| /* |
| * 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; |
| } |
| } |
| } |