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javax.servlet.jsp.jstl.core
Class LoopTagSupport

java.lang.Object
  javax.servlet.jsp.tagext.TagSupport
      javax.servlet.jsp.jstl.core.LoopTagSupport

All Implemented Interfaces:

java.io.Serializable, LoopTag, IterationTag, JspTag, Tag, TryCatchFinally

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, JSTL 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 Summary

protected int	begin Starting index ('begin' attribute)
protected boolean	beginSpecified Boolean flag indicating whether 'begin' was specified.
protected ValueExpression	deferredExpression The deferred expression if any
protected int	end Ending index of the iteration ('end' attribute).
protected boolean	endSpecified Boolean flag indicating whether 'end' was specified.
protected java.lang.String	itemId Attribute-exposing control
protected java.lang.String	statusId Attribute-exposing control
protected int	step Iteration step ('step' attribute)
protected boolean	stepSpecified Boolean flag indicating whether 'step' was specified.

Fields inherited from class javax.servlet.jsp.tagext.TagSupport

id, pageContext

Fields inherited from interface javax.servlet.jsp.tagext.IterationTag

EVAL_BODY_AGAIN

Fields inherited from interface javax.servlet.jsp.tagext.Tag

EVAL_BODY_INCLUDE, EVAL_PAGE, SKIP_BODY, SKIP_PAGE

Constructor Summary

LoopTagSupport()
Constructs a new LoopTagSupport.

Method Summary

int	doAfterBody() Continues the iteration when appropriate -- that is, if we (a) have more items and (b) don't run over our 'end' (given our 'step').
void	doCatch(java.lang.Throwable t) Rethrows the given Throwable.
void	doFinally() Removes any attributes that this LoopTagSupport set.
int	doStartTag() Begins iterating by processing the first item.
java.lang.Object	getCurrent() Retrieves the current item in the iteration.
protected java.lang.String	getDelims()
LoopTagStatus	getLoopStatus() Retrieves a 'status' object to provide information about the current round of the iteration.
protected abstract boolean	hasNext() Returns information concerning the availability of more items over which to iterate.
protected abstract java.lang.Object	next() Returns the next object over which the tag should iterate.
protected abstract void	prepare() Prepares for a single tag invocation.
void	release() Releases any resources this LoopTagSupport may have (or inherit).
void	setVar(java.lang.String id) Sets the 'var' attribute.
void	setVarStatus(java.lang.String statusId) Sets the 'varStatus' attribute.
protected void	validateBegin() Ensures the "begin" property is sensible, throwing an exception expected to propagate up if it isn't
protected void	validateEnd() Ensures the "end" property is sensible, throwing an exception expected to propagate up if it isn't
protected void	validateStep() Ensures the "step" property is sensible, throwing an exception expected to propagate up if it isn't

Methods inherited from class javax.servlet.jsp.tagext.TagSupport

doEndTag, findAncestorWithClass, getId, getParent, getValue, getValues, removeValue, setId, setPageContext, setParent, setValue

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface javax.servlet.jsp.tagext.Tag

doEndTag, getParent, setPageContext, setParent

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 JSTL 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 java.lang.String itemId

Attribute-exposing control

statusId

protected java.lang.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 java.lang.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:

java.util.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

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

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()

doStartTag

public int doStartTag()
               throws JspException

Begins iterating by processing the first item.

Specified by:

doStartTag in interface Tag

Overrides:

doStartTag in class TagSupport

Returns:

SKIP_BODY

Throws:

JspException - if an error occurs while processing this tag

See Also:

Tag#doStartTag()

doAfterBody

public int doAfterBody()
                throws JspException

Continues the iteration when appropriate -- that is, if we (a) have more items and (b) don't run over our 'end' (given our 'step').

Specified by:

doAfterBody in interface IterationTag

Overrides:

doAfterBody in class TagSupport

Returns:

SKIP_BODY

Throws:

JspException - if an error occurs while processing this tag

See Also:

IterationTag#doAfterBody()

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(java.lang.Throwable t)
             throws java.lang.Throwable

Rethrows the given Throwable.

Specified by:

doCatch in interface TryCatchFinally

Parameters:

t - The throwable exception navigating through this tag.

Throws:

java.lang.Throwable - if the exception is to be rethrown further up the nest chain.

getCurrent

public java.lang.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 java.lang.String getDelims()

setVar

public void setVar(java.lang.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(java.lang.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

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

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