1、 1 外文资料 所译外文资料： 作者： Dan Malks 书名： Professional JSP 出版时间 : 2000.7.26 所译章节 : Chapter 12 12.1 Introductory Good 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 allo
2、ws 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 a
3、rchitecture for your Web application is crucial to this separation. 12.2 JSP architecture We 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 s
4、hows this process in outline； the individual parts of the diagram will be explained in turn later in this article. JSP architecture: 2 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
5、 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 im
6、plementation 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
7、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 w
8、ill 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,
9、 such as JavaBeans, to create Java-based Web-enabled systems. We will examine architectural issues as they relate to JSP and servlets and discuss some effective designs while looking at the 3 tradeoffs of each. Before jumping directly into a discussion of specific architectures, though, we will brie
10、fly examine the need to develop a variety of architectures. 12.3 Code factoring and role separation One of the main reasons why the JavaServer Pages technology has evolved into what it is today (and its still evolving) is the overwhelming technical need to simplify application design by separating d
11、ynamic 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 archit
12、ectures, 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 applica
13、tion 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 sof
14、tware 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 presen
15、tation 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? Lets investigate this further. On any Web-based project, multiple rol
16、es 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
17、 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
