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jeremybenn |
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent.atomic;
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/**
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* An <tt>AtomicMarkableReference</tt> maintains an object reference
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* along with a mark bit, that can be updated atomically.
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* <p>
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* <p> Implementation note. This implementation maintains markable
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* references by creating internal objects representing "boxed"
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* [reference, boolean] pairs.
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*
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* @since 1.5
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* @author Doug Lea
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* @param <V> The type of object referred to by this reference
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*/
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public class AtomicMarkableReference<V> {
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private static class ReferenceBooleanPair<T> {
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private final T reference;
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private final boolean bit;
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ReferenceBooleanPair(T r, boolean i) {
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reference = r; bit = i;
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}
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}
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private final AtomicReference<ReferenceBooleanPair<V>> atomicRef;
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/**
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* Creates a new <tt>AtomicMarkableReference</tt> with the given
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* initial values.
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*
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* @param initialRef the initial reference
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* @param initialMark the initial mark
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*/
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public AtomicMarkableReference(V initialRef, boolean initialMark) {
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atomicRef = new AtomicReference<ReferenceBooleanPair<V>> (new ReferenceBooleanPair<V>(initialRef, initialMark));
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}
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/**
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* Returns the current value of the reference.
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*
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* @return the current value of the reference
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*/
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public V getReference() {
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return atomicRef.get().reference;
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}
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/**
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* Returns the current value of the mark.
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*
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* @return the current value of the mark
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*/
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public boolean isMarked() {
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return atomicRef.get().bit;
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}
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/**
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* Returns the current values of both the reference and the mark.
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* Typical usage is <tt>boolean[1] holder; ref = v.get(holder); </tt>.
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*
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* @param markHolder an array of size of at least one. On return,
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* <tt>markholder[0]</tt> will hold the value of the mark.
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* @return the current value of the reference
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*/
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public V get(boolean[] markHolder) {
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ReferenceBooleanPair<V> p = atomicRef.get();
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markHolder[0] = p.bit;
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return p.reference;
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}
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/**
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* Atomically sets the value of both the reference and mark
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* to the given update values if the
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* current reference is <tt>==</tt> to the expected reference
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* and the current mark is equal to the expected mark.
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* May fail spuriously and does not provide ordering guarantees,
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* so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
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*
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* @param expectedReference the expected value of the reference
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* @param newReference the new value for the reference
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* @param expectedMark the expected value of the mark
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* @param newMark the new value for the mark
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* @return true if successful
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*/
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public boolean weakCompareAndSet(V expectedReference,
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V newReference,
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boolean expectedMark,
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boolean newMark) {
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ReferenceBooleanPair<V> current = atomicRef.get();
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return expectedReference == current.reference &&
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expectedMark == current.bit &&
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((newReference == current.reference && newMark == current.bit) ||
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atomicRef.weakCompareAndSet(current,
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new ReferenceBooleanPair<V>(newReference,
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newMark)));
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}
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/**
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* Atomically sets the value of both the reference and mark
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* to the given update values if the
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* current reference is <tt>==</tt> to the expected reference
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* and the current mark is equal to the expected mark.
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*
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* @param expectedReference the expected value of the reference
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* @param newReference the new value for the reference
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* @param expectedMark the expected value of the mark
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* @param newMark the new value for the mark
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* @return true if successful
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*/
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public boolean compareAndSet(V expectedReference,
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V newReference,
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boolean expectedMark,
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boolean newMark) {
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ReferenceBooleanPair<V> current = atomicRef.get();
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return expectedReference == current.reference &&
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expectedMark == current.bit &&
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((newReference == current.reference && newMark == current.bit) ||
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atomicRef.compareAndSet(current,
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new ReferenceBooleanPair<V>(newReference,
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newMark)));
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}
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/**
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* Unconditionally sets the value of both the reference and mark.
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*
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* @param newReference the new value for the reference
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* @param newMark the new value for the mark
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*/
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public void set(V newReference, boolean newMark) {
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ReferenceBooleanPair<V> current = atomicRef.get();
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if (newReference != current.reference || newMark != current.bit)
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atomicRef.set(new ReferenceBooleanPair<V>(newReference, newMark));
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}
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/**
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* Atomically sets the value of the mark to the given update value
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* if the current reference is <tt>==</tt> to the expected
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* reference. Any given invocation of this operation may fail
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* (return <tt>false</tt>) spuriously, but repeated invocation
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* when the current value holds the expected value and no other
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* thread is also attempting to set the value will eventually
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* succeed.
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*
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* @param expectedReference the expected value of the reference
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* @param newMark the new value for the mark
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* @return true if successful
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*/
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public boolean attemptMark(V expectedReference, boolean newMark) {
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ReferenceBooleanPair<V> current = atomicRef.get();
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return expectedReference == current.reference &&
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(newMark == current.bit ||
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atomicRef.compareAndSet
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(current, new ReferenceBooleanPair<V>(expectedReference,
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newMark)));
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}
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}
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