In JavaServer Pages technology, actions are elements that can create and access programming language objects and affect the output stream. The JSP specification defines 6 standard actions that must be provided by any compliant JSP implementation.

In addition to the standard actions, JSP v1.1 technology supports the development of reusable modules called custom actions. A custom action is invoked by using a custom tag in a JSP page. A tag library is a collection of custom tags.

Some examples of tasks that can be performed by custom actions include form processing, accessing databases and other enterprise services such as email and directories, and flow control. Before the availability of custom actions, JavaBeans components in conjunction with scriplets were the main mechanism for performing such processing. The disadvantage of using this approach is that it makes JSP pages more complex and difficult to maintain.

Custom actions alleviate this problem by bringing the benefits of another level of componentization to JSP pages. Custom actions encapsulate recurring tasks so that they can be reused across more than one application and increase productivity by encouraging division of labor between library developers and library users. JSP tag libraries are created by developers who are proficient at the Java programming language and expert in accessing data and other services. JSP tag libraries are used by Web application designers who can focus on presentation issues rather than being concerned with how to access databases and other enterprise services.

Some features of custom tags are:

• They can be customized via attributes passed from the calling page.
  
  
• They have access to all the objects available to JSP pages.
  
  
• They can modify the response generated by the calling page.
  
  
• They can communicate with each other. You can create and initialize a JavaBeans component, create a variable that refers to that bean in one tag, and then use the bean in another tag.
  
  
• They can be nested within one another, allowing for complex interactions within a JSP page.
  
  

The next two sections describe the tasks involved in using and defining tags. The tutorial concludes with a discussion of two tag library examples. The examples include complete binary and source code in two Web application archives.

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