Package com.saxonica.ee.optim

Class MultithreadedFocusTrackingIterator

  • All Implemented Interfaces:
    java.io.Closeable, java.lang.AutoCloseable, LastPositionFinder, FocusIterator, SequenceIterator, GroundedIterator, LookaheadIterator
    public class MultithreadedFocusTrackingIterator
extends FocusTrackingIterator
    This is a thread-safe version of the parent class FocusTrackingIterator. It is identical to the superclass except that the methods a synchronized. It is used when there is any possibility that the iterator will be used from multiple threads.

    The class is not designed to allow arbitrary multi-threaded operation. For example, one thread might exhaust the input after another thread has called hasNext(), causing the first thread to get no results from a call of next(). Instead, the synchronization is designed only to allow a call on getLength() (caused by an XPath invocation of last()) to occur asynchronously with the thread that advances the iterator. This happens specifically when an xsl:result-document instruction is executed asynchronously, and the code within the body of the xsl:result-document instruction calls last(). In this situation the main thread (the one that advances the focus, and spawns new threads for xsl:result-document) must be suspended while the value of last() is computed, which is done by reading the iterator to completion; when the main thread resumes, there will be a new base iterator positioned over the materialized sequence that was read into memory as a side-effect of calling last().

    See bug 3927.

    • Constructor Detail

      • MultithreadedFocusTrackingIterator

        public MultithreadedFocusTrackingIterator​(SequenceIterator base)

    • Method Detail

      • next

        public Item next()
          throws XPathException
        Get the next item in the sequence. This method changes the state of the iterator, in particular it affects the result of subsequent calls of position() and current().
        Specified by:
        next in interface SequenceIterator
        Overrides:
        next in class FocusTrackingIterator
        Returns:
        the next item, or null if there are no more items. Once a call on next() has returned null, no further calls should be made. The preferred action for an iterator if subsequent calls on next() are made is to return null again, and all implementations within Saxon follow this rule.
        Throws:
        XPathException - if an error occurs retrieving the next item
        Since:
        8.4

      • current

        public Item current()
        Get the current value in the sequence (the one returned by the most recent call on next()). This will be null before the first call of next(). This method does not change the state of the iterator.
        Specified by:
        current in interface FocusIterator
        Overrides:
        current in class FocusTrackingIterator
        Returns:
        the current item, the one most recently returned by a call on next(). Returns null if next() has not been called, or if the end of the sequence has been reached.
        Since:
        8.4

      • position

        public int position()
        Get the current position. This will usually be zero before the first call on next(), otherwise it will be the number of times that next() has been called. Once next() has returned null, the preferred action is for subsequent calls on position() to return -1, but not all existing implementations follow this practice. (In particular, the EmptyIterator is stateless, and always returns 0 as the value of position(), whether or not next() has been called.)

        This method does not change the state of the iterator.

        Specified by:
        position in interface FocusIterator
        Overrides:
        position in class FocusTrackingIterator
        Returns:
        the current position, the position of the item returned by the most recent call of next(). This is 1 after next() has been successfully called once, 2 after it has been called twice, and so on. If next() has never been called, the method returns zero. If the end of the sequence has been reached, the value returned will always be <= 0; the preferred value is -1.
        Since:
        8.4

      • getLength

        public int getLength()
              throws XPathException
        Get the position of the last item in the sequence. The method is stateless in its external effect: that is, it does not change the values returned by position(), next(), hasNext(), etc. However, it creates a new base iterator which means that the result of calling getUnderlyingIterator() may change.
        Specified by:
        getLength in interface FocusIterator
        Specified by:
        getLength in interface LastPositionFinder
        Overrides:
        getLength in class FocusTrackingIterator
        Returns:
        the position of the last item
        Throws:
        XPathException - if a failure occurs reading the sequence

      • hasNext

        public boolean hasNext()
        Determine whether there are more items to come. Note that this operation is stateless and it is not necessary (or usual) to call it before calling next(). It is used only when there is an explicit need to tell if we are at the last element.

        This method must not be called unless the result of getProperties() on the iterator includes the bit setting SequenceIterator.Property.LOOKAHEAD

        Specified by:
        hasNext in interface LookaheadIterator
        Overrides:
        hasNext in class FocusTrackingIterator
        Returns:
        true if there are more items in the sequence
        Throws:
        java.lang.ClassCastException - if the base iterator does not support lookahead processing

      • getResidue

        public GroundedValue getResidue()
                         throws XPathException
        Return a GroundedValue containing all the remaining items in the sequence returned by this SequenceIterator, starting at the current position. This should be an "in-memory" value, not a Closure.
        Specified by:
        getResidue in interface GroundedIterator
        Overrides:
        getResidue in class FocusTrackingIterator
        Returns:
        the corresponding Value
        Throws:
        XPathException - in the cases of subclasses (such as the iterator over a MemoClosure) which cause evaluation of expressions while materializing the value.
        java.lang.ClassCastException - if the iterator does not have the SequenceIterator.Property.GROUNDED property.

      • close

        public void close()
        Close the iterator. This indicates to the supplier of the data that the client does not require any more items to be delivered by the iterator. This may enable the supplier to release resources. After calling close(), no further calls on the iterator should be made; if further calls are made, the effect of such calls is undefined.

        (Currently, closing an iterator is important only when the data is being "pushed" in another thread. Closing the iterator terminates that thread and means that it needs to do no additional work. Indeed, failing to close the iterator may cause the push thread to hang waiting for the buffer to be emptied.)

        Specified by:
        close in interface java.lang.AutoCloseable
        Specified by:
        close in interface java.io.Closeable
        Specified by:
        close in interface SequenceIterator
        Overrides:
        close in class FocusTrackingIterator
        Since:
        9.1

      • getSiblingPosition

        public int getSiblingPosition​(NodeInfo node,
                              NodeTest nodeTest,
                              int max)
        Get the sibling position of a node: specifically, count how many preceding siblings of a node satisfy the nodetest. This method appears here because it can potentially make use of cached information. When an instruction such as xsl:apply-templates select="*" (which selects a set of sibling nodes) is used in conjunction with patterns such as match="*[position() mod 2 = 1], then calculation of the position of one node in the sequence of siblings can take advantage of the fact that the position of the immediately preceding sibling is already known.

        This optimization was suggested by the late Sebastian Rahtz, one of Saxon's earliest power users, and it is dedicated to his memory.

        Overrides:
        getSiblingPosition in class FocusTrackingIterator
        Parameters:
        node - the starting node, which is assumed to satisfy the node test
        nodeTest - the node test
        max - the maximum number of nodes to be counted
        Returns:
        the number of preceding siblings that satisfy the node test, plus one, unless the number exceeds max, in which case return some number greater than or equal to max.