Package com.saxonica.ee.stream.feed

Class ItemFeed

java.lang.Object
net.sf.saxon.event.Outputter
net.sf.saxon.event.ProxyOutputter
com.saxonica.ee.stream.feed.ItemFeed
All Implemented Interfaces:
Result, Receiver
Direct Known Subclasses:
ApplyTemplatesAction, ApplyTemplatesFeed, AtomicItemFeed, BooleanFnFeed, BufferingApplyTemplatesFeed, BufferingFeed, BufferingFilterExpressionFeed, BufferingForEachExpressionFeed, CallableFeed, CardinalityCheckingFeed, ComplexNodeEventFeed, ConditionalBlockAdjunct.ConditionalBlockFeed, DecomposingFeed, DelegatingFeed, ExistenceFeed, FeedToEventBuffer, ForEachAction, ForEachGroupPartitionAction, ForExpressionAdjunct.ForExpressionFeed, GroundedItemFeed, ItemCheckingFeed, LiteralAdjunct.LiteralItemFeed, MotionlessFeed, NextIterationFeed, NoCloseFeed, NoOpenFeed, NoOpenOrCloseFeed, ShallowCopyEventFeed, SimpleNodeConstructorFeed, SinkFeed, TryCatchAdjunct.TryCatchFeed
public abstract class ItemFeed extends ProxyOutputter
An ItemFeed evaluates an expression in "composed push" mode. Its streaming input is supplied via a sequence of event calls (start element, end element, etc) that represent the value of the streamed operand of the expression; other operands are evaluated in pull mode in the usual way.

Expressions are thus evaluated "bottom up": instead of a parent expression triggering the evaluation of its children, the appearance of data needed by a child expression causes evaluation of the child expression, which then triggers evaluation of its parent, and so on.

One consequence of this is that special mechanisms are needed for error handling. In particular, it's not possible for a child expression to notify its parent expression of dynamic errors simply by throwing a Java exception: the error information needs to be passed up the pipeline in the same way as regular results.

Some expressions (inspection expressions) are only interested in top-level events: for example count(), exists, and instance of only need to know about a startElement event, they are not interested in the content of the element, only in its existence. Other expressions (absorption expression) need access to the content of the element: in most cases, the typed value of the element, but in some cases (such as copy-of() or snapshot() a finer-grained representation of the content. In such cases the startSelectedParentNode(FleetingParentNode, Location) method (which is notified of a top-level event in the input stream) returns a Receiver, which is then notified of all the contained events up to the corresponding endElement.

Some expressions (transmission expression) pass data on to their parent expressions directly, and in these cases the decision whether to ask for full details of the content is delegated to the parent expression, or beyond.

An expression whose result is known before all the input is available (for example, exists() can call a supplied Terminator to indicate that no more input is required. In some cases (where the expression would otherwise read an entire document) this enables parsing of the streamed input file to be abandoned.

The events passed up the pipeline will usually correspond to nodes in the streamed source document, or to atomic values computed from those nodes. In unusual edge cases, however, a consuming expression may generate new element or document nodes, which are still delivered incrementally because their content is established by streaming the source document. Events representing these constructed elements are passed up the pipeline as if they came from the source document. (Note, because this situation is unusual and somewhat contrived, it probably has incomplete test coverage and may not be fully implemented for all kinds of expression).

The start of an element may be represented either as a single event containing all the attributes and namespaces, or as a sequence of separate events (start element, attributes, namespaces, then startContent). Events coming directly from parsing the streamed input will use the more efficient "bundled" form, but events representing constructed nodes may be "unbundled" because attributes and namespaces can be constructed incrementally as the streamed input is processed. An ItemFeed for a parent expression should therefore be prepared to accept either form.

  • Constructor Details

    • ItemFeed

      public ItemFeed(ItemFeed result, XPathContext context)

    • ItemFeed

      public ItemFeed(Expression exp, ItemFeed result, XPathContext context)

    • ItemFeed

      public ItemFeed(Outputter result, XPathContext context, boolean thisIsAnExceptionalCase)
      Special constructor for the last ItemFeed in the pipeline, which feeds into an Outputter rather than into another ItemFeed. Note that the getResult() method in this situation will throw an exception.
      Parameters:
      result - the resulting outputter
      context - the dynamic evaluation context
      thisIsAnExceptionalCase - value is ignored; the parameter is there simply to flag this constructor as a special case

  • Method Details

    • processItems

      public static void processItems(SequenceIterator iter, Outputter result) throws XPathException
      Convenience method to process all the items selected by an iterator and push them to the next feed in the push pipeline
      Parameters:
      iter - iterator over the selected items
      result - the next feed in the pipeline
      Throws:
      XPathException - if any dynamic error occurs

    • setExpression

      public void setExpression(Expression exp)
      Set the expression being evaluated by this ItemFeed
      Parameters:
      exp - the relevant expression

    • getExpression

      public Expression getExpression()
      Get the expression being evaluated by this ItemFeed. May be null if not known or not relevant
      Returns:
      the expression being evaluated

    • setTerminator

      public void setTerminator(Terminator terminator)
      Provide a callback that can be used to request early termination of the streaming of this input document
      Parameters:
      terminator - the class that can be called to request termination

    • setHasFailed

      public void setHasFailed()
      Mark this feed as having failed (that is, as having detected and reported a dynamic error). Once the feed is marked as having failed, it should not attempt to process any further input.

    • getTerminator

      public Terminator getTerminator()
      Get the Terminator which can be used to request early termination of the Feed
      Returns:
      the Terminator that was passed to the open() call, or null if open() has not been called

    • getResultFeed

      public ItemFeed getResultFeed()
      Get the result, that is, the ItemFeed to which the items in the result of the expression should be supplied
      Returns:
      the Outputter that consumes the results of this expression

    • getContext

      public XPathContext getContext()
      Get the dynamic evaluation context
      Returns:
      the dynamic evaluation context

    • open

      public void open(Terminator terminator) throws XPathException
      Start evaluating the expression. The default implementation does nothing.
      Parameters:
      terminator - used to achieve early exit
      Throws:
      XPathException - if a dynamic error occurs

    • startSelectedParentNode

      public Receiver startSelectedParentNode(FleetingParentNode node, Location locationId) throws XPathException
      Signal that an element or document node has been found that matches the selection that this Watch is looking for. This method is called by the WatchManager while processing the startElement or startDocument event that matches the selection.
      Parameters:
      node - the element or document node whose start event has been matched
      locationId - the location associated with the element or document node (may be the location of the instruction that created it)
      Returns:
      a Receiver to be notified of all events starting with the startElement/startDocument event for the matched element, and ending with the endElement event for that element; or null if this feature is not required.
      Throws:
      XPathException - May be raised if a dynamic error occurs

    • append

      public abstract void append(Item item) throws XPathException
      Supply one item towards the streamed input of the expression
      Specified by:
      append in interface Receiver
      Overrides:
      append in class ProxyOutputter
      Parameters:
      item - the item to be supplied
      Throws:
      XPathException - if the operation fails

    • append

      public final void append(Item item, Location locationId, int properties) throws XPathException
      Supply one item towards the streamed input of the expression. This implementation calls append(Item). Subclasses do not have to cater for this event
      Specified by:
      append in interface Receiver
      Overrides:
      append in class ProxyOutputter
      Parameters:
      item - the item to be supplied
      locationId - the location of the calling instruction, for diagnostics
      properties - if the item is an element node, this indicates whether its namespaces need to be copied. Values are ReceiverOption.ALL_NAMESPACES; the default (0) means
      Throws:
      XPathException - if an error occurs

    • endSelectedParentNode

      public void endSelectedParentNode(Location locationId) throws XPathException
      Signal that the endElement event has occurred for the element whose startElement event caused the Watch to be activated.
      Parameters:
      locationId - the location of the element
      Throws:
      XPathException - May be raised if a constraint implemented by this Watch is violated

    • close

      public void close() throws XPathException
      Finish evaluating the expression.
      Specified by:
      close in interface Receiver
      Overrides:
      close in class ProxyOutputter
      Throws:
      XPathException - if a dynamic error occurs

    • dynamicError

      public void dynamicError(XPathException error) throws XPathException
      Report a dynamic error. This requires searching up the "feed" pipeline to see if there is a try/catch expression interested in being notified of this error; if there is none, the error is thrown as an exception
      Parameters:
      error - the exception representing the dynamic error
      Throws:
      XPathException - if there is no try/catch expression wanting to catch this error

    • hasFailed

      public boolean hasFailed()