/*
 * 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;

An attribute which allows to store a value reference. It may be updated atomically and so is thread-safe.
Type parameters:
<T> –   the type of the value it holds./**
 * An attribute which allows to store a value reference. It may be updated atomically and so is thread-safe.
 *
 * @param <T>   the type of the value it holds.
 */
public interface Attribute<T> {

    
Returns the key of this attribute.
/**
     * Returns the key of this attribute.
     */
    AttributeKey<T> key();

    
Returns the current value, which may be null /**
     * Returns the current value, which may be {@code null}
     */
    T get();

    
Sets the value
/**
     * Sets the value
     */
    void set(T value);

    
 Atomically sets to the given value and returns the old value which may be null if non was set before. /**
     *  Atomically sets to the given value and returns the old value which may be {@code null} if non was set before.
     */
    T getAndSet(T value);

    
 Atomically sets to the given value if this Attribute's value is null. If it was not possible to set the value as it contains a value it will just return the current value. /**
     *  Atomically sets to the given value if this {@link Attribute}'s value is {@code null}.
     *  If it was not possible to set the value as it contains a value it will just return the current value.
     */
    T setIfAbsent(T value);

    
Removes this attribute from the AttributeMap and returns the old value. Subsequent get() calls will return null. If you only want to return the old value and clear the Attribute while still keep it in the AttributeMap use getAndSet(Object) with a value of null. 
 Be aware that even if you call this method another thread that has obtained a reference to this Attribute via AttributeMap.attr(AttributeKey<Object>) will still operate on the same instance. That said if now another thread or even the same thread later will call AttributeMap.attr(AttributeKey<Object>) again, a new Attribute instance is created and so is not the same as the previous one that was removed. Because of this special caution should be taken when you call remove() or getAndRemove(). 
Deprecated:
please consider using getAndSet(Object) (with value of null)./**
     * Removes this attribute from the {@link AttributeMap} and returns the old value. Subsequent {@link #get()}
     * calls will return {@code null}.
     *
     * If you only want to return the old value and clear the {@link Attribute} while still keep it in the
     * {@link AttributeMap} use {@link #getAndSet(Object)} with a value of {@code null}.
     *
     * <p>
     * Be aware that even if you call this method another thread that has obtained a reference to this {@link Attribute}
     * via {@link AttributeMap#attr(AttributeKey)} will still operate on the same instance. That said if now another
     * thread or even the same thread later will call {@link AttributeMap#attr(AttributeKey)} again, a new
     * {@link Attribute} instance is created and so is not the same as the previous one that was removed. Because of
     * this special caution should be taken when you call {@link #remove()} or {@link #getAndRemove()}.
     *
     * @deprecated please consider using {@link #getAndSet(Object)} (with value of {@code null}).
     */
    @Deprecated
    T getAndRemove();

    
Atomically sets the value to the given updated value if the current value == the expected value. If it the set was successful it returns true otherwise false. /**
     * Atomically sets the value to the given updated value if the current value == the expected value.
     * If it the set was successful it returns {@code true} otherwise {@code false}.
     */
    boolean compareAndSet(T oldValue, T newValue);

    
Removes this attribute from the AttributeMap. Subsequent get() calls will return @{code null}. If you only want to remove the value and clear the Attribute while still keep it in AttributeMap use set(Object) with a value of null. 
 Be aware that even if you call this method another thread that has obtained a reference to this Attribute via AttributeMap.attr(AttributeKey<Object>) will still operate on the same instance. That said if now another thread or even the same thread later will call AttributeMap.attr(AttributeKey<Object>) again, a new Attribute instance is created and so is not the same as the previous one that was removed. Because of this special caution should be taken when you call remove() or getAndRemove(). 
Deprecated:
please consider using set(Object) (with value of null)./**
     * Removes this attribute from the {@link AttributeMap}. Subsequent {@link #get()} calls will return @{code null}.
     *
     * If you only want to remove the value and clear the {@link Attribute} while still keep it in
     * {@link AttributeMap} use {@link #set(Object)} with a value of {@code null}.
     *
     * <p>
     * Be aware that even if you call this method another thread that has obtained a reference to this {@link Attribute}
     * via {@link AttributeMap#attr(AttributeKey)} will still operate on the same instance. That said if now another
     * thread or even the same thread later will call {@link AttributeMap#attr(AttributeKey)} again, a new
     * {@link Attribute} instance is created and so is not the same as the previous one that was removed. Because of
     * this special caution should be taken when you call {@link #remove()} or {@link #getAndRemove()}.
     *
     * @deprecated please consider using {@link #set(Object)} (with value of {@code null}).
     */
    @Deprecated
    void remove();
}