java8.util.LBQSpliterator Maven / Gradle / Ivy
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/*
* Copyright (c) 2012, 2014, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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*/
package java8.util;
import java.util.Comparator;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.locks.ReentrantLock;
import java8.util.function.Consumer;
/**
* A customized variant of Spliterators.IteratorSpliterator for
* LinkedBlockingQueues.
*
* The returned spliterator is weakly consistent.
*
* The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
*
* The {@code Spliterator} implements {@code trySplit} to permit limited
* parallelism.
*
* @param the type of elements held in the LinkedBlockingQueue
*/
final class LBQSpliterator implements Spliterator {
private static final int MAX_BATCH = 1 << 25; // max batch array size
private final LinkedBlockingQueue queue;
private final ReentrantLock putLock;
private final ReentrantLock takeLock;
private Object current; // current node; null until initialized
private int batch; // batch size for splits
private boolean exhausted; // true when no more nodes
private long est; // size estimate
private LBQSpliterator(LinkedBlockingQueue queue) {
this.queue = queue;
this.est = queue.size();
this.putLock = getPutLock(queue);
this.takeLock = getTakeLock(queue);
}
static Spliterator spliterator(LinkedBlockingQueue queue) {
return new LBQSpliterator(queue);
}
@Override
public int characteristics() {
return Spliterator.ORDERED | Spliterator.NONNULL
| Spliterator.CONCURRENT;
}
@Override
public long estimateSize() {
return est;
}
@Override
public void forEachRemaining(Consumer action) {
Objects.requireNonNull(action);
LinkedBlockingQueue q = this.queue;
if (!exhausted) {
exhausted = true;
Object p = current;
do {
E e = null;
fullyLock();
try {
if (p == null) {
p = getHeadNext(q);
}
while (p != null) {
e = getNodeItem(p);
p = getNextNode(p);
if (e != null) {
break;
}
}
} finally {
fullyUnlock();
}
if (e != null) {
action.accept(e);
}
} while (p != null);
}
}
@Override
public Comparator getComparator() {
return Spliterators.getComparator(this);
}
@Override
public long getExactSizeIfKnown() {
return Spliterators.getExactSizeIfKnown(this);
}
@Override
public boolean hasCharacteristics(int characteristics) {
return Spliterators.hasCharacteristics(this, characteristics);
}
@Override
public boolean tryAdvance(Consumer action) {
Objects.requireNonNull(action);
LinkedBlockingQueue q = this.queue;
if (!exhausted) {
E e = null;
fullyLock();
try {
if (current == null) {
current = getHeadNext(q);
}
while (current != null) {
e = getNodeItem(current);
current = getNextNode(current);
if (e != null) {
break;
}
}
} finally {
fullyUnlock();
}
if (current == null) {
exhausted = true;
}
if (e != null) {
action.accept(e);
return true;
}
}
return false;
}
@Override
public Spliterator trySplit() {
Object h;
final LinkedBlockingQueue q = this.queue;
int b = batch;
int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
if (!exhausted
&& ((h = current) != null || (h = getHeadNext(q)) != null)
&& getNextNode(h) != null) {
Object[] a = new Object[n];
int i = 0;
Object p = current;
fullyLock();
try {
if (p != null || (p = getHeadNext(q)) != null) {
do {
if ((a[i] = getNodeItem(p)) != null) {
++i;
}
} while ((p = getNextNode(p)) != null && i < n);
}
} finally {
fullyUnlock();
}
if ((current = p) == null) {
est = 0L;
exhausted = true;
} else if ((est -= i) < 0L) {
est = 0L;
}
if (i > 0) {
batch = i;
return Spliterators.spliterator(a, 0, i, Spliterator.ORDERED
| Spliterator.NONNULL | Spliterator.CONCURRENT);
}
}
return null;
}
/**
* Lock to prevent both puts and takes.
*/
private void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* Unlock to allow both puts and takes.
*/
private void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
private static ReentrantLock getPutLock(LinkedBlockingQueue queue) {
return (ReentrantLock) U.getObject(queue, PUT_LOCK_OFF);
}
private static ReentrantLock getTakeLock(LinkedBlockingQueue queue) {
return (ReentrantLock) U.getObject(queue, TAKE_LOCK_OFF);
}
/**
* Returns queue.head.next as an Object
*/
private static Object getHeadNext(LinkedBlockingQueue queue) {
return getNextNode(U.getObject(queue, HEAD_OFF));
}
/**
* Returns node.next as an Object
*/
private static Object getNextNode(Object node) {
return U.getObject(node, NODE_NEXT_OFF);
}
/**
* Returns node.item as a T
*/
private static T getNodeItem(Object node) {
return (T) U.getObject(node, NODE_ITEM_OFF);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long HEAD_OFF;
private static final long NODE_ITEM_OFF;
private static final long NODE_NEXT_OFF;
private static final long PUT_LOCK_OFF;
private static final long TAKE_LOCK_OFF;
static {
try {
Class nc = Class
.forName("java.util.concurrent.LinkedBlockingQueue$Node");
HEAD_OFF = U.objectFieldOffset(LinkedBlockingQueue.class
.getDeclaredField("head"));
NODE_ITEM_OFF = U.objectFieldOffset(nc.getDeclaredField("item"));
NODE_NEXT_OFF = U.objectFieldOffset(nc.getDeclaredField("next"));
PUT_LOCK_OFF = U.objectFieldOffset(LinkedBlockingQueue.class
.getDeclaredField("putLock"));
TAKE_LOCK_OFF = U.objectFieldOffset(LinkedBlockingQueue.class
.getDeclaredField("takeLock"));
} catch (Exception e) {
throw new Error(e);
}
}
}