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

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.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
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

Fields

Modifier and Type	Field and Description
protected static int	EMPTY
protected int	state

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

depth, expression, inputIterator, savedXPathContext

Constructor Summary

Constructors

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

Method Summary

Methods

Modifier and Type	Method and Description
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, zero-based
SequenceIterator	iterate() Evaluate the expression in a given context to return an iterator over a sequence
void	process(XPathContext context) Process the expression by writing the value to the current Receiver
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, make, saveContext, setExpression, setSavedXPathContext

Methods inherited from class java.lang.Object

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

Field Detail

state

protected int state

EMPTY

protected static final int EMPTY

See Also:

Constant Field Values

Constructor Detail

MemoClosure

public MemoClosure()

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

Method Detail

iterate

public SequenceIterator iterate()
                         throws XPathException

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

Specified by:

iterate in interface Sequence

Overrides:

iterate in class Closure

Returns:

an iterator over all the items

Throws:

XPathException - in the situation where the sequence is evaluated lazily, and constructing an iterator over the items causes a dynamic error.

process

public void process(XPathContext context)
             throws XPathException

Process the expression by writing the value to the current Receiver

Overrides:

process in class Closure

Parameters:

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

Throws:

XPathException - if an error occurs evaluating the input expression

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, 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