S eparation of concerns is one of the oldest concepts in computer science. The term was coined by Dijkstra in 1974. 1 It is important because it simplifi es software, making it easier to develop and maintain. Separation of concerns is commonly achieved by decomposing an application into components. There are, however, crosscutting concerns, which span (or cut across) multiple components. These kinds of concerns cannot be handled by traditional forms of modularization and can make the application more complex and diffi cult to maintain. Examples of crosscutting concerns in enterprise Java applications include transaction management, security, persistence, and application assembly. Scattering the code that handles those concerns across multiple components is undesirable, however. This is because doing so would make each component more complex. Also, duplicating code in multiple modules can cause maintenance problems. Consequently, there has been a fl urry of activity in recent years to develop frameworks and new forms of modularity that try to untangle these crosscutting concerns from the application's business logic. In this article we look at the evolution of enterprise Java frameworks that tackle crosscutting concerns. We address how dissatisfaction with the fi rst-generation frameworks, which were based on the EJB (Enterprise JavaBeans) programming model, prompted the development of dramatically better frameworks. These newergeneration frameworks are based on the POJO (Plain Old Java Object) programming model. Even though the focus of this article is Java, it contains many useful lessons for developers of frameworks and applications written in other languages.
A major component of most enterprise applications is the code that transfers objects in and out of a relational database. The easiest solution is often to use an ORM (object-relational mapping) framework, which allows the developer to declaratively define the mapping between the object model and database schema and express database-access operations in terms of objects. This high-level approach significantly reduces the amount of database-access code that needs to be written and boosts developer productivity.
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