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java.lang.Object | +--net.sf.saxon.value.Value | +--net.sf.saxon.value.SequenceValue | +--net.sf.saxon.value.Closure
A Closure represents a value that has not yet been evaluated: the value is represented by an expression, together with saved values of all the context variables that the expression depends on.
The Closure maintains a reservoir containing those items in the value that have already been read. When a new iterator is requested to read the value, this iterator first examines and returns any items already placed in the reservoir by previous users of the Closure. When the reservoir is exhausted, it then uses an underlying Input Iterator to read further values of the underlying expression. If the value is not read to completion (for example, if the first user did exists($expr), then the Input Iterator is left positioned where this user abandoned it. The next user will read any values left in the reservoir by the first user, and then pick up iterating the base expression where the first user left off. Eventually, all the values of the expression will find their way into the reservoir, and future users simply iterate over the reservoir contents. Alternatively, of course, the values may be left unread.
Delayed evaluation is used only for expressions with a static type that allows more than one item, so the evaluateItem() method will not normally be used, but it is supported for completeness.
The expression may depend on local variables and on the context item; these values are held in the saved XPathContext object that is kept as part of the Closure, and they will always be read from that object. The expression may also depend on global variables; these are unchanging, so they can be read from the Bindery in the normal way. Expressions that depend on other contextual information, for example the values of position(), last(), current(), current-group(), should not be evaluated using this mechanism: they should always be evaluated eagerly. This means that the Closure does not need to keep a copy of these context variables.
Nested Class Summary | |
class |
Closure.ProgressiveIterator
A ProgressiveIterator starts by reading any items already held in the reservoir; when the reservoir is exhausted, it reads further items from the inputIterator, copying them into the reservoir as they are read. |
Method Summary | |
java.lang.Object |
convertToJava(java.lang.Class target,
Configuration config)
Convert to Java object (for passing to external functions) |
void |
display(int level,
NamePool pool)
Diagnostic display of the expression |
Item |
evaluateItem(XPathContext context)
Evaluate as a singleton. |
int |
getCardinality()
Get the cardinality |
ItemType |
getItemType()
Get the static item type |
int |
getSpecialProperties()
Get the static properties of this expression (other than its type). |
Item |
itemAt(int n)
Get the n'th item in the sequence (starting from 0). |
SequenceIterator |
iterate(XPathContext context)
Evaluate the expression in a given context to return an iterator over a sequence |
static Value |
make(Expression expression,
XPathContext context)
Construct a Closure by supplying the expression and the set of context variables. |
Methods inherited from class net.sf.saxon.value.SequenceValue |
conversionPreference, effectiveBooleanValue, evaluateAsString, getStringValue, nodeToJavaPreference |
Methods inherited from class net.sf.saxon.value.Value |
analyze, asItem, asValue, collapseWhitespace, getDependencies, getSubExpressions, inverse, normalizeWhitespace, promote, simplify, stringToNumber, trimWhitespace |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Method Detail |
public static Value make(Expression expression, XPathContext context) throws XPathException
XPathException
public ItemType getItemType()
getItemType
in interface Expression
getItemType
in class SequenceValue
public int getCardinality()
getCardinality
in interface Expression
getCardinality
in class SequenceValue
public int getSpecialProperties()
getSpecialProperties
in interface Expression
getSpecialProperties
in class Value
public Item evaluateItem(XPathContext context) throws XPathException
evaluateItem
in interface Expression
evaluateItem
in class SequenceValue
context
- The context in which the expression is to be evaluated
XPathException
- if any dynamic error occurs evaluating the
expressionpublic SequenceIterator iterate(XPathContext context) throws XPathException
context
- the evaluation context. This is ignored; we use the context saved
as part of the Closure instead.
XPathException
- if any dynamic error occurs evaluating the
expressionpublic Item itemAt(int n) throws XPathException
itemAt
in class SequenceValue
XPathException
public java.lang.Object convertToJava(java.lang.Class target, Configuration config) throws XPathException
convertToJava
in class SequenceValue
target
- The class required by the external functionconfig
- The configuration (needed for access to schema information)
XPathException
public void display(int level, NamePool pool)
SequenceValue
display
in interface Expression
display
in class SequenceValue
level
- indentation level for this expression
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