JSP Elements

JSP Elements

There are three types of JSP elements you can use: directive, action, and scripting. A new construct added in JSP 2.0 is an Expression Language (EL) expression; let's call this a forth element type, even though it's a bit different than the other three.

Directive elements

The directive elements, shown in Table 3-1, specify information about the page itself that remains the same between requests—for example, if session tracking is required or not, buffering requirements, and the name of a page that should be used to report errors, if any.

Standard action elements

Action elements typically perform some action based on information that is required at the exact time the JSP page is requested by a browser. An action can, for instance, access parameters sent with the request to do a database lookup. It can also dynamically generate HTML, such as a table filled with information retrieved from an external system.

 Custom action elements and the JSP Standard Tag Library

In addition to the standard actions, the JSP specification defines how to develop custom actions to extend the JSP language, either as Java classes or as text files with JSP elements. The JSP Standard Tag Library (JSTL) is such an extension, with the special status of being defined by a formal specification from Sun and typically bundled with the JSP container. JSTL contains action elements for the type of processing needed in most JSP applications, such as conditional processing, database access, internationalization, and more.

Scripting elements

Scripting elements, allow you to add small pieces of code (typically Java code) in a JSP page, such as an if statement to generate different HTML depending on a certain condition. Like actions, they are also executed when the page is requested. You should use scripting elements with extreme care: if you embed too much code in your JSP pages, you will end up with the same kind of maintenance problems as with servlets embedding HTML.

Expression Language expressions

A new feature in JSP 2.0 is the Expression Language (EL), originally developed as part of the JSTL specification. The EL is a simple language for accessing request data and data made available through application classes. EL expressions can be used directly in template text or to assign values to action element attributes. Its syntax is similar to JavaScript, but much more forgiving; it's constrained in terms of functionality, since it's not intended to be a full-fledged programming language.

Rather, it is a glue for tying together action elements and other application components.

JavaBeans components

JSP elements, such as action and scripting elements, are often used to work with JavaBeans components. Put succinctly, a JavaBeans component is a Java class that complies with certain coding conventions. JavaBeans components are typically used as containers for information that describes application entities, such as a customer or an order.