Class MemoClosure

All Implemented Interfaces:
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:
  • Field Details

  • Constructor Details

    • MemoClosure

      public MemoClosure()
      Constructor should not be called directly, instances should be made using the make() method.
  • Method Details

    • 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
    • process

      public void process(XPathContext context) throws XPathException
      Process the instruction, without returning any tail calls
      Specified by:
      process in interface Expression
      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
      Throws:
      XPathException
    • getLength

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