JavaServer Pages, 3rd Edition

Implementing a Custom Iteration Action

The JSTL <c:forEach> action is so flexible that it probably covers most cases, but to help develop application-specific iteration actions when needed, JSTL provides a base class for this as well. It’s named javax.servlet.jsp.jstl.core.LoopTagSupport :

public abstract class LoopTagSupport 
  extends javax.servlet.jsp.tagext.TagSupport
  implements javax.servlet.jsp.jstl.core.LoopTag,
    javax.servlet.jsp.tagext.IterationTag,
    javax.servlet.jsp.tagext.TryCatchFinally

The class has the following fields a subclass can access:

protected int begin
protected int end
protected int step
protected String itemId
protected String statusId
protected boolean beginSpecified
protected boolean endSpecified
protected boolean stepSpecified

These variables hold the value of the corresponding attributes. The variable names for the var and varStatus attributes (itemId and statusId) are, unfortunately, not in sync with the attribute names, due to an oversight when the attribute naming conventions where changed. Nobody’s perfect. For the int variables, there are also boolean variables that tell whether the corresponding attributes were set.

Here are the main methods a subclass must implement:

protected abstract void prepare(  ) 
  throws javax.servlet.jsp.JspTagException
protected abstract Object next(  ) throws javax.servlet.jsp.JspTagException
protected abstract boolean hasNext(  ) 
  throws javax.servlet.jsp.JspTagException

The prepare( ) method prepares for the iteration, for instance ...