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package com.sun.grizzly.connectioncache.impl.concurrent;
import com.sun.grizzly.connectioncache.spi.concurrent.ConcurrentQueue;
public class ConcurrentQueueBlockingImpl<V> implements ConcurrentQueue<V> {
// This implementation of ConcurrentQueue uses a single lock, which must be
// acquired to update the list. Every operation on this class updates the
// structure, so read/write locking is probably not useful.
//
// Trying to build a lock-free implementation runs into the usual problems:
// we need to atomically update more than one location at a time in the structure.
// Short of a transactional memory implementation, we would either need a complicated
// implementation implementing recursive fixup, or something like the Ladan-Mozes and
// Shavit algorithm (see "An Optimistic Approach to Lock-Free FIFO Queues"
// at http://people.csail.mit.edu/edya/publications/publicationsAndPatents.htm)
// that delays fixing up one direction in a double linked list. However, that
// algorithm does not consider general deletion, and I don't know whether that
// capability can be easily added or not.
// Any of these approaches are quite complicated, and so we won't go there yet.
// As always, first make it work, then make it fast(er), but only if necessary.
//
// 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<V> head = new Entry<V>( null ) ;
final Object lock = new Object() ;
int count = 0 ;
public ConcurrentQueueBlockingImpl() {
head.next = head ;
head.prev = head ;
}
private final class Entry<V> {
Entry<V> next = null ;
Entry<V> prev = null ;
private HandleImpl<V> handle ;
Entry( V value ) {
handle = new HandleImpl<V>( this, value ) ;
}
HandleImpl<V> handle() {
return handle ;
}
}
private final class HandleImpl<V> implements Handle<V> {
private Entry<V> entry ;
private final V value ;
private boolean valid ;
HandleImpl( Entry<V> entry, V value ) {
this.entry = entry ;
this.value = value ;
this.valid = true ;
}
Entry<V> entry() {
return entry ;
}
public V value() {
return value ;
}
/** Delete the element corresponding to this handle
* from the queue. Takes constant time.
* @return element corresponding to this handle was removed, (yes or no)
*/
public boolean remove() {
synchronized (lock) {
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() {
synchronized (lock) {
return count ;
}
}
/** Add a new element to the tail of the queue.
* Returns a handle for the element in the queue.
* @param arg element to offer to the queue
* @return a {@link Handle} for the element in the queue
*/
public Handle<V> offer( V arg ) {
if (arg == null)
throw new IllegalArgumentException( "Argument cannot be null" ) ;
Entry<V> entry = new Entry<V>( arg ) ;
synchronized (lock) {
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.
* @return element at the head of the queue
*/
public V poll() {
Entry<V> first = null ;
synchronized (lock) {
first = head.next ;
if (first == head)
return null ;
else {
// 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 ;
V value = first.handle().value() ;
return value ;
}
}