Class MemoClosure<T extends Item>
- java.lang.Object
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- net.sf.saxon.value.Value<T>
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- net.sf.saxon.value.Closure<T>
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- net.sf.saxon.value.MemoClosure<T>
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- All Implemented Interfaces:
ValueRepresentation<T>
public class MemoClosure<T extends Item> extends Closure<T>
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.
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Nested Class Summary
Nested Classes Modifier and Type Class 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.
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Field Summary
Fields Modifier and Type Field Description protected static int
EMPTY
protected int
state
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Fields inherited from class net.sf.saxon.value.Closure
depth, expression, inputIterator, savedXPathContext
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Fields inherited from class net.sf.saxon.value.Value
INDETERMINATE_ORDERING
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Fields inherited from interface net.sf.saxon.om.ValueRepresentation
EMPTY_VALUE_ARRAY
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Constructor Summary
Constructors Constructor Description MemoClosure()
Constructor should not be called directly, instances should be made using the make() method.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description int
getLength()
Get the length of the sequenceboolean
isFullyRead()
Determine whether the contents of the MemoClosure have been fully readT
itemAt(int n)
Get the n'th item in the sequence (starting from 0).SequenceIterator<T>
iterate()
Evaluate the expression in a given context to return an iterator over a sequenceValue
materialize()
Return a value containing all the items in the sequence returned by this SequenceIteratorvoid
process(XPathContext context)
Process the expression by writing the value to the current Receiver-
Methods inherited from class net.sf.saxon.value.Closure
getCardinality, getExpression, getItemType, getSavedXPathContext, make, makeIteratorClosure, reduce, saveContext, setExpression, setSavedXPathContext
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Methods inherited from class net.sf.saxon.value.Value
asItem, asItem, asIterator, asValue, checkPermittedContents, convertToJava, effectiveBooleanValue, equals, fromItem, getCanonicalLexicalRepresentation, getIterator, getSchemaComparable, getSequenceLength, getStringValue, getStringValueCS, hashCode, isIdentical
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Field Detail
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state
protected int state
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EMPTY
protected static final int EMPTY
- See Also:
- Constant Field Values
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Method Detail
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iterate
public SequenceIterator<T> iterate() throws XPathException
Evaluate the expression in a given context to return an iterator over a sequence- Overrides:
iterate
in classClosure<T extends Item>
- Returns:
- an iterator over the sequence of items
- Throws:
XPathException
- if a dynamic error occurs. This is possible only in the case of values that are materialized lazily, that is, where the iterate() method leads to computation of an expression that delivers the values.
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process
public void process(XPathContext context) throws XPathException
Process the expression by writing the value to the current Receiver- Overrides:
process
in classClosure<T extends Item>
- Parameters:
context
- The dynamic context, giving access to the current node, the current variables, etc.- Throws:
XPathException
- if an error occurs (for example if the value is a closure that needs to be evaluated)
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itemAt
public T 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 classValue<T extends Item>
- Parameters:
n
- position of the required item, counting from zero.- Returns:
- the n'th item in the sequence, where the first item in the sequence is numbered zero. If n is negative or >= the length of the sequence, returns null.
- Throws:
XPathException
- if an error occurs (for example if the value is a Closure that needs to be evaluated to find the Nth item)
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getLength
public int getLength() throws XPathException
Get the length of the sequence- Overrides:
getLength
in classValue<T extends Item>
- Returns:
- the number of items in the sequence
- Throws:
XPathException
- if an error occurs (for example if the value is a closure that needs to be read to determine its length)
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isFullyRead
public boolean isFullyRead()
Determine whether the contents of the MemoClosure have been fully read- Returns:
- true if the contents have been fully read
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materialize
public Value materialize() throws XPathException
Return a value containing all the items in the sequence returned by this SequenceIterator- Returns:
- the corresponding value
- Throws:
XPathException
- if a failure occurs reading the input
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