Hibernate is free software that is distributed under the GNU Lesser General Public License.
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 attempts to relieve the developer from manual result set handling and object conversion and keep the application portable to all supported SQL databases with little performance overhead.
Mapping
Mapping Java classes to database tables is accomplished through the configuration of an XML file or by using Java Annotations. When using an XML file, Hibernate can generate skeletalsource 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 managereflexive 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.
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.[1]
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.
Hibernate Query Language (HQL)
Hibernate provides an SQL inspired language called Hibernate Query Language (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.
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 CFML.
