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JSP Tag Libraries : Chapter 10: Iterating with Tags

10.1 Iterating with tags 101

Developing custom JSP tags that iterate over some set of values requires us to work, once again, with the familiar BodyTag interface. The BodyTag interface provides a method call protocol to control the execution of the tag's body— which we'll need in order to repeat the tag's body for every value in the JavaBean's indexed property.

NOTE In JSP1.2 a new IterationTag interface was defined and we can also create tags using this interface. You can find information regarding the IterationTag later in this chapter.

Figure 10.1 shows how a tag can implement iteration using the BodyTag method call protocol.

To further illustrate how iteration is accomplished with tags, we've translated the flow chart in figure 10.1 into (roughly) its Java equivalent.

t.doInitBody(); 
do { 
	// The JSP runtime execute 
	// the tag's body ... 
} while( t. doAfterBody() == BodyTag. EVAL_ BODY_ TAG); 

As figure 10. 1 and the code fragment show, two methods (table 10.1) take part in implementing the iteration:

Table 10.1 Iteration methods
JSP method	Description
doBodyInit()	Used to initialize preiteration JSP scripting variables and the tags' internal values. For example, if your tag exposes some iterator object as a JSP scripting variable, it will probably use doBodyInit() to export its initial value.
doAfterBody()	Controls the iteration with its return codes: To continue the iteration, doAfterBody() returns a value of BodyTag. EVAL_ BODY_ TAG (or IterationTag. EVAL_ BODY_ AGAIN in JSP1.2). To break the iteration , it returns a value BodyTag. SKIP_ BODY. This method is also where we re-export the iterator value (the current value of the property on which we are iterating), and where we write the result of the current iteration into the response.

NOTE You can skip the implementation of doBodyInit() and perform its work in doStartTag(). This will not have any effect on performance and may even simplify your tags. Better yet, since doStartTag() is not available in IterationTag, code that does not use it will be easier to port to this new tag. In any case, it is a good idea to separate the iteration handling from doStartTag() so that doStartTag() will only deal with service iinitialization (e. g., obtaining the object set that we are going to iterate) and doBodyInit() will deal with the initialization of the loop.

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