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.)