16.5 Summary

The creation of the J2EE multi-tier architecture was motivated by the business needs of Sun Microsystems. These business needs were influenced by the lessons of the CORBA model and by the competitive pressures of other proprietary distributed programming models, such as COM+ from Microsoft. J2EE features a server-side component framework for building enterprise-strength server-side Java applications, namely, Enterprise JavaBeans.

The J2EE/EJB specification is constantly expanding. Its ready-to-use services currently include transactions, security, naming, persistence, and resource management. These services enable the J2EE/EJB application programmer to focus on developing the business logic, thus removing the need to worry about low-level distribution details. J2EE/EJB achieves portability by using a common, portable language (Java) and by having precise contracts between components. It achieves performance and performance scalability via a number of mechanisms, including distributing applications across many processors (horizontal scaling), stateless session beans, and resource pools.

Despite the seeming simplicity of the J2EE/EJB programming model, there are many application-level architectural decisions that need to be carefully made. The various architectural tradeoffs must be analyzed and compared to derive an optimal design with respect to application quality requirements.