计算机 JSP web 外文翻译 外文文献

外文资料所译外文资料：①作者：Dan Malks②书名：Professional JSP③出版时间: 2000.7.26④所译章节: Chapter 1212.1IntroductoryGood Web application design tries to separate business objects, presentation, and manipulation of the objects into distinct layers. One benefit of using JavaServer Pages technology is that it allows us to separate the role of a Web designer more clearly from that of a software developer. While on a small-scale project, one individual may occupy both roles, on a larger project, they are likely to be separate and it is beneficial to separate their workflows as much as possible. Designing the architecture for your Web application is crucial to this separation.12.2 JSP architectureWe will examine a variety of ways to architect a system with JavaServer Pages, servlets, and JavaBeans. We will see a series of different architectures, each a development of the one before. The diagram below shows this process in outline; the individual parts of the diagram will be explained in turn later in this article.JSP architecture:When Sun introduced Java Server Pages, some were quick to claim that servlets had been replaced as the preferred request handling mechanism in Web-enabled enterprise architectures. Although JSP is a key component of the Java 2 Platform Enterprise Edition (J2EE) specification, serving as the preferred request handler and response mechanism, we must investigate further to understand its relationship with servlets.Other sections of Professional JSP explain the implementation details of JSP source translation and compilation into a servlets. Understanding that JSP is built on top of the servlet API, and uses servlet semantics, raises some interesting questions. Should we no longer develop stand-alone servlets in our Web-enabled systems? Is there some way to combine servlets and JSPs? If so, where do we place our Java code? Are there any other components involved in the request processing, such as JavaBeans? If so, where do they fit into the architecture and what type of role do they fulfill?It is important to understand that, although JSP technology will be a powerful successor to basic servlets, they have an evolutionary relationship and can be used in a cooperative and complementary manner.Given this premise, we will investigate how these two technologies, each a Java Standard Extension, can be used co-operatively along with other components, such as JavaBeans, to create Java-based Web-enabled systems. We will examine architecturalissues as they relate to JSP and servlets and discuss some effective designs while looking at the tradeoffs of each. Before jumping directly into a discussion of specific architectures, though, we will briefly examine the need to develop a variety of architectures.12.3 Code factoring and role separationOne of the main reasons why the JavaServer Pages technology has evolved into what it is today (and it's still evolving) is the overwhelming technical need to simplify application design by separating dynamic content from static template display data. The foundation for JSP was laid down with the initial development of the Java Web Server from Sun, which used page compilation and focused on embedding HTML inside Java code. As applications came to be based more on business objects and n-tier architectures, the focus changed to separating HTML from Java code, while still maintaining the integrity and flexibility the technology provided.In Chapter 5, JSP Sessions, in Professional JSP, we saw how beans and objects can be bound to different contexts just by defining a certain scope. Good application design builds on this idea and tries to separate the objects, the presentation, and the manipulation of the objects into distinct, distinguishable layers.Another benefit of using JSP is that it allows us to more cleanly separate the roles of a Web production/HTML designer individual from a software developer. Remember that a common development scenario with servlets was to embed the HTML presentation markup within the Java code of the servlet itself, which can be troublesome. In our discussion, we will consider the servlet solely as a container for Java code, while our entire HTML presentation template is encapsulated within a JSP source page. The question then arises as to how much Java code should remain embedded within our JSP source pages, and if it is taken out of the JSP source page, where should it reside?Let's investigate this further. On any Web-based project, multiple roles and responsibilities will exist. For example, an individual who designs HTML pages fulfills a Web production role while someone who writes software in the Java programming language fulfills a software development role.On small-scale projects these roles might be filled by the same individual, or two individuals working closely together. On a larger project, they will likely be filled by multiple individuals, who might not have overlapping skill sets, and are less productive if made too dependent on the workflow of the other.If code that could be factored out to a mediating servlet is included instead within HTML markup, then the potential exists for individuals in the software development role and those in the Web production role to become more dependent than necessary on the progress and workflow of the other. Such dependencies may create a more error-prone environment, where inadvertent changes to code by other team members become more common.This gives us some insight into one reason why we continue to develop basic servlets: they are an appropriate container for our common Java code that has been factored out of our JSP pages, giving our software development team an area of focus that is as loosely coupled to our JSP pages as possible. Certainly, there will be a need for these same individuals to work with the JSP source pages, but the dependency is reduced, and these pages become the focus of the Web-production team instead. Of course, if the same individual fulfills both roles, as is typical on a smaller project, such dependencies are not a major concern.So, we should try to minimize the Java code that we include within our JSP page, in order to uphold this cleaner separation of developer roles. As we have discussed, some of this Java code is appropriately factored to a mediating servlet. Code that is common to multiple requests, such as authentication, is a good candidate for a mediating servlet. Such code is included in one place, the servlet, instead of potentially being cut and pasted into multiple JSPs.We will also want to remove much of our business logic and data access code from our JSP page and encapsulate it within JavaBeans, called worker or helper beans. We start to see a pattern of code movement from our JSP into two areas: a servlet (or JSP) that sits in front of the main JSP, and JavaBeans that sit in back. We refer to this common pattern as "Factor Forward -- Factor Back," as shown in the figure below:Factor Forward -- Factor Back:Another way to think about what code should be localized and encapsulated is that our JSP page should reveal as little as possible of our Java code implementation details.Rather, the page should communicate our intent by revealing the delegating messages we send to worker beans, instructing them to get state from a model, or to complete some business processing.12.4 Redirecting and forwardingRedirecting and forwarding requests in JSPs and servlets takes place often, and it is important to understand the subtle difference between these two mechanisms even though they achieve the same goal (that is, a client asks for a resource on the server and a different resource is served to it):●When a servlet or JSP resource chooses to redirect the client (using aresponse.sendRedirect(url)) the request object does not reach the second resource directly since the underlying implementation is an HTTP redirect.The server sends an HTTP 302 message back to the client telling it that the resource has moved to another URL, and that the client should access it there.The bottom line is that the lifecycle of the initial request object that was accessed in the first JSP terminates with the end of the service method in the first JSP, or with the reply from the server.●In a forward mechanism the request object is forwarded to the second resource,thus maintaining any object bindings to the request and its state, without a round trip to the client on the network. This allows the first JSP to do some work internally and then send information to the second JSP asking it to do itsbit. (Servlets used a chaining mechanism to do this). See Chapter 5, JSP Sessions, in Professional JSP to get a clearer picture of scope. JSPs and servlets can use the forwarding mechanism to delegate tasks among themselves, in the process of separating dynamic and static content.Now, let's investigate how we build these systems.12.5 ArchitecturesBefore discussing specific architectures that we can use to build systems with servlets and JSP, it is worth mentioning two basic ways of using the JSP technology. Each of the architectures discussed in this chapter will be based on one of these approaches:●The first method is referred to here as the page-centric (or client-server)approach. This approach involves request invocations being made directly to JSP page.●In the second method, the dispatcher (or n-tier) approach, a basic servlet orJSP acts as a mediator or controller, delegating requests to JSP pages and JavaBeans.We will examine these approaches in light of a simple example, which will evolve to satisfy the requirements of various scenarios. The initial scenario involves providing a Web interface for guessing statistics about a soon-to-be-born baby. The guesses are stored, and can be reviewed later by the parents, to see who has guessed the closest. As the requirement scenarios become more sophisticated, such as adding the desire for a persistence mechanism, the solution scenarios will become more sophisticated, as well. Thus, our example will evolve and we will gain an understanding of how the various architectures that we discuss will help us build a system that satisfies these requirements in an elegant and effective manner.12.6 The page-centric approachApplications built using a client-server approach have been around for some time; they consist of one or more application programs running on client machines and connecting to a server-based application to work. (A good example would be a PowerBuilder or Oracle Forms-based system.) CGIs and pre-servlet applications were generally based on this simple 2-tier model, and with the introduction of servlets, 2-tier applications could also be created in Java.This model allows JSPs or servlets direct access to some resource like a database or legacy application to service a client's request: the early JSP specifications termed this a "Model 1" programming approach. The JSP page is where the incoming request is intercepted and processed, and the response is sent back to the client;JSPs only differed from servlets in this scenario by providing cleaner code and separating code from the content by placing data access in beans.Model 1 programming approach:The advantage of such an approach is that it is siple to program,and allows the page author to Generate dynamic content easily,based upon the request and the state resources.However this architecture does not scale up well for a large number of simultaneous clients since there would be a significant amount of request processing to be performed,and each request must establish or share a potentially scarce/expensive connection to the resource in question.(A good example would be JDBC connectons in servlets or JSPs and the need for connection pools.) Indiscriminate usage of this architecture usually leads to a significant amount of Java code embedded within the JSP page,this may not seem to be much of a problem for Java developers but it is certainly an issue if the JSP pages are maintained by designers:the code tends to get in the designe’s way,and you run the risk of your code becoming corrupted when others are tweaking the look and feel.译文12.1前言好的Web应用设计试图将业务对象，简报以及操作对象分为不同的层面。