Data persistence with Hibernate
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A persistence framework is middle ware that assists in the storage and retrieval of information between applications and databases, especially relational databases. It acts as a layer of abstraction for persisted data, bridging conceptual and technical differences between storage and utilisation.
Many persistence frameworks are also object-relational mapping (ORM) tools (e.g. Hibernate, MyBatis SQL Maps, Apache Cayenne, Entity Framework, Slick, and Java Ultra-Lite Persistence). Such frameworks map the objects in the application domain to data that needs to be persisted in a database. The mappings can be defined using either XML files or metadata annotations.
Hibernate ORM (or simply Hibernate) is an object–relational mapping tool for the Java programming language. It provides a framework for mapping an object-oriented domain model to a relational database. Hibernate handles object–relational impedance mismatch problems by replacing direct, persistent database accesses with high-level object handling functions.
Hibernate is free software that is distributed under the GNU Lesser General Public License 2.1.
Hibernate’s primary feature is mapping from Java classes to database tables, and mapping from Java data types to SQL data types. Hibernate also provides data query and retrieval facilities. It generates SQL calls and relieves the developer from the manual handling and object conversion of the result set.
The mapping of Java classes to database tables is implemented by the configuration of an XML file or by using Java Annotations. When using an XML file, Hibernate can generate skeleton source code for the persistence classes. This is auxiliary when annotations are used. Hibernate can use the XML file or the Java annotations to maintain the database schema.
There are provided facilities to arrange one-to-many and many-to-many relationships between classes. In addition to managing associations between objects, Hibernate can also manage reflexive associations wherein an object has a one-to-many relationship with other instances of the class type.
Hibernate supports the mapping of custom value types. This makes the following scenarios possible:
- Overriding the default SQL type when mapping a column to a property.
- Mapping Java Enums to columns as though they were regular properties.
- Mapping a single property to multiple columns.
Definition: Objects in an object-oriented application follow OOP principles, while objects in the back-end follow database normalization principles, resulting in different representation requirements. This problem is called “object–relational impedance mismatch”. Mapping is a way of resolving the object–relational impedance mismatch problem.
Mapping informs the ORM tool of what Java class object to store in which database table.
Hibernate provides a SQL inspired language called Hibernate Query Language (HQL) for writing SQL-like queries against Hibernate’s data objects. Criteria Queries are provided as an object-oriented alternative to HQL. Criteria Query is used to modify the objects and provide the restriction for the objects. HQL (Hibernate Query Language) is the object-oriented version of SQL. It generates database independent queries so that there is no need to write database-specific queries. Without this capability, changing the database would require individual SQL queries to be changed as well, leading to maintenance issues.
Hibernate provides transparent persistence for Plain Old Java Objects (POJOs). The only strict requirement for a persistent class is a no-argument constructor, though not necessarily public. Proper behavior in some applications also requires special attention to the equals() and hashCode() methods in the object classes. Hibernate recommends providing an identifier attribute, and this is planned to be a mandatory requirement in a future release.
Collections of data objects are typically stored in Java collection classes, such as implementations of the Set and List interfaces. Java generics, introduced in Java 5, are also 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 object to the other. For example, a parent Album class object can be configured to cascade its save and delete operations to its child Track class objects.
Hibernate can be used both in standalone Java applications and in Java EE applications using servlets, EJB session beans, and JBI service components. 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.