net.sf.saxon.value

Class MemoClosure

Implemented Interfaces:
Serializable, SequenceIterable, ValueRepresentation

public class MemoClosure
extends Closure

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.

Nested Class Summary

class
MemoClosure.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.

Field Summary

protected static int
EMPTY
protected int
state

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

depth, expression, inputIterator, savedXPathContext

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

EMPTY_CLASS_ARRAY, INDETERMINATE_ORDERING

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

EMPTY_VALUE_ARRAY

Constructor Summary

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

Method Summary

int
getLength()
Get the length of the sequence
boolean
isFullyRead()
Determine whether the contents of the MemoClosure have been fully read
Item
itemAt(int n)
Get the n'th item in the sequence (starting from 0).
SequenceIterator
iterate()
Evaluate the expression in a given context to return an iterator over a sequence
Value
materialize()
Return a value containing all the items in the sequence returned by this SequenceIterator
void
process(XPathContext context)
Process the expression by writing the value to the current Receiver

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

getCardinality, getItemType, iterate, make, makeIteratorClosure, process, reduce, saveContext

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

asItem, asItem, asIterator, asValue, checkPermittedContents, convertJavaObjectToXPath, convertToJava, convertToJava, effectiveBooleanValue, equals, fromItem, getCanonicalLexicalRepresentation, getCardinality, getItemType, getIterator, getLength, getSchemaComparable, getStringValue, getStringValueCS, itemAt, iterate, iterate, makeQNameValue, process, reduce, stringToNumber, toString

Field Details

EMPTY

protected static final int EMPTY
Field Value:
5

state

protected int state

Constructor Details

MemoClosure

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

Method Details

getLength

public int getLength()
            throws XPathException
Get the length of the sequence
Overrides:
getLength in interface Value

isFullyRead

public boolean isFullyRead()
Determine whether the contents of the MemoClosure have been fully read
Returns:
true if the contents have been fully read

itemAt

public Item itemAt(int n)
            throws XPathException
Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally
Overrides:
itemAt in interface Value

iterate

public SequenceIterator iterate()
            throws XPathException
Evaluate the expression in a given context to return an iterator over a sequence
Overrides:
iterate in interface Closure

materialize

public Value materialize()
            throws XPathException
Return a value containing all the items in the sequence returned by this SequenceIterator
Returns:
the corresponding value

process

public void process(XPathContext context)
            throws XPathException
Process the expression by writing the value to the current Receiver
Overrides:
process in interface Closure
Parameters:
context - The dynamic context, giving access to the current node, the current variables, etc.