t a book signing in the mid-1980s, not long after I got out of school with my newly minted computer science degree, I ran into E.F. Codd, the IBM researcher who originated the relational database model. As you might expect, Codd was none too enthusiastic about the term and practice of "object-oriented" development, just then being popularized by methodologists such as Grady Booch, who went on to create (with Jim Rumbaugh and Ivar Jacobsen) the Unified Modeling Language (UML). "You might as well describe birds as 'air-oriented' creatures," Codd complained.
But in the past decade and a half, object technology has been in the ascendancy. Database administrators, who ruled the mainframe and client/server development roost, have been forced to increasingly share power in IT shops as developers have scrambled to adopt object-oriented (OO) languages such as C++ and Java. For the past few years, object and data professionals have brokered an uneasy truce, especially with respect to technology such as Enterprise JavaBeans (EJBs) that are written in Java but typically stored in relational databases such as Oracle or IBM's DB2, which use non-object technology.
To overcome the well-known impedance mismatch problem that exists in relational DBMSs, where an application programmer is forced to work in a language (such as Java or C++) that has a syntax, semantics, and type system different from the data manipulation language (that is, SQL) of the DBMS, many developers have resorted to using some sort of object-relational mapping. Author and consultant Scott Ambler has done the majority of the heavy lifting in this area, with his advocacy of a vendor-neutral UML Persistence Model profile defined as part of the upcoming UML 2.0 standard. (For background, see his white paper "Mapping Objects to Relational Databases: What You Need to Know and Why," posted at IBM DeveloperWorks, or Software Development magazine's "Thinking Objectively" columns.
The EJB UML mapping
A profile is defined as a specialized use of UML within the UML specification, and some of the more interesting work currently going on inside the Java Community Process (the Sun-led participatory process that develops and revises Java technology specifications) revolves around the creation of an EJB UML-mapping profile. The JSR-000026 UML profile for EJB defines a set of extensions to UML that can be used to model software implemented with Enterprise JavaBeans in UML. These extensions will let Java IDE, app server and other enterprise tool vendors provide EJB modeling capabilities using UML within their tools, as well as forward and reverse engineering between UML models and EJB implementations. The specification defines an XML DTD for a file placed within the EJB-JAR that identifies a UML model stored in that EJB-JAR and its relationship to other EJBs in the same EJB-JAR. This will allow enterprise tool and framework vendors to use Java's automation and reflection APIs to access UML models stored in EJB-JARs.
What is especially compelling about this is that it gives EJB components the capability of self-describing their contents and capabilities, using either use case or other UML diagrams. The proposed profile will also support Extensible Meta-Data Interchange (XMI), the widely used meta-data representation format based on XML.
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What's Next for the UML? 
Tool Support 
