Hibernate [Electronic resources] : A Developers Notebook نسخه متنی

Chapter 2. Introduction to Mapping

Generating Some Class

Cooking Up a Schema

Connecting Hibernate to MySQL

Now that we're in a position to work with Hibernate, it's worth pausing
to reflect on why we wanted to in the first place, lest we remain lost in
the details of installation and configuration. Object-oriented languages
like Java provide a powerful and convenient abstraction for working with
information at runtime in the form of objects that instantiate classes.
These objects can link up with each other in a myriad of ways, and they
can embody rules and behavior as well as the raw data they represent.
But when the program ends, all the objects swiftly and silently vanish.

For information we need to keep around between runs, or share between
different programs and systems, relational databases have proven to be
hard to beat. They're scalable, reliable, efficient, and extremely flexible.
So what we need is a means of taking information from a SQL database
and turning it into Java objects, and vice versa.

There are many different ways of doing this, ranging from completely
manual database design and coding, to highly automated tools. The general
problem is known as Object/Relational Mapping, and Hibernate is a
lightweight O/R mapping service for Java.

The 'lightweight' designation means it is designed to be fairly simple to
learn and use, and to place reasonable demands on system resources,
compared to some of the other available tools. Despite this, it manages to
be broadly useful and deep. The designers have done a good job of figuring
out the kinds of things that real projects need to accomplish, and
supporting them well.

You can use Hibernate in many different ways, depending on what
you're starting with. If you've got a database that you need to interact
with, there are tools that can analyze the existing schema as a starting
point for your mapping, and help you write the Java classes to represent
the data. If you've got classes that you want to store in a new database,
you can start with the classes, get help building a mapping document,
and generate an initial database schema. We'll look at some of these
approaches later.

For now, we're going to see how you can start a brand new project, with
no existing classes or data, and have Hibernate help you build both.
When starting from scratch like this, the most convenient place to begin
is in the middle, with an abstract definition of the mapping we're going
to make between program objects and the database tables that will store
them.

In our examples we're going to be working with a database that could
power an interface to a large personal collection of music, allowing users
to search, browse, and listen in a natural way. (You might well have
guessed this from the names of the database files that were created at
the end of the first chapter.)