Enterprise JavaBeans 3.1, 6th Edition

Basic Relational Mapping

A developer can take two directions when implementing entity beans. Some applications start from a Java object model and derive a database schema from this model. Other applications have an existing database schema from which they have to derive a Java object model.

The Java Persistence specification provides enough flexibility to start from either direction. If you are creating a database schema from a Java object model, most persistence vendors have tools that can autogenerate database schemas based on the annotations or XML metadata you provide in your code. In this scenario, prototyping your application is fast and easy, as you do not have to define much metadata in order for the persistence engine to generate a schema for you. When you want to fine-tune your mappings, the Java Persistence specification has the necessary annotations and XML mappings to do this.

If you have an existing database schema, many vendors have tools that can generate Java entity code directly from it. Sometimes, though, this generated code is not very object-oriented and doesn’t map to your database very well. Luckily, the Java Persistence specification provides the necessary mapping capabilities to facilitate a solution to this problem.

You will find that your use of annotations and mapping XML will depend on the direction you are coming from. If you are autogenerating your schema from your entity classes, you probably will not need annotations such as @Table and @Column (covered ...