Package com.saxonica.ee.parallel

Class MultithreadedContextMappingIterator

java.lang.Object

com.saxonica.ee.parallel.MultithreadedContextMappingIterator

All Implemented Interfaces:

Closeable, AutoCloseable, SequenceIterator

public final class MultithreadedContextMappingIterator
extends Object
implements SequenceIterator

This class performs the same role as ContextMappingIterator, but using multiple threads.

It merges a sequence of sequences into a single flat sequence. It takes as inputs an iteration, and an "action" expression to be applied to each Item returned by that iteration, with that item as the context item. The action expression itself returns another iterator. The result is an iteration of the concatenation of all the iterations returned by the action expression.

Constructor Summary

Constructors

Constructor	Description
MultithreadedContextMappingIterator(PullEvaluator action, XPathContextMinor context, int threads)	Construct an Iterator that will apply a specified action expression to each Item returned by the current iterator of the supplied dynamic context.

Method Summary

Modifier and Type	Method	Description
void	close()	Close the iterator.
Item	next()	Get the next item in the sequence.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

MultithreadedContextMappingIterator

public MultithreadedContextMappingIterator(PullEvaluator action,
 XPathContextMinor context,
 int threads)
                                    throws XPathException

Construct an Iterator that will apply a specified action expression to each Item returned by the current iterator of the supplied dynamic context.

Parameters:

action - the mapping expression to be applied

context - the processing context. The action expression is applied to each item returned by context.getCurrentIterator() in turn.

threads - the number of threads to be used (in addition to the base evaluation thread) These threads are used to evaluate the action expression. The current thread is used to evaluate the base iterator and to gather the results of invoking the action expressions in the right order.

Throws:

XPathException

Method Details

next

public Item next()

Description copied from interface: SequenceIterator

Get the next item in the sequence. This method changes the state of the iterator.

Specified by:

next in interface SequenceIterator

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.

close

public void close()

Description copied from interface: SequenceIterator

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.

For example, the iterator returned by the unparsed-text-lines() function has a close() method that causes the underlying input stream to be closed, whether or not the file has been read to completion.

Closing an iterator is important 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.

Closing an iterator is not necessary if the iterator is read to completion: if a call on SequenceIterator.next() returns null, the iterator will be closed automatically. An explicit call on SequenceIterator.close() is needed only when iteration is abandoned prematurely.

It is not possible to guarantee that an iterator that is not read to completion or will be closed. For example, if a lazy-evaluated variable $var is passed to a user-written function, the function may access $var[1] only; we have no way of knowing whether further items will be read. For this reason, any SequenceIterator that holds resources which need to be closed should use the Cleaner mechanism. The Configuration holds a Cleaner, and resources held by a SequenceIterator should be registered with the Cleaner; if the SequenceIterator is then garbage-collected without being closed, the Cleaner will ensure that the underlying resources are closed. (An example of a SequenceIterator that uses this mechanism is the UnparsedTextIterator).

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Closeable

Specified by:

close in interface SequenceIterator