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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/publicdomain/zero/1.0/
*/
package java.util.concurrent.atomic;
/**
* An {@code AtomicMarkableReference} maintains an object reference
* along with a mark bit, that can be updated atomically.
*
* <p>Implementation note: This implementation maintains markable
* references by creating internal objects representing "boxed"
* [reference, boolean] pairs.
*
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicMarkableReference<V> {
private static class Pair<T> {
final T reference;
final boolean mark;
private Pair(T reference, boolean mark) {
this.reference = reference;
this.mark = mark;
}
static <T> Pair<T> of(T reference, boolean mark) {
return new Pair<T>(reference, mark);
}
}
private volatile Pair<V> pair;
/**
* Creates a new {@code AtomicMarkableReference} with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialMark the initial mark
*/
public AtomicMarkableReference(V initialRef, boolean initialMark) {
pair = Pair.of(initialRef, initialMark);
}
/**
* 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 mark.
*
* @return the current value of the mark
*/
public boolean isMarked() {
return pair.mark;
}
/**
* Returns the current values of both the reference and the mark.
* Typical usage is {@code boolean[1] holder; ref = v.get(holder); }.
*
* @param markHolder an array of size of at least one. On return,
* {@code markholder[0]} will hold the value of the mark.
* @return the current value of the reference
*/
public V get(boolean[] markHolder) {
Pair<V> pair = this.pair;
markHolder[0] = pair.mark;
return pair.reference;
}
/**
* Atomically sets the value of both the reference and mark
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current mark is equal to the expected mark.
*
* <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 expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedMark the expected value of the mark
* @param newMark the new value for the mark
* @return true if successful
*/
public boolean weakCompareAndSet(V expectedReference,
V newReference,
boolean expectedMark,
boolean newMark) {
return compareAndSet(expectedReference, newReference,
expectedMark, newMark);
}
/**
* Atomically sets the value of both the reference and mark
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current mark is equal to the expected mark.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedMark the expected value of the mark
* @param newMark the new value for the mark
* @return true if successful
*/
public boolean compareAndSet(V expectedReference,
V newReference,
boolean expectedMark,
boolean newMark) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
expectedMark == current.mark &&
((newReference == current.reference &&
newMark == current.mark) ||
casPair(current, Pair.of(newReference, newMark)));
}
/**
* Unconditionally sets the value of both the reference and mark.
*
* @param newReference the new value for the reference
* @param newMark the new value for the mark
*/
public void set(V newReference, boolean newMark) {
Pair<V> current = pair;
if (newReference != current.reference || newMark != current.mark)
this.pair = Pair.of(newReference, newMark);
}
/**
* Atomically sets the value of the mark 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 newMark the new value for the mark
* @return true if successful
*/
public boolean attemptMark(V expectedReference, boolean newMark) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
(newMark == current.mark ||
casPair(current, Pair.of(expectedReference, newMark)));
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long pairOffset =
objectFieldOffset(UNSAFE, "pair", AtomicMarkableReference.class);
private boolean casPair(Pair<V> cmp, Pair<V> val) {
return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
}
static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
}
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