/*

 * 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.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent.atomic;

import sun.misc.Unsafe;

/**

 * An object reference that may be updated atomically. See the {@link

 * java.util.concurrent.atomic} package specification for description

 * of the properties of atomic variables.

 * @since 1.5

 * @author Doug Lea

 * @param <V> The type of object referred to by this reference

 */

public class AtomicReference<V>  implements java.io.Serializable {

    private static final long serialVersionUID = -1848883965231344442L;

    private static final Unsafe unsafe = Unsafe.getUnsafe();

    private static final long valueOffset;

    static {

      try {

        valueOffset = unsafe.objectFieldOffset

            (AtomicReference.class.getDeclaredField("value"));

      } catch (Exception ex) { throw new Error(ex); }

    }

    private volatile V value;

    /**

     * Creates a new AtomicReference with the given initial value.

     *

     * @param initialValue the initial value

     */

    public AtomicReference(V initialValue) {

        value = initialValue;

    }

    /**

     * Creates a new AtomicReference with null initial value.

     */

    public AtomicReference() {

    }

    /**

     * Gets the current value.

     *

     * @return the current value

     */

    public final V get() {

        return value;

    }

    /**

     * Sets to the given value.

     *

     * @param newValue the new value

     */

    public final void set(V newValue) {

        value = newValue;

    }

    /**

     * Eventually sets to the given value.

     *

     * @param newValue the new value

     * @since 1.6

     */

    public final void lazySet(V newValue) {

        unsafe.putOrderedObject(this, valueOffset, newValue);

    }

    /**

     * Atomically sets the value to the given updated value

     * if the current value {@code ==} the expected value.

     * @param expect the expected value

     * @param update the new value

     * @return true if successful. False return indicates that

     * the actual value was not equal to the expected value.

     */

    public final boolean compareAndSet(V expect, V update) {

        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

    }

    /**

     * Atomically sets the value to the given updated value

     * if the current value {@code ==} the expected value.

     *

     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>

     * and does not provide ordering guarantees, so is only rarely an

     * appropriate alternative to {@code compareAndSet}.

     *

     * @param expect the expected value

     * @param update the new value

     * @return true if successful.

     */

    public final boolean weakCompareAndSet(V expect, V update) {

        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

    }

    /**

     * Atomically sets to the given value and returns the old value.

     *

     * @param newValue the new value

     * @return the previous value

     */

    public final V getAndSet(V newValue) {

        while (true) {

            V x = get();

            if (compareAndSet(x, newValue))

                return x;

        }

    }

    /**

     * Returns the String representation of the current value.

     * @return the String representation of the current value.

     */

    public String toString() {

        return String.valueOf(get());

    }

}