Package net.sf.saxon.value

Class MemoClosure

  • All Implemented Interfaces:
    java.io.Serializable, Expression, 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.

    See Also:
    Serialized Form

    • 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​(XPathContext context)
                         throws XPathException
        Evaluate the expression in a given context to return an iterator over a sequence
        Specified by:
        iterate in interface Expression
        Overrides:
        iterate in class Closure
        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

      • 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
        Throws:
        XPathException

      • isFullyRead

        public boolean isFullyRead()
        Determine whether the contents of the MemoClosure 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