io.reactivex.internal.queue.MpscLinkedQueue Maven / Gradle / Ivy
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/**
* Copyright (c) 2016-present, RxJava Contributors.
*
* Licensed 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.
*/
/*
* The code was inspired by the similarly named JCTools class:
* https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/atomic
*/
package io.reactivex.internal.queue;
import java.util.concurrent.atomic.AtomicReference;
import io.reactivex.annotations.Nullable;
import io.reactivex.internal.fuseable.SimplePlainQueue;
/**
* A multi-producer single consumer unbounded queue.
* @param the contained value type
*/
public final class MpscLinkedQueue implements SimplePlainQueue {
private final AtomicReference> producerNode;
private final AtomicReference> consumerNode;
public MpscLinkedQueue() {
producerNode = new AtomicReference>();
consumerNode = new AtomicReference>();
LinkedQueueNode node = new LinkedQueueNode();
spConsumerNode(node);
xchgProducerNode(node); // this ensures correct construction: StoreLoad
}
/**
* {@inheritDoc}
*
* IMPLEMENTATION NOTES:
* Offer is allowed from multiple threads.
* Offer allocates a new node and:
*
* - Swaps it atomically with current producer node (only one producer 'wins')
*
- Sets the new node as the node following from the swapped producer node
*
* This works because each producer is guaranteed to 'plant' a new node and link the old node. No 2 producers can
* get the same producer node as part of XCHG guarantee.
*
* @see java.util.Queue#offer(java.lang.Object)
*/
@Override
public boolean offer(final T e) {
if (null == e) {
throw new NullPointerException("Null is not a valid element");
}
final LinkedQueueNode nextNode = new LinkedQueueNode(e);
final LinkedQueueNode prevProducerNode = xchgProducerNode(nextNode);
// Should a producer thread get interrupted here the chain WILL be broken until that thread is resumed
// and completes the store in prev.next.
prevProducerNode.soNext(nextNode); // StoreStore
return true;
}
/**
* {@inheritDoc}
*
* IMPLEMENTATION NOTES:
* Poll is allowed from a SINGLE thread.
* Poll reads the next node from the consumerNode and:
*
* - If it is null, the queue is assumed empty (though it might not be).
*
- If it is not null set it as the consumer node and return it's now evacuated value.
*
* This means the consumerNode.value is always null, which is also the starting point for the queue. Because null
* values are not allowed to be offered this is the only node with it's value set to null at any one time.
*
* @see java.util.Queue#poll()
*/
@Nullable
@Override
public T poll() {
LinkedQueueNode currConsumerNode = lpConsumerNode(); // don't load twice, it's alright
LinkedQueueNode nextNode = currConsumerNode.lvNext();
if (nextNode != null) {
// we have to null out the value because we are going to hang on to the node
final T nextValue = nextNode.getAndNullValue();
spConsumerNode(nextNode);
return nextValue;
}
else if (currConsumerNode != lvProducerNode()) {
// spin, we are no longer wait free
while ((nextNode = currConsumerNode.lvNext()) == null) { } // NOPMD
// got the next node...
// we have to null out the value because we are going to hang on to the node
final T nextValue = nextNode.getAndNullValue();
spConsumerNode(nextNode);
return nextValue;
}
return null;
}
@Override
public boolean offer(T v1, T v2) {
offer(v1);
offer(v2);
return true;
}
@Override
public void clear() {
while (poll() != null && !isEmpty()) { } // NOPMD
}
LinkedQueueNode lvProducerNode() {
return producerNode.get();
}
LinkedQueueNode xchgProducerNode(LinkedQueueNode node) {
return producerNode.getAndSet(node);
}
LinkedQueueNode lvConsumerNode() {
return consumerNode.get();
}
LinkedQueueNode lpConsumerNode() {
return consumerNode.get();
}
void spConsumerNode(LinkedQueueNode node) {
consumerNode.lazySet(node);
}
/**
* {@inheritDoc}
*
* IMPLEMENTATION NOTES:
* Queue is empty when producerNode is the same as consumerNode. An alternative implementation would be to observe
* the producerNode.value is null, which also means an empty queue because only the consumerNode.value is allowed to
* be null.
*/
@Override
public boolean isEmpty() {
return lvConsumerNode() == lvProducerNode();
}
static final class LinkedQueueNode extends AtomicReference> {
private static final long serialVersionUID = 2404266111789071508L;
private E value;
LinkedQueueNode() {
}
LinkedQueueNode(E val) {
spValue(val);
}
/**
* Gets the current value and nulls out the reference to it from this node.
*
* @return value
*/
public E getAndNullValue() {
E temp = lpValue();
spValue(null);
return temp;
}
public E lpValue() {
return value;
}
public void spValue(E newValue) {
value = newValue;
}
public void soNext(LinkedQueueNode n) {
lazySet(n);
}
public LinkedQueueNode lvNext() {
return get();
}
}
}