net.sf.saxon.value
Class MemoClosure

java.lang.Object
  extended by net.sf.saxon.value.Value
      extended by net.sf.saxon.value.Closure
          extended by net.sf.saxon.value.MemoClosure
All 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.

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.

See Also:
Serialized Form

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
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, make, makeIteratorClosure, reduce, saveContext
 
Methods inherited from class net.sf.saxon.value.Value
asItem, asItem, asIterator, asValue, checkPermittedContents, convertToJava, effectiveBooleanValue, equals, fromItem, getCanonicalLexicalRepresentation, getIterator, getSchemaComparable, getStringValue, getStringValueCS, hashCode, isIdentical, iterate
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, 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

Overrides:
iterate in class Closure
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.

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

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 class Value
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

getLength

public int getLength()
              throws XPathException
Get the length of the sequence

Overrides:
getLength in class Value
Returns:
the number of items in the sequence
Throws:
XPathException

isFullyRead

public boolean isFullyRead()
Determine whether the contents of the MemoClosure have been fully read

Returns:
true if the contents have been fully read

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


Copyright (c) 2004-2010 Saxonica Limited. All rights reserved.