JavaServer Pages, 3rd Edition

What Is a Custom Tag Library?

The JSP standard actions, such as the <jsp:useBean> and <jsp:getProperty> actions used in Chapter 6, are HTML-like elements for commonly needed functions in a JSP page: creating beans, accessing bean properties, and invoking other JSP pages. But there’s a lot more you want to do that isn’t covered by the standard actions.

To extend the set of action elements a page author can use in the same familiar way, new actions can be developed, either by a programmer as Java classes or by a page author as tag files (a special kind of JSP file). In either case, these actions are called custom actions. A custom action can do pretty much anything: it has access to all information about the request, it can add content to the response body as well as set response headers, and it can use any Java API to access external resources such as databases, LDAP servers, or mail servers. The way the JSP container interacts with a custom action also makes it possible for a custom action to conditionally process its body and to abort the processing of the rest of the page. Custom actions can be created for application-specific functions to make it easier for page authors to develop the JSP pages. Some typical examples are shown later in this book.

A custom action is inserted into a page using an HTML-like (actually XML) element. The attribute values, and sometimes the body, you provide tell the action what to do and the data to use. In fact, you have already used a custom action; ...