jsp和数据库外文文献原文及翻译 第2页
In addition to deciding where in the MVC paradigm to interact with the database, you have several choices for integrating JDBC technology into your JSP pages. For example, you can insert JDBC using scriptlets, insert it using a tag library, or hide it within custom tags or other classes. We'll next look at examples of several approaches and discuss their use.
A tag library example using DBTags
One of the first things that a new JSP programmer hears, usually from well-intentioned friends and experts, is not to use scriptlets. Instead, they tell the new JSP programmer to use custom tags. Custom tags are a means by which the JSP platform's capabilities are extended: custom XML-style tags, tied to code libraries, implement the desired functionality. We'll see how well they work, in our next example.
The Jakarta TagLibs Project is a subproject of the Jakarta Project, the official reference implementation of the Java Servlet and JavaServer Pages technologies.
One of the packages developed under the auspices of the Jakarta TagLibs Project is the DBTags custom tag library (formerly known as the JDBC tag library). The JSP page in Listing 4 implements the same hit counter as in Listing 3, replacing the scriptlet with custom tags.
Most tag libraries written to date have been written by programmers for programmers; the semantics of those tags are geared toward other 原文请找腾讯752018766辣,文-论'文.网
http://www.751com.cn/ it difficult to separate using DBTags from rendering output into the desired form. Finally, notice that the logic differs between Listing 3 and Listing 4. The DBTags execute tag consumes the update count from any update statement sent via JDBC; only query results can be retrieved. That means we cannot find out how many rows were updated by the update statement. So we have to switch the update and insert statements; we always try to insert a new record, force DBTags to ignore any error, and then perform the update.
In fairness to the DBTags tag library, it is not a bad tag library for programmers. Aside from its consumption of the update count, the code provides a fairly good mapping to JDBC. Therein lies the problem, however: the tags provide little more than a direct translation of the JDBC package. Other than hiding some exception handling, the tag library doesn't really provide any abstraction over scriptlets. It certainly doesn't help separate presentation from function.
So, the real issue is not whether to use scriptlets or tags; that question is a consequence, not a cause, of the problem of separating function from presentation. The solution is to provide higher-level functionality to presentation-page authors at an appropriate level of discourse. The reason tags are considered better than scriptlets is that scriptlets, by definition, are programming, whereas tags can represent high-level concepts.
Hiding JDBC from presentation pages
When integrating JDBC with JSP technology, we want to hide as much of that integration from the presentation author as possible. Where we do expose database concepts, we want to expose them at a suitable level of abstraction. This approach leads to our next example.
In the example in Listing 5, we hide the JDBC integration from the presentation page. (A live version of this page is located on the JavaServer Pages Developers Guide Web site.)
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