Package net.sf.saxon.value

Class MemoClosure

java.lang.Object

net.sf.saxon.value.Closure

net.sf.saxon.value.MemoClosure

All Implemented Interfaces:

ContextOriginator, Sequence

public class MemoClosure
extends Closure
implements ContextOriginator

A MemoClosure 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 MemoClosure is designed for use when the value is only read several times. The value is saved on the first evaluation and remembered for later use.

The MemoClosure 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 MemoClosure. 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.

In Saxon-EE, a for-each loop can be multithreaded. If a variable declared outside the loop is evaluated as a MemoClosure, then a reference to the variable within the loop can result in concurrent attempts to evaluate the variable incrementally. This is prevented by synchronizing the evaluation methods.

Field Summary

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

depth, expression, inputEvaluator, inputIterator, learningEvaluator, savedXPathContext, serialNumber

Constructor Summary

Constructors

Constructor	Description
MemoClosure(Expression expr, PullEvaluator inputEvaluator, XPathContext context)

Method Summary

Modifier and Type	Method	Description
Sequence	getSequenceAsIs()
Item	itemAt(int n)	Get the n'th item in the sequence, zero-based
SequenceIterator	iterate()	Evaluate the expression in a given context to return an iterator over a sequence
Sequence	makeRepeatable()	Ensure that the sequence is in a form where it can be evaluated more than once.
GroundedValue	reduce()	Return a value containing all the items in the sequence returned by this SequenceIterator

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

getExpression, getSavedXPathContext, head, saveContext, setInputEvaluator, setLearningEvaluator, setSavedXPathContext

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface net.sf.saxon.om.Sequence

materialize

Constructor Details

MemoClosure

public MemoClosure(Expression expr,
 PullEvaluator inputEvaluator,
 XPathContext context)
            throws XPathException

Throws:

XPathException

Method Details

iterate

public SequenceIterator iterate()

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

Specified by:

iterate in interface Sequence

Specified by:

iterate in class Closure

Returns:

an iterator (specifically, a Saxon SequenceIterator, which is not a Iterator) over all the items

itemAt

public Item itemAt(int n)
            throws XPathException

Get the n'th item in the sequence, zero-based

Parameters:

n - the index of the required item, starting from zero

Returns:

the item at the relevant position

Throws:

XPathException - if an error occurs reading the base iterator

reduce

public GroundedValue reduce()
                     throws XPathException

Return a value containing all the items in the sequence returned by this SequenceIterator

Overrides:

reduce in class Closure

Returns:

the corresponding value

Throws:

XPathException - if a failure occurs reading the input

makeRepeatable

public Sequence makeRepeatable()

Description copied from interface: Sequence

Ensure that the sequence is in a form where it can be evaluated more than once. Some sequences (for example LazySequence and Closure can 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.

Specified by:

makeRepeatable in interface Sequence

Overrides:

makeRepeatable in class Closure

Returns:

An equivalent sequence that can be repeatedly evaluated

getSequenceAsIs

public Sequence getSequenceAsIs()