1.3 Persistence Models

A persistence model dictates how your components persist themselves to a data store. The EJB component model, for example, comes with a built-in persistence model in the form of container-managed persistence. When you use the container-managed persistence model with Enterprise JavaBeans, you do not have to write a line of database access code. Instead, you specify a database mapping when you deploy the application through deployment configuration files. The EJB container manages the rest.

Other persistence models may perform all or part of the work necessary to implement component persistence. Over the course of this book, we will examine the most popular persistence models for both the JavaBeans and EJB component models.

1.3.1 EJB Persistence

Until recently, container-managed persistence in EJB has proven to be insufficient for most application development. Most EJB developers write their own persistence logic for their beans—a practice called bean-managed persistence. Bean-managed EJB programmers use some other persistence model when constructing their components. The full gamut of EJB-specific persistence models include:

EJB 1.x CMP

This model is container-managed persistence (CMP) under the EJB 1.x specification. As I noted earlier, few people actually use this persistence model. It has difficulties with such basic persistence operations as searching and primary key management. It also does not provide a solid mapping of many-to-many relationships.

BMP

Bean-managed persistence (BMP) is not itself a persistence model. It instead means that you are using some non-EJB persistence model for storage of your EJB components.

EJB 2.x CMP

Because of the massive shortcomings of EJB 1.x CMP, EJB 2.0 introduced a new container-managed persistence model. Though it is not yet widely supported, it does promise to address the shortcomings of EJB 1.x CMP. To meet this challenge, however, it introduces a new query language. Because many alternative persistence models have evolved, it is unclear if EJB 2.x CMP will be accepted.

1.3.2 Other Persistence Models

Many persistence models have evolved both to address shortcomings in EJB persistence models and to support the persistence of non-EJB systems. If you have read my book Database Programming with JDBC and Java, then you have seen one such alternative persistence model. Among today's most popular alternatives is Java Data Objects (JDO). We will examine JDO and my custom persistence model as well as several others over the course of this book.