/*
* Copyright 1999-2004 The Apache Software Foundation.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.cocoon.taglib.core;
import org.apache.cocoon.environment.ObjectModelHelper;
import org.apache.cocoon.environment.Request;
import org.apache.cocoon.taglib.IterationTag;
import org.apache.cocoon.taglib.VarTagSupport;
import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
/**
* <p>Cocoon taglib allows developers to write custom iteration tags by
* implementing the LoopTag interface. (This is not to be confused with
* org.apache.cocoon.taglib.IterationTag)
* LoopTag establishes a mechanism for iteration tags to be recognized
* and for type-safe communication with custom subtags.
* </p>
*
* <p>Since most iteration tags will behave identically with respect to
* actual iterative behavior, however, Cocoon taglib 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.</p>
*
* <p>In particular, this base class provides support for:</p>
*
* <ul>
* <li> iteration control, based on protected next() and hasNext() methods
* <li> subsetting (begin, end, step functionality, including validation
* of subset parameters for sensibility)
* <li> item retrieval (getCurrent())
* <li> status retrieval (LoopTagStatus)
* <li> exposing attributes (set by 'var' and 'varStatus' attributes)
* </ul>
*
* <p>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, status). 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.</p>
*
* Migration from JSTL1.0
* @see javax.servlet.jsp.jstl.core.LoopTagSupport
*
* @author <a href="mailto:volker.schmitt@basf-it-services.com">Volker Schmitt</a>
* @version CVS $Id: LoopTagSupport.java,v 1.5 2004/03/05 13:02:24 bdelacretaz Exp $
*/
public abstract class LoopTagSupport extends VarTagSupport implements LoopTag, IterationTag //, TryCatchFinally
{
//*********************************************************************
// 'Protected' state
/*
* JavaBean-style properties and other state slaved to them. These
* properties can be set directly by accessors; they will not be
* modified by the LoopTagSupport implementation -- and should
* not be modified by subclasses outside accessors unless those
* subclasses are perfectly aware of what they're doing.
* (An example where such non-accessor modification might be sensible
* is in the doStartTag() method of an EL-aware subclass.)
*/
/** Starting index ('begin' attribute) */
protected int begin;
/**
* Ending index ('end' attribute). -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.
*/
protected int end;
/** Iteration step ('step' attribute) */
protected int step;
/** Boolean flag indicating whether 'begin' was specified. */
protected boolean beginSpecified;
/** Boolean flag indicating whether 'end' was specified. */
protected boolean endSpecified;
/** Boolean flag indicating whether 'step' was specified. */
protected boolean stepSpecified;
/** Attribute-exposing control */
protected String statusId;
//*********************************************************************
// 'Private' state (implementation details)
/*
* State exclusively internal to the default, reference implementation.
* (While this state is kept private to ensure consistency, 'status'
* and 'item' happen to have one-for-one, read-only, accesor methods
* as part of the LoopTag interface.)
*
* 'last' is kept separately for two reasons: (a) to avoid
* running a computation every time it's requested, and (b) to
* let LoopTagStatus.isLast() avoid throwing any exceptions,
* which would complicate subtag and scripting-variable use.
*
* Our 'internal index' begins at 0 and increases by 'step' each
* round; this is arbitrary, but it seemed a simple way of keeping
* track of the information we need. To avoid computing
* getIteratorStatus().getCount() by dividing index / step, we keep
* a separate 'count' and increment it by 1 each round (as a minor
* performance improvement).
*/
private LoopTagStatus status; // our LoopTagStatus
private Object item; // the current item
protected int index; // the current internal index
protected int count; // the iteration count
protected boolean last; // current round == last one?
//*********************************************************************
// Constructor
/**
* Constructs a new LoopTagSupport. As with TagSupport, subclasses
* should not provide other constructors and are expected to call
* the superclass constructor
*/
public LoopTagSupport() {
super();
init();
}
//*********************************************************************
// Abstract methods
/**
* <p>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.</p>
*
* <p>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 container as a SAXException; a standalone Iterator
* would not be able to do this. (This explains why LoopTagSupport
* does not simply call for an Iterator from its subtags.)</p>
*
* @return the java.lang.Object to use in the next round of iteration
* @exception org.xml.sax.SAXException
* for other, unexpected exceptions
*/
protected abstract Object next() throws SAXException;
/**
* <p>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.</p>
*
* <p>See <a href="#next()">next</a> for more information about the
* purpose and expectations behind this tag.</p>
*
* @return <tt>true</tt> if there is at least one more item to iterate
* over, <tt>false</tt> otherwise
* @exception org.xml.sax.SAXException
* @see #next()
*/
protected abstract boolean hasNext() throws SAXException;
/**
* <p>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.</p>
*
* @exception org.xml.sax.SAXException
*/
protected abstract void prepare() throws SAXException;
//*********************************************************************
// Lifecycle management and implementation of iterative behavior
// Releases any resources we may have (or inherit)
public void recycle() {
unExposeVariables(); // XXX if doFinally is supported this can removed
init();
super.recycle();
}
// Begins iterating by processing the first item.
public int doStartTag(String namespaceURI, String localName, String qName, Attributes atts) throws SAXException {
// make sure 'begin' isn't greater than 'end'
if (end != -1 && begin > end)
throw new SAXException("begin (" + begin + ") > end (" + end + ")");
// we're beginning a new iteration, so reset our counts (etc.)
index = 0;
count = 1;
last = false;
// let the subclass conduct any necessary preparation
prepare();
// throw away the first 'begin' items (if they exist)
discardIgnoreSubset(begin);
// get the item we're interested in
if (hasNext())
// index is 0-based, so we don't update it for the first item
item = next();
else
return SKIP_BODY;
/*
* now discard anything we have to "step" over.
* (we do this in advance to support LoopTagStatus.isLast())
*/
discard(step - 1);
// prepare to include our body...
exposeVariables();
calibrateLast();
return EVAL_BODY;
}
/*
* Continues the iteration when appropriate -- that is, if we (a) have
* more items and (b) don't run over our 'end' (given our 'step').
*/
public int doAfterBody() throws SAXException {
// re-sync the index, given our prior behind-the-scenes 'step'
index += step - 1;
// increment the count by 1 for each round
count++;
// everything's been prepared for us, so just get the next item
if (hasNext() && !atEnd()) {
index++;
item = next();
} else
return SKIP_BODY;
/*
* now discard anything we have to "step" over.
* (we do this in advance to support LoopTagStatus.isLast())
*/
discard(step - 1);
// prepare to re-iterate...
exposeVariables();
calibrateLast();
return EVAL_BODY_AGAIN;
}
/*
* Removes attributes that our tag 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.
*/
public void doFinally() {
/*
* Make sure to un-expose variables, restoring them to their
* prior values, if applicable.
*/
unExposeVariables();
}
/*
* Be transparent with respect to exceptions: rethrow anything we get.
*/
public void doCatch(Throwable t) throws Throwable {
throw t;
}
//*********************************************************************
// Accessor methods
/*
* Overview: The getXXX() methods we provide implement the Tag
* contract. setXXX() accessors are provided only for those
* properties (attributes) that must be known at translation time,
* on the premise that these accessors will vary less than the
* others in terms of their interface with the page author.
*/
/*
* (Purposely inherit JavaDoc and semantics from LoopTag.
* Subclasses can override this if necessary, but such a need is
* expected to be rare.)
*/
public Object getCurrent() {
return item;
}
/*
* (Purposely inherit JavaDoc and semantics from LoopTag.
* Subclasses can override this method for more fine-grained control
* over LoopTagStatus, but an effort has been made to simplify
* implementation of subclasses that are happy with reasonable default
* behavior.)
*/
public LoopTagStatus getIteratorStatus() {
// local implementation with reasonable default behavior
class Status implements LoopTagStatus {
/*
* All our methods are straightforward. We inherit
* our JavaDoc from LoopTagSupport; see that class
* for more information.
*/
public Object getCurrent() {
/*
* Access the item through getCurrent() instead of just
* returning the item our containing class stores. This
* should allow a subclass of LoopTagSupport to override
* getCurrent() without having to rewrite getIteratorStatus() too.
*/
return (LoopTagSupport.this.getCurrent());
}
public int getIndex() {
return index + begin; // our 'index' isn't getIndex()
}
public int getCount() {
return count;
}
public boolean isFirst() {
return (index == 0); // our 'index' isn't getIndex()
}
public boolean isLast() {
return (last); // use cached value
}
public Integer getBegin() {
if (beginSpecified)
return (new Integer(begin));
else
return null;
}
public Integer getEnd() {
if (endSpecified)
return (new Integer(end));
else
return null;
}
public Integer getStep() {
if (stepSpecified)
return (new Integer(step));
else
return null;
}
}
/*
* We just need one per invocation... Actually, for the current
* implementation, we just need one per instance, but I'd rather
* not keep the reference around once release() has been called.
*/
if (status == null)
status = new Status();
return status;
}
/*
* We only support setter methods for attributes that need to be
* offered as Strings or other literals; other attributes will be
* handled directly by implementing classes, since there might be
* both rtexprvalue- and EL-based varieties, which will have
* different signatures. (We can't pollute child classes by having
* base implementations of those setters here; child classes that
* have attributes with different signatures would end up having
* two incompatible setters, which is illegal for a JavaBean.
*/
// for tag attribute
public void setVarStatus(String statusId) {
this.statusId = statusId;
}
//*********************************************************************
// Protected utility methods
/*
* These methods validate attributes common to iteration tags.
* Call them if your own subclassing implementation modifies them
* -- e.g., if you set them through an expression language.
*/
/**
* Ensures the "begin" property is sensible, throwing an exception
* expected to propagate up if it isn't
*/
protected void validateBegin() throws SAXException {
if (begin < 0)
throw new SAXException("'begin' < 0");
}
/**
* Ensures the "end" property is sensible, throwing an exception
* expected to propagate up if it isn't
*/
protected void validateEnd() throws SAXException {
if (end < 0)
throw new SAXException("'end' < 0");
}
/**
* Ensures the "step" property is sensible, throwing an exception
* expected to propagate up if it isn't
*/
protected void validateStep() throws SAXException {
if (step < 1)
throw new SAXException("'step' <= 0");
}
//*********************************************************************
// Private utility methods
/** (Re)initializes state (during release() or construction) */
private void init() {
// defaults for internal bookkeeping
index = 0; // internal index always starts at 0
count = 1; // internal count always starts at 1
status = null; // we clear status on release()
item = null; // item will be retrieved for each round
last = false; // last must be set explicitly
beginSpecified = false; // not specified until it's specified :-)
endSpecified = false; // (as above)
stepSpecified = false; // (as above)
// defaults for interface with page author
begin = 0; // when not specified, 'begin' is 0 by spec.
end = -1; // when not specified, 'end' is not used
step = 1; // when not specified, 'step' is 1
statusId = null; // when not specified, no variable exported
}
/** Sets 'last' appropriately. */
private void calibrateLast() throws SAXException {
/*
* the current round is the last one if (a) there are no remaining
* elements, or (b) the next one is beyond the 'end'.
*/
last = !hasNext() || atEnd() || (end != -1 && (begin + index + step > end));
}
/**
* Exposes attributes (formerly scripting variables, but no longer!)
* if appropriate. Note that we don't really care, here, whether they're
* scripting variables or not.
*/
private void exposeVariables() throws SAXException {
/*
* We need to support null items returned from next(); we
* do this simply by passing such non-items through to the
* scoped variable as effectively 'null' (that is, by calling
* removeAttribute()).
*
* Also, just to be defensive, we handle the case of a null
* 'status' object as well.
*
* We call getCurrent() and getIteratorStatus() (instead of just using
* 'item' and 'status') to bridge to subclasses correctly.
* A subclass can override getCurrent() or getIteratorStatus() but still
* depend on our doStartTag() and doAfterBody(), which call this
* method (exposeVariables()), to expose 'item' and 'status'
* correctly.
*/
if (var != null) {
if (getCurrent() == null)
removeVariable(var);
else
setVariable(var, getCurrent());
}
if (statusId != null) {
if (getIteratorStatus() == null)
removeVariable(statusId);
else
setVariable(statusId, getIteratorStatus());
}
}
/**
* Removes page attributes that we have exposed and, if applicable,
* restores them to their prior values (and scopes).
*/
private void unExposeVariables() {
// "nested" variables are now simply removed
Request request = ObjectModelHelper.getRequest(objectModel);
if (var != null)
request.removeAttribute(var);
if (statusId != null)
request.removeAttribute(statusId);
}
/**
* Cycles through and discards up to 'n' items from the iteration.
* We only know "up to 'n'", not "exactly n," since we stop cycling
* if hasNext() returns false or if we hit the 'end' of the iteration.
* Note: this does not update the iteration index, since this method
* is intended as a behind-the-scenes operation. The index must be
* updated separately. (I don't really like this, but it's the simplest
* way to support isLast() without storing two separate inconsistent
* indices. We need to (a) make sure hasNext() refers to the next
* item we actually *want* and (b) make sure the index refers to the
* item associated with the *current* round, not the next one.
* C'est la vie.)
*/
private void discard(int n) throws SAXException {
/*
* copy index so we can restore it, but we need to update it
* as we work so that atEnd() works
*/
int oldIndex = index;
while (n-- > 0 && !atEnd() && hasNext()) {
index++;
next();
}
index = oldIndex;
}
/**
* Discards items ignoring subsetting rules. Useful for discarding
* items from the beginning (i.e., to implement 'begin') where we
* don't want factor in the 'begin' value already.
*/
private void discardIgnoreSubset(int n) throws SAXException {
while (n-- > 0 && hasNext())
next();
}
/**
* Returns true if the iteration has past the 'end' index (with
* respect to subsetting), false otherwise. ('end' must be set
* for atEnd() to return true; if 'end' is not set, atEnd()
* always returns false.)
*/
private boolean atEnd() {
return ((end != -1) && (begin + index >= end));
}
}