manipulated only with respect to other entities. For example, an Album object may represent an entity but the Tracks object associated with the Album objects would represent a component of the Album entity if it is assumed that Tracks can only be saved or retrieved from the database through the Album object.