com.sun.xml.ws.transport.tcp.connectioncache.impl.concurrent.ConcurrentQueueImpl Maven / Gradle / Ivy
/*
* Copyright (c) 1997, 2018 Oracle and/or its affiliates. All rights reserved.
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Distribution License v. 1.0, which is available at
* http://www.eclipse.org/org/documents/edl-v10.php.
*
* SPDX-License-Identifier: BSD-3-Clause
*/
package com.sun.xml.ws.transport.tcp.connectioncache.impl.concurrent;
import com.sun.xml.ws.transport.tcp.connectioncache.spi.concurrent.ConcurrentQueue;
public class ConcurrentQueueImpl implements ConcurrentQueue {
// This implementation of ConcurrentQueue is unsynchronized, for use in
// other implementations that manage concurrency with locks.
//
// Structure: Head points to a node containing a null value, which is a special marker.
// head.next is the first element, head.prev is the last. The queue is empty if
// head.next == head.prev == head.
final Entry head = new Entry( null ) ;
int count = 0 ;
public ConcurrentQueueImpl() {
head.next = head ;
head.prev = head ;
}
private final class Entry {
Entry next = null ;
Entry prev = null ;
private HandleImpl handle ;
Entry( V value ) {
handle = new HandleImpl( this, value ) ;
}
HandleImpl handle() {
return handle ;
}
}
private final class HandleImpl implements Handle {
private Entry entry ;
private final V value ;
private boolean valid ;
HandleImpl( Entry entry, V value ) {
this.entry = entry ;
this.value = value ;
this.valid = true ;
}
Entry entry() {
return entry ;
}
public V value() {
return value ;
}
/** Delete the element corresponding to this handle
* from the queue. Takes constant time.
*/
public boolean remove() {
if (!valid) {
return false ;
}
valid = false ;
entry.next.prev = entry.prev ;
entry.prev.next = entry.next ;
count-- ;
entry.prev = null ;
entry.next = null ;
entry.handle = null ;
entry = null ;
valid = false ;
return true ;
}
}
public int size() {
return count ;
}
/** Add a new element to the tail of the queue.
* Returns a handle for the element in the queue.
*/
public Handle offer( V arg ) {
if (arg == null)
throw new IllegalArgumentException( "Argument cannot be null" ) ;
Entry entry = new Entry( arg ) ;
entry.next = head ;
entry.prev = head.prev ;
head.prev.next = entry ;
head.prev = entry ;
count++ ;
return entry.handle() ;
}
/** Return an element from the head of the queue.
* The element is removed from the queue.
*/
public V poll() {
Entry first = null ;
V value = null ;
first = head.next ;
if (first == head)
return null ;
else {
value = first.handle().value() ;
// assert that the following expression returns true!
first.handle().remove() ;
}
// Once first is removed from the queue, it is invisible to other threads,
// so we don't need to synchronize here.
first.next = null ;
first.prev = null ;
return value ;
}
}