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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
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/*
 * 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 java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;


An AtomicStampedReference maintains an object reference along with an integer "stamp", that can be updated atomically. 
Implementation note: This implementation maintains stamped
references by creating internal objects representing "boxed"
[reference, integer] pairs.

Author:Doug Lea
Type parameters:
  • <V> – The type of object referred to by this reference
Since:1.5
/**
 * An {@code AtomicStampedReference} maintains an object reference
 * along with an integer "stamp", that can be updated atomically.
 *
 * <p>Implementation note: This implementation maintains stamped
 * references by creating internal objects representing "boxed"
 * [reference, integer] pairs.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of object referred to by this reference
 */
public class AtomicStampedReference<V> {

    private static class Pair<T> {
        final T reference;
        final int stamp;
        private Pair(T reference, int stamp) {
            this.reference = reference;
            this.stamp = stamp;
        }
        static <T> Pair<T> of(T reference, int stamp) {
            return new Pair<T>(reference, stamp);
        }
    }

    private volatile Pair<V> pair;

    
Creates a new AtomicStampedReference with the given initial values. 
Params:
  • initialRef – the initial reference
  • initialStamp – the initial stamp
/**
     * Creates a new {@code AtomicStampedReference} with the given
     * initial values.
     *
     * @param initialRef the initial reference
     * @param initialStamp the initial stamp
     */
    public AtomicStampedReference(V initialRef, int initialStamp) {
        pair = Pair.of(initialRef, initialStamp);
    }

    
Returns the current value of the reference.

Returns:the current value of the reference
/**
     * Returns the current value of the reference.
     *
     * @return the current value of the reference
     */
    public V getReference() {
        return pair.reference;
    }

    
Returns the current value of the stamp.

Returns:the current value of the stamp
/**
     * Returns the current value of the stamp.
     *
     * @return the current value of the stamp
     */
    public int getStamp() {
        return pair.stamp;
    }

    
Returns the current values of both the reference and the stamp. Typical usage is int[1] holder; ref = v.get(holder); . 
Params:
  • stampHolder – an array of size of at least one. On return, stampHolder[0] will hold the value of the stamp.
Returns:the current value of the reference
/**
     * Returns the current values of both the reference and the stamp.
     * Typical usage is {@code int[1] holder; ref = v.get(holder); }.
     *
     * @param stampHolder an array of size of at least one.  On return,
     * {@code stampHolder[0]} will hold the value of the stamp.
     * @return the current value of the reference
     */
    public V get(int[] stampHolder) {
        Pair<V> pair = this.pair;
        stampHolder[0] = pair.stamp;
        return pair.reference;
    }

    
Atomically sets the value of both the reference and stamp to the given update values if the current reference is == to the expected reference and the current stamp is equal to the expected stamp. 
May fail
spuriously and does not provide ordering guarantees, so is only rarely an appropriate alternative to compareAndSet. 
Params:
  • expectedReference – the expected value of the reference
  • newReference – the new value for the reference
  • expectedStamp – the expected value of the stamp
  • newStamp – the new value for the stamp
Returns:true if successful
/**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is {@code ==} to the expected reference
     * and the current stamp is equal to the expected stamp.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, so is
     * only rarely an appropriate alternative to {@code compareAndSet}.
     *
     * @param expectedReference the expected value of the reference
     * @param newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean weakCompareAndSet(V   expectedReference,
                                     V   newReference,
                                     int expectedStamp,
                                     int newStamp) {
        return compareAndSet(expectedReference, newReference,
                             expectedStamp, newStamp);
    }

    
Atomically sets the value of both the reference and stamp to the given update values if the current reference is == to the expected reference and the current stamp is equal to the expected stamp. 
Params:
  • expectedReference – the expected value of the reference
  • newReference – the new value for the reference
  • expectedStamp – the expected value of the stamp
  • newStamp – the new value for the stamp
Returns:true if successful
/**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is {@code ==} to the expected reference
     * and the current stamp is equal to the expected stamp.
     *
     * @param expectedReference the expected value of the reference
     * @param newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean compareAndSet(V   expectedReference,
                                 V   newReference,
                                 int expectedStamp,
                                 int newStamp) {
        Pair<V> current = pair;
        return
            expectedReference == current.reference &&
            expectedStamp == current.stamp &&
            ((newReference == current.reference &&
              newStamp == current.stamp) ||
             casPair(current, Pair.of(newReference, newStamp)));
    }

    
Unconditionally sets the value of both the reference and stamp.

Params:
  • newReference – the new value for the reference
  • newStamp – the new value for the stamp
/**
     * Unconditionally sets the value of both the reference and stamp.
     *
     * @param newReference the new value for the reference
     * @param newStamp the new value for the stamp
     */
    public void set(V newReference, int newStamp) {
        Pair<V> current = pair;
        if (newReference != current.reference || newStamp != current.stamp)
            this.pair = Pair.of(newReference, newStamp);
    }

    
Atomically sets the value of the stamp to the given update value if the current reference is == to the expected reference. Any given invocation of this operation may fail (return false) spuriously, but repeated invocation when the current value holds the expected value and no other thread is also attempting to set the value will eventually succeed. 
Params:
  • expectedReference – the expected value of the reference
  • newStamp – the new value for the stamp
Returns:true if successful
/**
     * Atomically sets the value of the stamp to the given update value
     * if the current reference is {@code ==} to the expected
     * reference.  Any given invocation of this operation may fail
     * (return {@code false}) spuriously, but repeated invocation
     * when the current value holds the expected value and no other
     * thread is also attempting to set the value will eventually
     * succeed.
     *
     * @param expectedReference the expected value of the reference
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean attemptStamp(V expectedReference, int newStamp) {
        Pair<V> current = pair;
        return
            expectedReference == current.reference &&
            (newStamp == current.stamp ||
             casPair(current, Pair.of(expectedReference, newStamp)));
    }

    // VarHandle mechanics
    private static final VarHandle PAIR;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
                                   Pair.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    private boolean casPair(Pair<V> cmp, Pair<V> val) {
        return PAIR.compareAndSet(this, cmp, val);
    }
}