/*
* 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/licenses/publicdomain
*/
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
/** {@collect.stats}
* {@description.open}
* 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.
* {@description.close}
* @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;
/** {@collect.stats}
* {@description.open}
* Creates a new AtomicReference with the given initial value.
* {@description.close}
*
* @param initialValue the initial value
*/
public AtomicReference(V initialValue) {
value = initialValue;
}
/** {@collect.stats}
* {@description.open}
* Creates a new AtomicReference with null initial value.
* {@description.close}
*/
public AtomicReference() {
}
/** {@collect.stats}
* {@description.open}
* Gets the current value.
* {@description.close}
*
* @return the current value
*/
public final V get() {
return value;
}
/** {@collect.stats}
* {@description.open}
* Sets to the given value.
* {@description.close}
*
* @param newValue the new value
*/
public final void set(V newValue) {
value = newValue;
}
/** {@collect.stats}
* {@description.open}
* Eventually sets to the given value.
* {@description.close}
*
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(V newValue) {
unsafe.putOrderedObject(this, valueOffset, newValue);
}
/** {@collect.stats}
* {@description.open}
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
* {@description.close}
* @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);
}
/** {@collect.stats}
* {@description.open}
* 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}.
* {@description.close}
*
* @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);
}
/** {@collect.stats}
* {@description.open}
* Atomically sets to the given value and returns the old value.
* {@description.close}
*
* @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;
}
}
/** {@collect.stats}
* {@description.open}
* Returns the String representation of the current value.
* {@description.close}
* @return the String representation of the current value.
*/
public String toString() {
return String.valueOf(get());
}
}