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/*
* 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;
/**
* 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
* @param <V> The type of object referred to by this reference
*/
public class AtomicStampedReference<V> {
private static class ReferenceIntegerPair<T> {
private final T reference;
private final int integer;
ReferenceIntegerPair(T r, int i) {
reference = r; integer = i;
}
}
private final AtomicReference<ReferenceIntegerPair<V>> atomicRef;
/**
* Creates a new <tt>AtomicStampedReference</tt> with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialStamp the initial stamp
*/
public AtomicStampedReference(V initialRef, int initialStamp) {
atomicRef = new AtomicReference<ReferenceIntegerPair<V>>
(new ReferenceIntegerPair<V>(initialRef, initialStamp));
}
/**
* Returns the current value of the reference.
*
* @return the current value of the reference
*/
public V getReference() {
return atomicRef.get().reference;
}
/**
* Returns the current value of the stamp.
*
* @return the current value of the stamp
*/
public int getStamp() {
return atomicRef.get().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 V get(int[] stampHolder) {
ReferenceIntegerPair<V> p = atomicRef.get();
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.
* May fail spuriously and does not provide ordering guarantees,
* so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
*
* @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(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
ReferenceIntegerPair<V> current = atomicRef.get();
return expectedReference == current.reference &&
expectedStamp == current.integer &&
((newReference == current.reference &&
newStamp == current.integer) ||
atomicRef.weakCompareAndSet(current,
new ReferenceIntegerPair<V>(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(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
ReferenceIntegerPair<V> current = atomicRef.get();
return expectedReference == current.reference &&
expectedStamp == current.integer &&
((newReference == current.reference &&
newStamp == current.integer) ||
atomicRef.compareAndSet(current,
new ReferenceIntegerPair<V>(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(V newReference, int newStamp) {
ReferenceIntegerPair<V> current = atomicRef.get();
if (newReference != current.reference || newStamp != current.integer)
atomicRef.set(new ReferenceIntegerPair<V>(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(V expectedReference, int newStamp) {
ReferenceIntegerPair<V> current = atomicRef.get();
return expectedReference == current.reference &&
(newStamp == current.integer ||
atomicRef.compareAndSet(current,
new ReferenceIntegerPair<V>(expectedReference,
newStamp)));
}
}
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