Introduction xv In this book in particular,we take a very low level,EJB-specific patterns focus.That is,rather than document general abstract patterns that could potentially be applied across technologies,we focus on how to get things done with EJB,discussing EJB-specific problems,and EJB-specific intricacies of implementation.Thus,in this book we part from many other patterns books in often showing exact implementations of a pattern (where the pattern is not project specific).The goal is to provide you,the EJB developer/architect with all the information you need to readily begin using these patterns in your EJB/J2EE-based applications. Origins of Patterns For many people,patterns were first introduced to them via the landmark book,Design Patterns:Elements of Reusable Object-Oriented Software (Gamma, et al.,1994).Not the first work on software patterns,this book had the positive effect of bringing the concept and use of patterns for software development into the mainstream. The actual origin of patterns begins long before Design Patterns was pub- lished in 1994.Patterns were first described by Christopher Alexander,applied to the construction/architecture of towns and buildings in the 1970s.In A Pat- tern Language (1977),Alexander writes:"Each pattern describes a problem which occurs over and over again in our environment,and then describes the core of the solution to that problem,in such a way that you can use this solu- tion a million times over,without ever doing it the same way twice." Patterns are a great way of organizing knowledge and solutions in any domain of life,not just civil engineering and software design.What makes pat. terns well suited for cataloging knowledge is their structure and hands nature.Good patterns show ways to solve problems and are structured in a style that lends itself well to explaining the aspects of the problem and solution at work. Pattern Style Used in This Book The patterns in this book are described with a style very similar to the original style used by Alexander,called Alexandrian form.The form I use consists of a pattern written up in a prose-like form,separated by a problem and solution statement as follows: Context:One or two sentences to describe the context in which the pattern applies. Problem:A question that illustrates thethe problem this pattern is solving. 米装举 In this book in particular, we take a very low level, EJB-specific patterns focus. That is, rather than document general abstract patterns that could potentially be applied across technologies, we focus on how to get things done with EJB, discussing EJB-specific problems, and EJB-specific intricacies of implementation. Thus, in this book we part from many other patterns books in often showing exact implementations of a pattern (where the pattern is not￾project specific). The goal is to provide you, the EJB developer/architect with all the information you need to readily begin using these patterns in your EJB/J2EE-based applications. Origins of Patterns For many people, patterns were first introduced to them via the landmark book, Design Patterns: Elements of Reusable Object-Oriented Software (Gamma, et al., 1994). Not the first work on software patterns, this book had the positive effect of bringing the concept and use of patterns for software development into the mainstream. The actual origin of patterns begins long before Design Patterns was pub￾lished in 1994. Patterns were first described by Christopher Alexander, applied to the construction/architecture of towns and buildings in the 1970s. In A Pat￾tern Language (1977), Alexander writes: “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solu￾tion a million times over, without ever doing it the same way twice.” Patterns are a great way of organizing knowledge and solutions in any domain of life, not just civil engineering and software design. What makes pat￾terns well suited for cataloging knowledge is their structure and hands-on nature. Good patterns show ways to solve problems and are structured in a style that lends itself well to explaining the aspects of the problem and solution at work. Pattern Style Used in This Book The patterns in this book are described with a style very similar to the original style used by Alexander, called Alexandrian form. The form I use consists of a pattern written up in a prose-like form, separated by a problem and solution statement as follows: Context: One or two sentences to describe the context in which the pattern applies. Problem: A question that illustrates the the problem this pattern is solving. * * * Introduction xv