Hibernate (framework)
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. Mapping. 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: 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 Query Language (HQL ). 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. Persistence. 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 . Proper behavior in some applications also requires special attention to the and 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 and 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 class object can be configured to cascade its and operations to its child class objects. Integration. 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. 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 an entity and can be manipulated only with respect to that entity. 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. Unlike J2EE, Hibernate can switch databases. History. Hibernate was started in  2001 by Gavin King with colleagues from Cirrus Technologies as an alternative to using EJB2-style entity beans. The original goal was to offer better persistence capabilities than those offered by EJB2; by simplifying the complexities and supplementing certain missing features. In early 2003, the Hibernate development team began Hibernate2 releases, which offered many significant improvements over the first release. JBoss, Inc. (now part of Red Hat) later hired the lead Hibernate developers in order to further its development. In 2005, Hibernate version 3.0 was released. Key features included a new Interceptor/Callback architecture, user defined filters, and JDK 5.0  Annotations (Java's metadata feature). , Hibernate 3 (version 3.5.0 and up) was a certified implementation of the Java Persistence API 2.0 specification via a wrapper for the Core module which provides conformity with the JSR 317 standard. In Dec 2011, Hibernate Core 4.0.0 Final was released. This includes new features such as multi-tenancy support, introduction of ServiceRegistry (a major change in how Hibernate builds and manages "services"), better session opening from SessionFactory, improved integration via "org.hibernate.integrator.spi.Integrator" and auto discovery, internationalization support, message codes in logging, and a more distinction between the API, SPI or implementation classes. In December  2012, Hibernate ORM 4.1.9 Final was released. In Mar 2013, Hibernate ORM 4.2 Final was released. In December 2013, Hibernate ORM 4.3.0 Final was released. It features Java Persistence API 2.1. In September 2015, Hibernate ORM 5.0.2 Final was released. It has improved bootstrapping, hibernate-java8, hibernate-spatial, Karaf support. In November 2018, Hibernate ORM 5.1.17 Final was released. This is the final release of the 5.1 series. In October 2018, Hibernate ORM 5.3 Final was released. It featured Java Persistence API 2.2 inheritance caching. In December 2018, Hibernate ORM 5.4.0 Final was released. Application programming interface. The Hibernate API  is provided in the Java package org.hibernate. org.hibernate.Session interface. The org.hibernate.Session interface represents a Hibernate session, i.e., the main point of the manipulation performed on the database entities. The latter activities include (among the other things) managing the persistence state (transient, persisted, detached) of the objects, fetching the persisted ones from the database and the management of the transaction demarcation. A session is intended to last as long as the logical transaction on the database. Due to the latter feature, Session implementations are not expected to be thread safe nor to be used by multiple clients. Software components . The Hibernate software includes the following components: