What is Object/Relational Mapping?
Hibernate is concerned with helping your application to achieve persistence. So what is persistence? Persistence simply means that we would like our application's data to outlive the applications process. In Java terms, we would like the state of (some of) our objects to live beyond the scope of the JVM so that the same state is available later.
Specifically, Hibernate is concerned with data persistence as it applies to relational databases (RDBMS). In the world of Object-Oriented applications, there is often a discussion about using an object database (ODBMS) as opposed to a RDBMS. We are not going to explore that discussion here. Suffice it to say that RDBMS remain a very popular persistence mechanism and will so for the foreseeable future.
The Object-Relational Impedance Mismatch
'Object-Relational Impedance Mismatch' (sometimes called the 'paradigm mismatch') is just a fancy way of saying that object models and relational models do not work very well together. RDBMSs represent data in a tabular format (a spreadsheet is a good visualization for those not familiar with RDBMSs), whereas object-oriented languages, such as Java, represent it as an interconnected graph of objects. Loading and storing graphs of objects using a tabular relational database exposes us to 5 mismatch problems...
Sometimes you will have an object model which has more classes than the number of corresponding tables in the database (we says the object model is more granular than the relational model). Take for example the notion of an Address...
2. Subtypes (inheritance)
Inheritance is a natural paradigm in object-oriented programming languages. However, RDBMSs do not define anything similar on the whole (yes some databases do have subtype support but it is completely non-standardized)...
A RDBMS defines exactly one notion of 'sameness': the primary key. Java, however, defines both object identity (a==b) and object equality (a.equals(b)).
Associations are represented as unidirectional references in Object Oriented languages whereas RDBMSs use the notion of foreign keys. If you need bidirectional relationships in Java, you must define the association twice.
Likewise, you cannot determine the multiplicity of a relationship by looking at the object domain model.
5. Data navigation
The way you access data in Java is fundamentally different than the way you do it in a relational database. In Java, you navigate from one association to an other walking the object network.
This is not an efficient way of retrieving data from a relational database. You typically want to minimize the number of SQL queries and thus load several entities via JOINs and select the targeted entities before you start walking the object network.
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