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From the Book: In the world of software, a pattern is a tangible manifestation of an organization's tribal memory. A pattern provides a common solution to a common problem and so, within the culture of one specific organization or within one domain, naming and then specifying a pattern represents the codification of a common solution, drawn from proven, prior experience. Having a good language of patterns at your disposal is like having an extended team of experts sitting at your side during development: by applying one of their patterns, you in effect take the benefit of their hard-won knowledge. As such, the best patterns are not so much invented as they are discovered and then harvested from existing, successful systems. Thus, at its most mature state, a pattern is full of things that work, absent of things that don't work, and revealing of the wisdom and rationale of its designers. Deep, really useful, patterns are typically ancient: you see one and will often remark, "Hey, I've done that before." However, the very naming of the pattern gives you a vocabulary that you didn't have previously and so helps you apply that pattern in ways you otherwise might have not have realized. Ultimately, the effect of such a pattern will be to make your system simpler. Patterns not only help you build simpler systems that work, but they also help you build beautiful programs. In a culture of time starvation, writing beautiful software is often impossible. That's sad, for as professionals, we strive to build things of quality. By applying a good set of patterns, it is possible to bring a degree of elegance in to your systems that might otherwise have been lacking. The authors of Core J2EE Patterns have harvested a really useful set of patterns. Don't get me wrong: J2EE is certainly an important platform, enabling teams to build some very powerful systems. However, reality is, there is still a wide semantic gap between the abstractions and services that J2EE provides and the final application that a team must build. Patterns such as specified in this book represent solutions that appear again and again in filling that gap. By applying these patterns, you thus carry out the primary means of reducing software risk: you write less software. Rather than discovering these solutions on your own, apply these patterns, which have already proven their utility in existing systems. More than just naming a set of patterns, the authors make them approachable by specifying their semantics using the UML. Additionally, they show you how to apply these patterns and how to refactor your system to take advantage of them. Again, it's just like having a team of experts sitting at your side. Grady Booch Chief Scientist Rational Software Corporation