net.sf.saxon.value
Class Closure

java.lang.Object
  extended bynet.sf.saxon.value.Value
      extended bynet.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 interface net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
 
Constructor Summary
protected Closure()
          Private constructor: instances must be created using the make() method
 
Method Summary
 void display(int level, NamePool pool, java.io.PrintStream out)
          Convert to Java object (for passing to external functions)
 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()
          Get the static item type
 int getSpecialProperties()
          Get the static properties of this expression (other than its type).
 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, boolean save)
          Construct a Closure by supplying the expression and the set of context variables.
 void process(XPathContext context)
          Process the instruction, without returning any tail calls
 
Methods inherited from class net.sf.saxon.value.Value
analyze, asItem, asValue, checkPermittedContents, collapseWhitespace, convert, convertJavaObjectToXPath, convertToJava, effectiveBooleanValue, equals, evaluateAsString, evaluateItem, getDependencies, getIterator, getLength, getParentExpression, getStringValue, hashCode, itemAt, iterateSubExpressions, makeQNameValue, normalizeWhitespace, promote, simplify, stringToNumber, toString, trimWhitespace
 
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

protected Closure()
Private constructor: instances must be created using the make() method

Method Detail

make

public static Value make(Expression expression,
                         XPathContext context,
                         boolean save)
                  throws XPathException
Construct a Closure by supplying the expression and the set of context variables.

Throws:
XPathException

getItemType

public ItemType getItemType()
Get the static item type

Specified by:
getItemType in interface Expression
Overrides:
getItemType in class Value
Returns:
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

display

public void display(int level,
                    NamePool pool,
                    java.io.PrintStream out)
Convert to Java object (for passing to external functions)

Specified by:
display in interface Expression
Overrides:
display in class Value