Hibernate - Overview

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.

Unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always. The LGPL open source license allows the use of Hibernate and NHibernate in open source and commercial projects.

What an object-relational mapper does?

If you are working with object-orientated programming and relational databases, you will surely have noticed that these are two different paradigms. The relational model deals with relations,tuples and sets - it is very mathematical by nature. The object-orientated paradigm however deals with objects, their atributes and associations to each other. As soon as you want to make objects persistent using a relational database you will notice: There is a rift between these two paradigms, the so called object-relational gap. A object-relational mapper (or ORM as a shorthand) will help you bridge that gap.

Well, how does this gap manifest? If you are passing objects around in your application and sometimes reach the point where you want to persist them, you will typically open a JDBC connection, create an SQL statement and copy all your property values over to the PreparedStatement or into the SQL string you are building. This may be easy for a small value object - but consider this for an object with many properties. Thats not the only problem. What about associations? If your Cat object you want to store has a List of kittens contained? Do you store them too? Automatically? Manually? What about foreign key constraints?

The same applies for loading - let's assume you load a Cat object from the database and it has a collection of kittens. Do you load the kittens too? Not load them yet but later? If you load the kittens, consider each of the kitten object has an association to yet more objects. In this case, such eager loading may easily load your complete object tree. Not loading the kittens is however not really better however - you will need explicit reloading later if you probably want to access the kittens.

As you can see, the object-relational gap quickly becomes very wide if you have large object models. And there are a lot more things to consider like lazy loading, circular references, caching, etc. In fact, there have been studies that showed that about 35% of an application code was produced by the mapping between application data and the datastore.

So what can an ORM do for you? A ORM basically intends to takes most of that burden of your shoulder. With a good ORM, you have to define the way you map your classes to tables once - which property maps to which column, which class to which table, etc.