10.1 Introduction

Java Blend, a product of Sun Microsystems, is representative of a class of systems that provide object/relational mapping. In these systems, an application writer creates and operates on Java objects that can be identified as persistent. The Java classes that support persistent objects are mapped to SQL tables, so that a persistent object is actually stored in one or more rows. When an application operates upon a persistent Java object, SQL statements are automatically executed to reflect any changes to the corresponding rows.

Java Blend 1.0 was released by Sun in June 1998. Java Blend provides a mapping tool that allows a user to begin with a database schema and generate Java classes. The classes that Java Blend generates may be augmented by additional methods, turning them into useful business objects. The user may also begin with Java classes and generate a database schema.

Java Blend allows an application to retrieve persistent objects in a nonprocedural way by supporting OQL (Object Query Language), which has been defined by the Object Data Management Group (ODMG). ^[1] OQL bears some similarity to SQL, but we'll see that it provides specific capabilities that are matched to the ODMG and Java data models.

Most of this chapter is devoted to describing Java Blend first, its underlying concepts and then the specific process of building a Java Blend application. Once we've completed this description, we will spend a little while discussing ODMG 2.0. Java Blend is one of several products on the market...