Class MemoClosure

  • All Implemented Interfaces:
    ContextOriginator, Sequence

    public class MemoClosure
    extends Closure
    implements ContextOriginator
    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.

    • Method Detail

      • iterate

        public SequenceIterator iterate()
        Evaluate the expression in a given context to return an iterator over a sequence
        Specified by:
        iterate in interface Sequence
        Specified by:
        iterate in class Closure
        an iterator (specifically, a Saxon SequenceIterator, which is not a Iterator) over all the items
      • itemAt

        public Item itemAt​(int n)
                    throws XPathException
        Get the n'th item in the sequence, zero-based
        n - the index of the required item, starting from zero
        the item at the relevant position
        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
        reduce in class Closure
        the corresponding value
        XPathException - if a failure occurs reading the input
      • makeRepeatable

        public Sequence makeRepeatable()
        Description copied from interface: Sequence
        Ensure that the sequence is in a form where it can be evaluated more than once. Some sequences (for example LazySequence and Closure can only be evaluated once, and this operation causes these to be grounded. However, making it repeatable is not the same as making it grounded; it does not flush out all errors. Indeed, lazy evaluation relies on this property, because an expression that has been lifted out of a loop must not be evaluated unless the loop is executed at least once, to prevent spurious errors.
        Specified by:
        makeRepeatable in interface Sequence
        makeRepeatable in class Closure
        An equivalent sequence that can be repeatedly evaluated
      • getSequenceAsIs

        public Sequence getSequenceAsIs()