Class MemoClosure
- All Implemented Interfaces:
Serializable
,Expression
,ValueRepresentation
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.
- See Also:
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Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionfinal class
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
FieldsFields inherited from class net.sf.saxon.value.Closure
depth, expression, inputIterator, savedXPathContext
Fields inherited from class net.sf.saxon.value.Value
EMPTY_CLASS_ARRAY
Fields inherited from interface net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD
Fields inherited from interface net.sf.saxon.om.ValueRepresentation
EMPTY_VALUE_ARRAY
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Constructor Summary
ConstructorsConstructorDescriptionConstructor should not be called directly, instances should be made using the make() method. -
Method Summary
Modifier and TypeMethodDescriptionint
Get the length of the sequenceboolean
Determine whether the contents of the MemoClosure have been fully readitemAt
(int n) Get the n'th item in the sequence (starting from 0).iterate
(XPathContext context) Evaluate the expression in a given context to return an iterator over a sequenceReturn a value containing all the items in the sequence returned by this SequenceIteratorvoid
process
(XPathContext context) Process the instruction, without returning any tail callsMethods inherited from class net.sf.saxon.value.Closure
display, getCardinality, getImplementationMethod, getItemType, getSpecialProperties, isIndexable, make, makeIteratorClosure, reduce
Methods inherited from class net.sf.saxon.value.Value
asItem, asIterator, asValue, checkPermittedContents, convert, convertJavaObjectToXPath, convertToJava, effectiveBooleanValue, equals, evaluateAsString, evaluateItem, getDependencies, getIterator, getParentExpression, getStringValue, getStringValueCS, hashCode, iterateSubExpressions, makeQNameValue, optimize, promote, simplify, stringToNumber, toString, typeCheck
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Field Details
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state
protected int state -
EMPTY
protected static final int EMPTY- See Also:
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Constructor Details
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MemoClosure
public MemoClosure()Constructor should not be called directly, instances should be made using the make() method.
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Method Details
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iterate
Evaluate the expression in a given context to return an iterator over a sequence- Specified by:
iterate
in interfaceExpression
- Overrides:
iterate
in classClosure
- Parameters:
context
- the evaluation context. This is ignored; we use the context saved as part of the Closure instead.- Returns:
- a SequenceIterator that can be used to iterate over the result of the expression
- Throws:
XPathException
- if any dynamic error occurs evaluating the expression
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process
Process the instruction, without returning any tail calls- Specified by:
process
in interfaceExpression
- Overrides:
process
in classClosure
- Parameters:
context
- The dynamic context, giving access to the current node, the current variables, etc.- Throws:
XPathException
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itemAt
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
- Throws:
XPathException
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getLength
Get the length of the sequence- Overrides:
getLength
in classValue
- Throws:
XPathException
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isFullyRead
public boolean isFullyRead()Determine whether the contents of the MemoClosure have been fully read -
materialize
Return a value containing all the items in the sequence returned by this SequenceIterator- Returns:
- the corresponding value
- Throws:
XPathException
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