Package jakarta.servlet.jsp.jstl.core

Class LoopTagSupport

  • All Implemented Interfaces:
    LoopTag, IterationTag, JspTag, Tag, TryCatchFinally, Serializable
    public abstract class LoopTagSupport
extends TagSupport
implements LoopTag, IterationTag, TryCatchFinally

    Base support class to facilitate implementation of iteration tags.

    Since most iteration tags will behave identically with respect to actual iterative behavior, the Jakarta Standard Tag Library provides this base support class to facilitate implementation. Many iteration tags will extend this and merely implement the hasNext() and next() methods to provide contents for the handler to iterate over.

    In particular, this base class provides support for:

    • Iteration control, based on protected prepare(), next(), and hasNext() methods
    • Subsetting (begin, end, step functionality, including validation of subset parameters for sensibility)
    • item retrieval (getCurrent())
    • status retrieval (LoopTagStatus)
    • exposing attributes (set by var and varStatus attributes)

    In providing support for these tasks, LoopTagSupport contains certain control variables that act to modify the iteration. Accessors are provided for these control variables when the variables represent information needed or wanted at translation time (e.g., var, varStatus). For other variables, accessors cannot be provided here since subclasses may differ on their implementations of how those accessors are received. For instance, one subclass might accept a String and convert it into an object of a specific type by using an expression evaluator; others might accept objects directly. Still others might not want to expose such information to outside control.

    Author:
    Shawn Bayern
    See Also:
    Serialized Form

    • Field Detail

      • begin

        protected int begin
        Starting index ('begin' attribute)

      • end

        protected int end
        Ending index of the iteration ('end' attribute). A value of -1 internally indicates 'no end specified', although accessors for the core Jakarta Standard Tag Library tags do not allow this value to be supplied directly by the user.

      • step

        protected int step
        Iteration step ('step' attribute)

      • beginSpecified

        protected boolean beginSpecified
        Boolean flag indicating whether 'begin' was specified.

      • endSpecified

        protected boolean endSpecified
        Boolean flag indicating whether 'end' was specified.

      • stepSpecified

        protected boolean stepSpecified
        Boolean flag indicating whether 'step' was specified.

      • itemId

        protected String itemId
        Attribute-exposing control

      • statusId

        protected String statusId
        Attribute-exposing control

      • deferredExpression

        protected ValueExpression deferredExpression
        The deferred expression if any

    • Constructor Detail

      • LoopTagSupport

        public LoopTagSupport()
        Constructs a new LoopTagSupport. As with TagSupport, subclasses should not implement constructors with arguments, and no-arguments constructors implemented by subclasses must call the superclass constructor.

    • Method Detail

      • next

        protected abstract Object next()
                        throws JspTagException

        Returns the next object over which the tag should iterate. This method must be provided by concrete subclasses of LoopTagSupport to inform the base logic about what objects it should iterate over.

        It is expected that this method will generally be backed by an Iterator, but this will not always be the case. In particular, if retrieving the next object raises the possibility of an exception being thrown, this method allows that exception to propagate back to the JSP container as a JspTagException; a standalone Iterator would not be able to do this. (This explains why LoopTagSupport does not simply call for an Iterator from its subtags.)

        Returns:
        the java.lang.Object to use in the next round of iteration
        Throws:
        NoSuchElementException - if next() is called but no new elements are available
        JspTagException - for other, unexpected exceptions

      • hasNext

        protected abstract boolean hasNext()
                            throws JspTagException

        Returns information concerning the availability of more items over which to iterate. This method must be provided by concrete subclasses of LoopTagSupport to assist the iterative logic provided by the supporting base class.

        See next for more information about the purpose and expectations behind this tag.

        Returns:
        true if there is at least one more item to iterate over, false otherwise
        Throws:
        JspTagException - thrown on error
        See Also:
        next()

      • prepare

        protected abstract void prepare()
                         throws JspTagException

        Prepares for a single tag invocation. Specifically, allows subclasses to prepare for calls to hasNext() and next(). Subclasses can assume that prepare() will be called once for each invocation of doStartTag() in the superclass.

        Throws:
        JspTagException - thrown on error

      • release

        public void release()
        Releases any resources this LoopTagSupport may have (or inherit).
        Specified by:
        release in interface Tag
        Overrides:
        release in class TagSupport
        See Also:
        Tag.release()

      • doFinally

        public void doFinally()
        Removes any attributes that this LoopTagSupport set.

        These attributes are intended to support scripting variables with NESTED scope, so we don't want to pollute attribute space by leaving them lying around.

        Specified by:
        doFinally in interface TryCatchFinally

      • doCatch

        public void doCatch​(Throwable t)
             throws Throwable
        Rethrows the given Throwable.
        Specified by:
        doCatch in interface TryCatchFinally
        Parameters:
        t - The throwable exception navigating through this tag.
        Throws:
        Throwable - if the exception is to be rethrown further up the nest chain.

      • getCurrent

        public Object getCurrent()
        Description copied from interface: LoopTag
        Retrieves the current item in the iteration. Behaves idempotently; calling getCurrent() repeatedly should return the same Object until the iteration is advanced. (Specifically, calling getCurrent() does not advance the iteration.)
        Specified by:
        getCurrent in interface LoopTag
        Returns:
        the current item as an object

      • getLoopStatus

        public LoopTagStatus getLoopStatus()
        Description copied from interface: LoopTag
        Retrieves a 'status' object to provide information about the current round of the iteration.
        Specified by:
        getLoopStatus in interface LoopTag
        Returns:
        The LoopTagStatus for the current LoopTag.

      • getDelims

        protected String getDelims()

      • setVar

        public void setVar​(String id)
        Sets the 'var' attribute.
        Parameters:
        id - Name of the exported scoped variable storing the current item of the iteration.

      • setVarStatus

        public void setVarStatus​(String statusId)
        Sets the 'varStatus' attribute.
        Parameters:
        statusId - Name of the exported scoped variable storing the status of the iteration.

      • validateBegin

        protected void validateBegin()
                      throws JspTagException
        Ensures the "begin" property is sensible, throwing an exception expected to propagate up if it isn't
        Throws:
        JspTagException - thrown on error

      • validateEnd

        protected void validateEnd()
                    throws JspTagException
        Ensures the "end" property is sensible, throwing an exception expected to propagate up if it isn't
        Throws:
        JspTagException - thrown on error

      • validateStep

        protected void validateStep()
                     throws JspTagException
        Ensures the "step" property is sensible, throwing an exception expected to propagate up if it isn't
        Throws:
        JspTagException - thrown on error