org.apache.spark.util.NextIterator.scala Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.spark.util
/** Provides a basic/boilerplate Iterator implementation. */
private[spark] abstract class NextIterator[U] extends Iterator[U] {
private var gotNext = false
private var nextValue: U = _
private var closed = false
protected var finished = false
/**
* Method for subclasses to implement to provide the next element.
*
* If no next element is available, the subclass should set `finished`
* to `true` and may return any value (it will be ignored).
*
* This convention is required because `null` may be a valid value,
* and using `Option` seems like it might create unnecessary Some/None
* instances, given some iterators might be called in a tight loop.
*
* @return U, or set 'finished' when done
*/
protected def getNext(): U
/**
* Method for subclasses to implement when all elements have been successfully
* iterated, and the iteration is done.
*
* Note: `NextIterator` cannot guarantee that `close` will be
* called because it has no control over what happens when an exception
* happens in the user code that is calling hasNext/next.
*
* Ideally you should have another try/catch, as in HadoopRDD, that
* ensures any resources are closed should iteration fail.
*/
protected def close()
/**
* Calls the subclass-defined close method, but only once.
*
* Usually calling `close` multiple times should be fine, but historically
* there have been issues with some InputFormats throwing exceptions.
*/
def closeIfNeeded() {
if (!closed) {
// Note: it's important that we set closed = true before calling close(), since setting it
// afterwards would permit us to call close() multiple times if close() threw an exception.
closed = true
close()
}
}
override def hasNext: Boolean = {
if (!finished) {
if (!gotNext) {
nextValue = getNext()
if (finished) {
closeIfNeeded()
}
gotNext = true
}
}
!finished
}
override def next(): U = {
if (!hasNext) {
throw new NoSuchElementException("End of stream")
}
gotNext = false
nextValue
}
}