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SCA is designed to support applications assembled from services written in multiple programming languages This chapter provides the background and understanding necessary to implement services in arguably the most important of those languages for enterprise development: Java SCA includes a full-featured programming model for implementing services in Java The primary goal of this programming model is to provide the capabilities necessary in Java to build loosely coupled services Moreover, it attempts to do so in a way that is simpler to use than existing Java-based alternatives, including EJB and Spring This chapter focuses on the basics of loosely coupled services, including service contract design, asynchronous communications, and component life cycle Speci cally, this chapter covers the following: Designing service contracts
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SCA includes a fullfeatured programming model for implementing services in Java
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Implementing asynchronous interactions and callback patterns Managing component life cycle, state, and concurrency
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After completing this chapter, you will have a solid grounding in implementing Java-based services and an understanding of best practices to apply when designing those services
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As we discussed in the rst chapter, a key goal of SCA is reuse: Application functionality and code that can be shared by multiple clients is more valuable than functionality and code that cannot This goal is far from novel Many technologies claim to promote code reuse Arguably, the most successful technologies in this respect have been object-oriented languages, which did much to promote intra-process reuse, or calls between code hosted in the same process By organizing code into classes and interfaces, objectoriented languages allowed complex applications to be assembled from smaller, reusable units that are easier to maintain and evolve Yet code would be even more valuable if reuse were not limited to a process or application In other words, if clients could connect remotely with existing or separately deployed code, the code would be even more valuable In the 1990s and early 2000s, DCE, CORBA, DCOM, and Java EE attempted to replicate the success of object-oriented technology in distributed applications by applying many of the same principles to remote communications In particular, these technologies were built around the concept of distributed objects : units of code that could be invoked remotely to perform a task The goal of these frameworks was to enable objects to be invoked across process boundaries similar to the way that objectoriented enabled objects could be invoked locally
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One of the key lessons learned from distributed objects is that applications must be carefully designed not to introduce bottlenecks by making too many remote calls or by placing unnecessary requirements on them such as transactionality
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Unfortunately, practical experience highlighted a number of problems with this approach The most important of these was that local and remote invocations are different and those differences cannot be managed away by middleware Remote communication introduces latency that affects application performance This is compounded when additional qualities of service are required, such as transactions and security One of the key lessons learned from distributed objects is that applications must be carefully designed not to introduce bottlenecks by making too many remote calls or by placing unnecessary requirements on them, such as transactionality SCA rejects the notion that object-oriented principles are to be employed at all levels of application design A core tenet of SCA is that
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development of remote services is unique For remote communications, developers rely on the techniques of loose coupling that we describe in this chapter Most applications, however, cannot be restricted to remote invocations In order to achieve scalability, performance, and avoid unnecessary complexity, application code will need to make many more local calls than remote ones In these cases, SCA stipulates that developers apply principles of good object-oriented design In addition to loosely coupled remote services, we also detail the facilities provided by the SCA Java programming model for creating services intended for use in a single process, which follow traditional object-oriented patterns Protocol Abstraction and Location Transparency Protocol abstraction and location transparency are commonly confused Understanding the distinction between the two is fundamental to understanding the SCA programming model SCA simpli es development by handling the intricacies of remote communications What it doesn t do is oversimplify the nature of those communications and the impact they have on application code Protocol abstraction involves separating the speci cs of how remote invocations are performed from application code by requiring the hosting container to manage communications For example, the following service invocation could be made using web services or an alternative protocol such as RMI the host container handles the speci cs of owing calls while the code remains unchanged (see Listing 31)
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