public class Closure extends java.lang.Object implements Sequence, ContextOriginatorA 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.
Constructors Constructor Description
Closure()Constructor should not be called directly, instances should be made using the make() method.
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description
head()Get the first item in the sequence.
iterate()Evaluate the expression in a given context to return an iterator over a sequence
make(Expression expression, XPathContext context, int ref)Construct a Closure by supplying the expression and the set of context variables.
makeRepeatable()Ensure that the sequence is in a form where it can be evaluated more than once.
reduce()Reduce a value to its simplest form.
saveContext(Expression expression, XPathContext context)
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
public static Sequence make(Expression expression, XPathContext context, int ref) throws XPathExceptionConstruct a Closure by supplying the expression and the set of context variables.
expression- the expression to be lazily evaluated
context- the dynamic context of the expression including for example the variables on which it depends
ref- the number of references to the value being lazily evaluated; this affects the kind of Closure that is created
- the Closure, a virtual value that can later be materialized when its content is required
XPathException- if a dynamic error occurs
public void saveContext(Expression expression, XPathContext context) throws XPathException
public Item head() throws XPathExceptionGet the first item in the sequence.
public Expression getExpression()
public XPathContextMajor getSavedXPathContext()
public void setExpression(Expression expression)
public void setSavedXPathContext(XPathContextMajor savedXPathContext)
public SequenceIterator iterate() throws XPathExceptionEvaluate the expression in a given context to return an iterator over a sequence
public GroundedValue reduce() throws XPathExceptionReduce 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
- the simplified value
XPathException- if an error occurs doing the lazy evaluation
public Sequence makeRepeatable() throws XPathExceptionDescription copied from interface:
SequenceEnsure that the sequence is in a form where it can be evaluated more than once. Some sequences (for example
Closurecan only be evaluated once, and this operation causes these to be grounded. However, making it repeatable is not the same as making it grounded; it does not flush out all errors. Indeed, lazy evaluation relies on this property, because an expression that has been lifted out of a loop must not be evaluated unless the loop is executed at least once, to prevent spurious errors.