net.sf.saxon.value
Class Closure

java.lang.Object
  net.sf.saxon.value.Value
      net.sf.saxon.value.Closure

All Implemented Interfaces:

Expression, java.io.Serializable, ValueRepresentation

Direct Known Subclasses:

MemoClosure

public class Closure

extends Value

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.

This Closure is designed for use when the value is only read once. If the value is read more than once, a new iterator over the underlying expression is obtained each time: this may (for example in the case of a filter expression) involve significant re-calculation.

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.

See Also:

Serialized Form

Field Summary

protected int	depth
protected Expression	expression
protected SequenceIterator	inputIterator
protected XPathContextMajor	savedXPathContext

Fields inherited from class net.sf.saxon.value.Value

EMPTY_CLASS_ARRAY

Fields inherited from interface net.sf.saxon.expr.Expression

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

Fields inherited from interface net.sf.saxon.om.ValueRepresentation

EMPTY_VALUE_ARRAY

Constructor Summary

Closure()
Constructor should not be called directly, instances should be made using the make() method.

Method Summary

void	display(int level, NamePool pool, java.io.PrintStream out) Diagnostic display of the expression
int	getCardinality() Get the cardinality
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
ItemType	getItemType(TypeHierarchy th) Get the static item type
int	getSpecialProperties() Get the static properties of this expression (other than its type).
boolean	isIndexable() Determine whether this Closure is indexable
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, int ref) Construct a Closure by supplying the expression and the set of context variables.
static Closure	makeIteratorClosure(SequenceIterator iterator) Construct a Closure over an existing SequenceIterator.
void	process(XPathContext context) Process the instruction, without returning any tail calls
Value	reduce() Reduce a value to its simplest form.

Methods inherited from class net.sf.saxon.value.Value

asItem, asIterator, asValue, checkPermittedContents, collapseWhitespace, convert, convertJavaObjectToXPath, convertToJava, effectiveBooleanValue, equals, evaluateAsString, evaluateItem, getDependencies, getIterator, getLength, getParentExpression, getStringValue, hashCode, itemAt, iterateSubExpressions, makeQNameValue, normalizeWhitespace, optimize, promote, schemaEquals, simplify, stringToNumber, toString, trimWhitespace, typeCheck

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

expression

protected Expression expression

savedXPathContext

protected XPathContextMajor savedXPathContext

depth

protected int depth

inputIterator

protected SequenceIterator inputIterator

Constructor Detail

Closure

public Closure()

Constructor should not be called directly, instances should be made using the make() method.

Method Detail

makeIteratorClosure

public static Closure makeIteratorClosure(SequenceIterator iterator)

Construct a Closure over an existing SequenceIterator. This is used when an extension function returns a SequenceIterator as its result (it replaces the old SequenceIntent class for this purpose). There is no known expression in this case. Note that the caller must ensure this is a "clean" iterator: it must be positioned at the start, and must not be shared by anyone else.

make

public static Value make(Expression expression,
                         XPathContext context,
                         int ref)
                  throws XPathException

Construct a Closure by supplying the expression and the set of context variables.

Throws:

XPathException

getItemType

public ItemType getItemType(TypeHierarchy th)

Get the static item type

Specified by:

getItemType in interface Expression

Overrides:

getItemType in class Value

Parameters:

th -

Returns:

for the default implementation: AnyItemType (not known)

getCardinality

public int getCardinality()

Get the cardinality

Specified by:

getCardinality in interface Expression

Overrides:

getCardinality in class Value

getSpecialProperties

public int getSpecialProperties()

Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.

Specified by:

getSpecialProperties in interface Expression

Overrides:

getSpecialProperties in class Value

Returns:

StaticProperty.NON_CREATIVE

getImplementationMethod

public int getImplementationMethod()

An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided. This implementation provides both iterate() and process() methods natively.

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

Evaluate the expression in a given context to return an iterator over a sequence

Parameters:

context - the evaluation context. This is ignored; we use the context saved as part of the Closure instead.

Returns:

a SequenceIterator that can be used to iterate over the result of the expression

Throws:

XPathException - if any dynamic error occurs evaluating the expression

process

public void process(XPathContext context)
             throws XPathException

Process the instruction, without returning any tail calls

Specified by:

process in interface Expression

Overrides:

process in class Value

Parameters:

context - The dynamic context, giving access to the current node, the current variables, etc.

Throws:

XPathException

reduce

public Value reduce()
             throws XPathException

Reduce a value to its simplest form. If the value is a closure or some other form of deferred value such as a FunctionCallPackage, then it is reduced to a SequenceExtent. If it is a SequenceExtent containing a single item, then it is reduced to that item. One consequence that is exploited by class FilterExpression is that if the value is a singleton numeric value, then the result will be an instance of NumericValue

Overrides:

reduce in class Value

Throws:

XPathException

isIndexable

public boolean isIndexable()

Determine whether this Closure is indexable

display

public void display(int level,
                    NamePool pool,
                    java.io.PrintStream out)

Description copied from class: Value

Diagnostic display of the expression

Specified by:

display in interface Expression

Overrides:

display in class Value