Package net.sf.saxon.expr

Class AxisExpression

  • All Implemented Interfaces:
    ExportAgent, Locatable, IdentityComparable, Traceable
    public final class AxisExpression
extends Expression
    An AxisExpression is always obtained by simplifying a PathExpression. It represents a PathExpression that starts at the context node, and uses a simple node-test with no filters. For example "*", "title", "./item", "@*", or "ancestor::chapter*".

    An AxisExpression delivers nodes in axis order (not in document order). To get nodes in document order, in the case of a reverse axis, the expression should be wrapped in a call on reverse().

    • Constructor Detail

      • AxisExpression

        public AxisExpression​(int axis,
                      NodeTest nodeTest)
        Constructor for an AxisExpression whose origin is the context item
        Parameters:
        axis - The axis to be used in this AxisExpression: relevant constants are defined in class AxisInfo.
        nodeTest - The conditions to be satisfied by selected nodes. May be null, indicating that any node on the axis is acceptable
        See Also:
        AxisInfo

    • Method Detail

      • setAxis

        public void setAxis​(int axis)
        Set the axis
        Parameters:
        axis - the new axis

      • getExpressionName

        public java.lang.String getExpressionName()
        Get a name identifying the kind of expression, in terms meaningful to a user.
        Overrides:
        getExpressionName in class Expression
        Returns:
        a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in explain() output displaying the expression.

      • simplify

        public Expression simplify()
                    throws XPathException
        Simplify an expression
        Overrides:
        simplify in class Expression
        Returns:
        the simplified expression (or the original if unchanged, or if modified in-situ)
        Throws:
        XPathException - if an error is discovered during expression rewriting

      • typeCheck

        public Expression typeCheck​(ExpressionVisitor visitor,
                            ContextItemStaticInfo contextInfo)
                     throws XPathException
        Type-check the expression
        Overrides:
        typeCheck in class Expression
        Parameters:
        visitor - an expression visitor
        contextInfo - Information available statically about the context item: whether it is (possibly) absent; its static type; its streaming posture.
        Returns:
        the original expression, rewritten to perform necessary run-time type checks, and to perform other type-related optimizations
        Throws:
        XPathException - if an error is discovered during this phase (typically a type error)

      • getContextItemType

        public ItemType getContextItemType()
        Get the static type of the context item for this AxisExpression. May be null if not known.
        Returns:
        the statically-inferred type, or null if not known

      • optimize

        public Expression optimize​(ExpressionVisitor visitor,
                           ContextItemStaticInfo contextInfo)
        Perform optimisation of an expression and its subexpressions.

        This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

        Overrides:
        optimize in class Expression
        Parameters:
        visitor - an expression visitor
        contextInfo - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
        Returns:
        the original expression, rewritten if appropriate to optimize execution

      • getCost

        public double getCost()
        Return the estimated cost of evaluating an expression. This is a very crude measure based on the syntactic form of the expression (we have no knowledge of data values). We take the cost of evaluating a simple scalar comparison or arithmetic expression as 1 (one), and we assume that a sequence has length 5. The resulting estimates may be used, for example, to reorder the predicates in a filter expression so cheaper predicates are evaluated first.
        Overrides:
        getCost in class Expression
        Returns:
        the estimated cost

      • equals

        public boolean equals​(java.lang.Object other)
        Is this expression the same as another expression?
        Overrides:
        equals in class Expression
        Parameters:
        other - the other operand; the result is false if this is not an Expression
        Returns:
        true if the other operand is an expression and if it can be determined that the two expressions are equivalent, in the sense that they will always return the same result.

      • computeHashCode

        protected int computeHashCode()
        get HashCode for comparing two expressions
        Overrides:
        computeHashCode in class Expression
        Returns:
        a computed hash code

      • copy

        public Expression copy​(RebindingMap rebindings)
        Copy an expression. This makes a deep copy.
        Specified by:
        copy in class Expression
        Parameters:
        rebindings - a mutable list of (old binding, new binding) pairs that is used to update the bindings held in any local variable references that are copied.
        Returns:
        the copy of the original expression

      • computeSpecialProperties

        protected int computeSpecialProperties()
        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.
        Overrides:
        computeSpecialProperties in class Expression
        Returns:
        the special properties, as a bit-significant integer

      • getItemType

        public final ItemType getItemType()
        Determine the data type of the items returned by this expression
        Specified by:
        getItemType in class Expression
        Returns:
        Type.NODE or a subtype, based on the NodeTest in the axis step, plus information about the content type if this is known from schema analysis

      • getStaticUType

        public UType getStaticUType​(UType contextItemType)
        Get the static type of the expression as a UType, following precisely the type inference rules defined in the XSLT 3.0 specification.
        Overrides:
        getStaticUType in class Expression
        Parameters:
        contextItemType - the static type of the context item for the expression evaluation
        Returns:
        the static item type of the expression according to the XSLT 3.0 defined rules

      • getIntrinsicDependencies

        public int getIntrinsicDependencies()
        Determine the intrinsic dependencies of an expression, that is, those which are not derived from the dependencies of its subexpressions. For example, position() has an intrinsic dependency on the context position, while (position()+1) does not. The default implementation of the method returns 0, indicating "no dependencies".
        Overrides:
        getIntrinsicDependencies in class Expression
        Returns:
        a set of bit-significant flags identifying the "intrinsic" dependencies. The flags are documented in class net.sf.saxon.value.StaticProperty

      • isSubtreeExpression

        public boolean isSubtreeExpression()
        Determine whether the expression can be evaluated without reference to the part of the context document outside the subtree rooted at the context node.
        Overrides:
        isSubtreeExpression in class Expression
        Returns:
        true if the expression has no dependencies on the context node, or if the only dependencies on the context node are downward selections using the self, child, descendant, attribute, and namespace axes.

      • getAxis

        public int getAxis()
        Get the axis
        Returns:
        the axis number, for example AxisInfo.CHILD

      • getNodeTest

        public NodeTest getNodeTest()
        Get the NodeTest. Returns null if the AxisExpression can return any node.
        Returns:
        the node test, or null if all nodes are returned

      • addToPathMap

        public PathMap.PathMapNodeSet addToPathMap​(PathMap pathMap,
                                           PathMap.PathMapNodeSet pathMapNodeSet)
        Add a representation of this expression to a PathMap. The PathMap captures a map of the nodes visited by an expression in a source tree.
        Overrides:
        addToPathMap in class Expression
        Parameters:
        pathMap - the PathMap to which the expression should be added
        pathMapNodeSet - the PathMapNodeSet to which the paths embodied in this expression should be added
        Returns:
        the pathMapNode representing the focus established by this expression, in the case where this expression is the first operand of a path expression or filter expression

      • isContextPossiblyUndefined

        public boolean isContextPossiblyUndefined()
        Ask whether there is a possibility that the context item will be undefined
        Returns:
        true if this is a possibility

      • getImplementationMethod

        public int getImplementationMethod()
        Description copied from class: Expression
        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 directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.
        Specified by:
        getImplementationMethod in class Expression
        Returns:
        the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

      • iterate

        public SequenceIterator iterate​(XPathContext context)
                         throws XPathException
        Evaluate the path-expression in a given context to return a NodeSet
        Overrides:
        iterate in class Expression
        Parameters:
        context - the evaluation context
        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

      • iterate

        public AxisIterator iterate​(NodeInfo origin)
        Iterate the axis from a given starting node, without regard to context
        Parameters:
        origin - the starting node
        Returns:
        the iterator over the axis

      • export

        public void export​(ExpressionPresenter destination)
            throws XPathException
        Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.
        Specified by:
        export in interface ExportAgent
        Specified by:
        export in class Expression
        Parameters:
        destination - the expression presenter used to display the structure
        Throws:
        XPathException - if the export fails, for example if an expression is found that won't work in the target environment.

      • toString

        public java.lang.String toString()
        Represent the expression as a string. The resulting string will be a valid XPath 3.0 expression with no dependencies on namespace bindings other than the binding of the prefix "xs" to the XML Schema namespace.
        Overrides:
        toString in class Expression
        Returns:
        the expression as a string in XPath 3.0 syntax

      • toShortString

        public java.lang.String toShortString()
        Description copied from class: Expression
        Produce a short string identifying the expression for use in error messages
        Overrides:
        toShortString in class Expression
        Returns:
        a short string, sufficient to identify the expression

      • getStreamerName

        public java.lang.String getStreamerName()
        Description copied from class: Expression
        Get the (partial) name of a class that supports streaming of this kind of expression
        Overrides:
        getStreamerName in class Expression
        Returns:
        the partial name of a class that can be instantiated to provide streaming support in Saxon-EE, or null if there is no such class

      • getPreconditions

        public java.util.Set<Expression> getPreconditions()
        Find any necessary preconditions for the satisfaction of this expression as a set of boolean expressions to be evaluated on the context node
        Returns:
        A set of conditions, or null if none have been computed