atomicstampedreference.java

来自「SRI international 发布的OAA框架软件」· Java 代码 · 共 171 行

/*
 * 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 edu.emory.mathcs.backport.java.util.concurrent.atomic;

/**
 * An <tt>AtomicStampedReference</tt> 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
 */
public class AtomicStampedReference {

    private static class ReferenceIntegerPair {
        private final Object reference;
        private final int integer;
        ReferenceIntegerPair(Object r, int i) {
            reference = r; integer = i;
        }
    }

    private final AtomicReference atomicRef;

    /**
     * Creates a new <tt>AtomicStampedReference</tt> with the given
     * initial values.
     *
     * @param initialRef the initial reference
     * @param initialStamp the initial stamp
     */
    public AtomicStampedReference(Object initialRef, int initialStamp) {
        atomicRef = new AtomicReference(
            new ReferenceIntegerPair(initialRef, initialStamp));
    }

    /**
     * Returns the current value of the reference.
     *
     * @return the current value of the reference
     */
    public Object getReference() {
        return getPair().reference;
    }

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

    /**
     * Returns the current values of both the reference and the stamp.
     * Typical usage is <tt>int[1] holder; ref = v.get(holder); </tt>.
     *
     * @param stampHolder an array of size of at least one.  On return,
     * <tt>stampholder[0]</tt> will hold the value of the stamp.
     * @return the current value of the reference
     */
    public Object get(int[] stampHolder) {
        ReferenceIntegerPair p = getPair();
        stampHolder[0] = p.integer;
        return p.reference;
    }

    /**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is <tt>==</tt> to the expected reference
     * and the current stamp is equal to the expected stamp.  Any given
     * invocation of this operation may fail (return
     * <tt>false</tt>) 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 newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return true if successful
     */
    public boolean weakCompareAndSet(Object expectedReference,
                                     Object newReference,
                                     int    expectedStamp,
                                     int    newStamp) {
        ReferenceIntegerPair current = getPair();
        return  expectedReference == current.reference &&
            expectedStamp == current.integer &&
            ((newReference == current.reference &&
              newStamp == current.integer) ||
             atomicRef.weakCompareAndSet(current,
                                     new ReferenceIntegerPair(newReference,
                                                              newStamp)));
    }

    /**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is <tt>==</tt> 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 true if successful
     */
    public boolean compareAndSet(Object expectedReference,
                                 Object newReference,
                                 int    expectedStamp,
                                 int    newStamp) {
        ReferenceIntegerPair current = getPair();
        return  expectedReference == current.reference &&
            expectedStamp == current.integer &&
            ((newReference == current.reference &&
              newStamp == current.integer) ||
             atomicRef.compareAndSet(current,
                                     new ReferenceIntegerPair(newReference,
                                                              newStamp)));
    }


    /**
     * 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(Object newReference, int newStamp) {
        ReferenceIntegerPair current = getPair();
        if (newReference != current.reference || newStamp != current.integer)
            atomicRef.set(new ReferenceIntegerPair(newReference, newStamp));
    }

    /**
     * Atomically sets the value of the stamp to the given update value
     * if the current reference is <tt>==</tt> to the expected
     * reference.  Any given invocation of this operation may fail
     * (return <tt>false</tt>) 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 true if successful
     */
    public boolean attemptStamp(Object expectedReference, int newStamp) {
        ReferenceIntegerPair current = getPair();
        return  expectedReference == current.reference &&
            (newStamp == current.integer ||
             atomicRef.compareAndSet(current,
                                     new ReferenceIntegerPair(expectedReference,
                                                              newStamp)));
    }

    private ReferenceIntegerPair getPair() {
        return (ReferenceIntegerPair)atomicRef.get();
    }
}