--- a/jdk/src/share/classes/java/util/concurrent/atomic/AtomicStampedReference.java Tue Jan 11 13:42:34 2011 -0800
+++ b/jdk/src/share/classes/java/util/concurrent/atomic/AtomicStampedReference.java Wed Jan 12 14:40:36 2011 +0000
@@ -39,7 +39,7 @@
* An {@code AtomicStampedReference} maintains an object reference
* along with an integer "stamp", that can be updated atomically.
*
- * <p> Implementation note. This implementation maintains stamped
+ * <p>Implementation note: This implementation maintains stamped
* references by creating internal objects representing "boxed"
* [reference, integer] pairs.
*
@@ -47,17 +47,21 @@
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
-public class AtomicStampedReference<V> {
+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 static class Pair<T> {
+ final T reference;
+ final int stamp;
+ private Pair(T reference, int stamp) {
+ this.reference = reference;
+ this.stamp = stamp;
+ }
+ static <T> Pair<T> of(T reference, int stamp) {
+ return new Pair<T>(reference, stamp);
}
}
- private final AtomicReference<ReferenceIntegerPair<V>> atomicRef;
+ private volatile Pair<V> pair;
/**
* Creates a new {@code AtomicStampedReference} with the given
@@ -67,8 +71,7 @@
* @param initialStamp the initial stamp
*/
public AtomicStampedReference(V initialRef, int initialStamp) {
- atomicRef = new AtomicReference<ReferenceIntegerPair<V>>
- (new ReferenceIntegerPair<V>(initialRef, initialStamp));
+ pair = Pair.of(initialRef, initialStamp);
}
/**
@@ -77,7 +80,7 @@
* @return the current value of the reference
*/
public V getReference() {
- return atomicRef.get().reference;
+ return pair.reference;
}
/**
@@ -86,7 +89,7 @@
* @return the current value of the stamp
*/
public int getStamp() {
- return atomicRef.get().integer;
+ return pair.stamp;
}
/**
@@ -98,9 +101,9 @@
* @return the current value of the reference
*/
public V get(int[] stampHolder) {
- ReferenceIntegerPair<V> p = atomicRef.get();
- stampHolder[0] = p.integer;
- return p.reference;
+ Pair<V> pair = this.pair;
+ stampHolder[0] = pair.stamp;
+ return pair.reference;
}
/**
@@ -119,18 +122,12 @@
* @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)));
+ public boolean weakCompareAndSet(V expectedReference,
+ V newReference,
+ int expectedStamp,
+ int newStamp) {
+ return compareAndSet(expectedReference, newReference,
+ expectedStamp, newStamp);
}
/**
@@ -145,18 +142,17 @@
* @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 &&
+ public boolean compareAndSet(V expectedReference,
+ V newReference,
+ int expectedStamp,
+ int newStamp) {
+ Pair<V> current = pair;
+ return
+ expectedReference == current.reference &&
+ expectedStamp == current.stamp &&
((newReference == current.reference &&
- newStamp == current.integer) ||
- atomicRef.compareAndSet(current,
- new ReferenceIntegerPair<V>(newReference,
- newStamp)));
+ newStamp == current.stamp) ||
+ casPair(current, Pair.of(newReference, newStamp)));
}
@@ -167,9 +163,9 @@
* @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));
+ Pair<V> current = pair;
+ if (newReference != current.reference || newStamp != current.stamp)
+ this.pair = Pair.of(newReference, newStamp);
}
/**
@@ -186,11 +182,32 @@
* @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)));
+ Pair<V> current = pair;
+ return
+ expectedReference == current.reference &&
+ (newStamp == current.stamp ||
+ casPair(current, Pair.of(expectedReference, newStamp)));
+ }
+
+ // Unsafe mechanics
+
+ private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
+ private static final long pairOffset =
+ objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.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;
+ }
}
}