Web programming
Units WEB1P and WEB2P
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Web programmingUnits WEB1P and WEB2P |
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Hibernate is a powerful, high performance object/relational persistence and query service. Hibernate lets you develop persistent classes following object-oriented idiom - including association, inheritance, polymorphism, composition, and collections. Hibernate allows you to express queries in its own portable SQL extension (HQL), as well as in native SQL, or with an object-oriented Criteria and Example API.
Hibernate is one of a number of object-relational tools. These are intended to bridge the gap between objects that you represent in your program, and the relations that you conventionally represent them as in a database. Others include:
Hibernate is so good, most of its features have been incorporated into the Java Persistence API (JPA) and become a standard part of the Java language. It is now possible to use the features of Hibernate directly or (more usually) access them via the JPA interface. In the latter case, Hibernate is the persistence provider, providing an implementation of an entity manager for JPA-managed objects.
What in Hibernate is called a session, in JPA is called an Entity Manager.
The reference implementation for JPA is Oracle Toplink.
Hibernate/JPA are used to manage objects of so-called entity classes. In JPA an entity class is denoted by the annotation @Entity. An entity class normally maps onto a single database table.
Simple properties (e.g. String, int/Integer) of the entity class are by default regarded as columns of the table. Properties of the entity class that are references to other objects are normally regarded as being foreign key relationships to other database tables (though Hibernate/JPA will manage all the foreign key relationships for you).
Relationships between objects are normally annotated @OneToOne, @OneToMany or @ManyToMany. Each of these can take attributes defining details of the relationship. Multi-ended relationships are normally based on one of the Java Collection types (e.g. Set, List or Map). Inverse relationships (back from the referred-to object to the referrer) can also be managed.
Hibernate defines and supports the following object states:
Transient - an object is transient if it has just been instantiated using the new operator, and it is not associated with a Hibernate Session. It has no persistent representation in the database and no identifier value has been assigned. Transient instances will be destroyed by the garbage collector if the application doesn't hold a reference anymore. Use the Hibernate Session to make an object persistent (and let Hibernate take care of the SQL statements that need to be executed for this transition).
Persistent - a persistent instance has a representation in the database and an identifier value. It might just have been saved or loaded, however, it is by definition in the scope of a Session. Hibernate will detect any changes made to an object in persistent state and synchronize the state with the database when the unit of work completes. Developers don't execute manual UPDATE statements, or DELETE statements when an object should be made transient.
Detached - a detached instance is an object that has been persistent, but its Session has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a new Session at a later point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call them application transactions, i.e. a unit of work from the point of view of the user.
Operations are available in a Hibernate session or JPA Entity Manager to move between these states. For example, em.persist(obj) will make a transient object persistent (i.e. its value will be stored in the database).
JPA provides two means of querying a database to return a single or list of objects. These are EJB3-QL (similar to SQL but object-oriented) and criteria. Hibernate provides both of these (its almost identical query language is called HQL) plus a third method - query by example.
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Last updated by Prof Jim Briggs of the School of Computing at the University of Portsmouth |
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| The web programming units include some material that was formerly part of the WPRMP, WECPP, WPSSM and WEMAM units. |