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/*
* Copyright 2012 The Netty Project
*
* The Netty Project 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 io.netty.util;
import io.netty.util.internal.PlatformDependent;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
* Default {@link AttributeMap} implementation which use simple synchronization per bucket to keep the memory overhead
* as low as possible.
*/
public class DefaultAttributeMap implements AttributeMap {
@SuppressWarnings("rawtypes")
private static final AtomicReferenceFieldUpdater updater;
static {
@SuppressWarnings("rawtypes")
AtomicReferenceFieldUpdater referenceFieldUpdater =
PlatformDependent.newAtomicReferenceFieldUpdater(DefaultAttributeMap.class, "attributes");
if (referenceFieldUpdater == null) {
referenceFieldUpdater = AtomicReferenceFieldUpdater
.newUpdater(DefaultAttributeMap.class, AtomicReferenceArray.class, "attributes");
}
updater = referenceFieldUpdater;
}
private static final int BUCKET_SIZE = 4;
private static final int MASK = BUCKET_SIZE - 1;
// Initialize lazily to reduce memory consumption; updated by AtomicReferenceFieldUpdater above.
@SuppressWarnings("UnusedDeclaration")
private volatile AtomicReferenceArray> attributes;
@SuppressWarnings("unchecked")
@Override
public Attribute attr(AttributeKey key) {
if (key == null) {
throw new NullPointerException("key");
}
AtomicReferenceArray> attributes = this.attributes;
if (attributes == null) {
// Not using ConcurrentHashMap due to high memory consumption.
attributes = new AtomicReferenceArray>(BUCKET_SIZE);
if (!updater.compareAndSet(this, null, attributes)) {
attributes = this.attributes;
}
}
int i = index(key);
DefaultAttribute head = attributes.get(i);
if (head == null) {
// No head exists yet which means we may be able to add the attribute without synchronization and just
// use compare and set. At worst we need to fallback to synchronization
head = new DefaultAttribute(key);
if (attributes.compareAndSet(i, null, head)) {
// we were able to add it so return the head right away
return (Attribute) head;
} else {
head = attributes.get(i);
}
}
synchronized (head) {
DefaultAttribute curr = head;
for (;;) {
if (!curr.removed && curr.key == key) {
return (Attribute) curr;
}
DefaultAttribute next = curr.next;
if (next == null) {
DefaultAttribute attr = new DefaultAttribute(head, key);
curr.next = attr;
attr.prev = curr;
return attr;
} else {
curr = next;
}
}
}
}
@Override
public boolean hasAttr(AttributeKey key) {
if (key == null) {
throw new NullPointerException("key");
}
AtomicReferenceArray> attributes = this.attributes;
if (attributes == null) {
// no attribute exists
return false;
}
int i = index(key);
DefaultAttribute head = attributes.get(i);
if (head == null) {
// No attribute exists which point to the bucket in which the head should be located
return false;
}
// check on the head can be done without synchronization
if (head.key == key && !head.removed) {
return true;
}
synchronized (head) {
// we need to synchronize on the head
DefaultAttribute curr = head.next;
while (curr != null) {
if (!curr.removed && curr.key == key) {
return true;
}
curr = curr.next;
}
return false;
}
}
private static int index(AttributeKey key) {
return key.id() & MASK;
}
@SuppressWarnings("serial")
private static final class DefaultAttribute extends AtomicReference implements Attribute {
private static final long serialVersionUID = -2661411462200283011L;
// The head of the linked-list this attribute belongs to, which may be itself
private final DefaultAttribute head;
private final AttributeKey key;
// Double-linked list to prev and next node to allow fast removal
private DefaultAttribute prev;
private DefaultAttribute next;
// Will be set to true one the attribute is removed via getAndRemove() or remove()
private volatile boolean removed;
DefaultAttribute(DefaultAttribute head, AttributeKey key) {
this.head = head;
this.key = key;
}
DefaultAttribute(AttributeKey key) {
head = this;
this.key = key;
}
@Override
public AttributeKey key() {
return key;
}
@Override
public T setIfAbsent(T value) {
while (!compareAndSet(null, value)) {
T old = get();
if (old != null) {
return old;
}
}
return null;
}
@Override
public T getAndRemove() {
removed = true;
T oldValue = getAndSet(null);
remove0();
return oldValue;
}
@Override
public void remove() {
removed = true;
set(null);
remove0();
}
private void remove0() {
synchronized (head) {
// We only update the linked-list structure if prev != null because if it is null this
// DefaultAttribute acts also as head. The head must never be removed completely and just be
// marked as removed as all synchronization is done on the head itself for each bucket.
// The head itself will be GC'ed once the DefaultAttributeMap is GC'ed. So at most 5 heads will
// be removed lazy as the array size is 5.
if (prev != null) {
prev.next = next;
if (next != null) {
next.prev = prev;
}
}
}
}
}
}