Class Closure

  • All Implemented Interfaces:
    java.io.Serializable, Expression, ValueRepresentation
    Direct Known Subclasses:
    MemoClosure

    public class Closure
    extends Value
    A Closure 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.

    This Closure is designed for use when the value is only read once. If the value is read more than once, a new iterator over the underlying expression is obtained each time: this may (for example in the case of a filter expression) involve significant re-calculation.

    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

      • Closure

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

      • makeIteratorClosure

        public static Closure makeIteratorClosure​(SequenceIterator iterator)
        Construct a Closure over an existing SequenceIterator. This is used when an extension function returns a SequenceIterator as its result (it replaces the old SequenceIntent class for this purpose). There is no known expression in this case. Note that the caller must ensure this is a "clean" iterator: it must be positioned at the start, and must not be shared by anyone else.
      • getSpecialProperties

        public int getSpecialProperties()
        Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.
        Specified by:
        getSpecialProperties in interface Expression
        Overrides:
        getSpecialProperties in class Value
        Returns:
        StaticProperty.NON_CREATIVE
      • getImplementationMethod

        public int getImplementationMethod()
        An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided. This implementation provides both iterate() and process() methods natively.
      • iterate

        public SequenceIterator iterate​(XPathContext context)
                                 throws XPathException
        Evaluate the expression in a given context to return an iterator over a sequence
        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 Value
        Parameters:
        context - The dynamic context, giving access to the current node, the current variables, etc.
        Throws:
        XPathException
      • reduce

        public Value reduce()
                     throws XPathException
        Reduce a value to its simplest form. If the value is a closure or some other form of deferred value such as a FunctionCallPackage, then it is reduced to a SequenceExtent. If it is a SequenceExtent containing a single item, then it is reduced to that item. One consequence that is exploited by class FilterExpression is that if the value is a singleton numeric value, then the result will be an instance of NumericValue
        Overrides:
        reduce in class Value
        Throws:
        XPathException
      • isIndexable

        public boolean isIndexable()
        Determine whether this Closure is indexable
      • display

        public void display​(int level,
                            java.io.PrintStream out,
                            Configuration config)
        Description copied from class: Value
        Diagnostic display of the expression
        Specified by:
        display in interface Expression
        Overrides:
        display in class Value
        Parameters:
        level - indentation level for this expression
        out - Output destination