This class differs from the primitive double {@code ==} operator - * and from {@link Double#equals} in that it uses purely bitwise - * equality in methods such as {@link #compareAndSet}, as if - * implemented by: + *
This class compares primitive {@code double}
+ * values in methods such as {@link #compareAndSet} by comparing their
+ * bitwise representation using {@link Double#doubleToRawLongBits},
+ * which differs from both the primitive double {@code ==} operator
+ * and from {@link Double#equals}, as if implemented by:
* May fail spuriously
* and does not provide ordering guarantees, so is only rarely an
@@ -156,7 +124,7 @@ public class AtomicDouble extends Number
*
* @param expect the expected value
* @param update the new value
- * @return true if successful.
+ * @return {@code true} if successful
*/
public final boolean weakCompareAndSet(double expect, double update) {
return compareAndSet(expect, update);
@@ -170,11 +138,12 @@ public class AtomicDouble extends Number
*/
public final double getAndAdd(double delta) {
while (true) {
- long currentBits = value;
- long nextBits = doubleToRawLongBits(longBitsToDouble(currentBits) + delta);
- if (unsafe.compareAndSwapLong(this, valueOffset,
- currentBits, nextBits))
- return longBitsToDouble(currentBits);
+ long current = value;
+ double currentVal = longBitsToDouble(current);
+ double nextVal = currentVal + delta;
+ long next = doubleToRawLongBits(nextVal);
+ if (unsafe.compareAndSwapLong(this, valueOffset, current, next))
+ return currentVal;
}
}
@@ -185,23 +154,24 @@ public class AtomicDouble extends Number
* @return the updated value
*/
public final double addAndGet(double delta) {
- for (;;) {
- double current = get();
- double next = current + delta;
- if (compareAndSet(current, next))
- return next;
+ while (true) {
+ long current = value;
+ double currentVal = longBitsToDouble(current);
+ double nextVal = currentVal + delta;
+ long next = doubleToRawLongBits(nextVal);
+ if (unsafe.compareAndSwapLong(this, valueOffset, current, next))
+ return nextVal;
}
}
/**
* Returns the String representation of the current value.
- * @return the String representation of the current value.
+ * @return the String representation of the current value
*/
public String toString() {
return Double.toString(get());
}
-
/**
* Returns the value of this {@code AtomicDouble} as an {@code int}
* after a narrowing primitive conversion.
@@ -211,7 +181,8 @@ public class AtomicDouble extends Number
}
/**
- * Returns the value of this {@code AtomicDouble} as a {@code long}.
+ * Returns the value of this {@code AtomicDouble} as a {@code long}
+ * after a narrowing primitive conversion.
*/
public long longValue() {
return (long)get();
@@ -219,20 +190,30 @@ public class AtomicDouble extends Number
/**
* Returns the value of this {@code AtomicDouble} as a {@code float}
- * after a widening primitive conversion.
+ * after a narrowing primitive conversion.
*/
public float floatValue() {
return (float)get();
}
/**
- * Returns the value of this {@code AtomicDouble} as a {@code double}
- * after a widening primitive conversion.
+ * Returns the value of this {@code AtomicDouble} as a {@code double}.
*/
public double doubleValue() {
return get();
}
+ // Unsafe mechanics
+ private static final sun.misc.Unsafe unsafe = getUnsafe();
+ private static final long valueOffset;
+
+ static {
+ try {
+ valueOffset = unsafe.objectFieldOffset
+ (AtomicDouble.class.getDeclaredField("value"));
+ } catch (Exception ex) { throw new Error(ex); }
+ }
+
/**
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
* Replace with a simple call to Unsafe.getUnsafe when integrating
{@code
* boolean bitEquals(double x, double y) {
* long xBits = Double.doubleToRawLongBits(x);
@@ -35,43 +37,6 @@ import static java.lang.Double.longBitsT
public class AtomicDouble extends Number implements java.io.Serializable {
static final long serialVersionUID = -8405198993435143622L;
- // setup to use Unsafe.compareAndSwapLong for updates
- private static final sun.misc.Unsafe unsafe = getUnsafe();
- private static final long valueOffset;
-
- /**
- * Records whether the underlying JVM supports lockless
- * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
- * method works in either case, some constructions should be
- * handled at Java level to avoid locking user-visible locks.
- */
- static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
-
- /**
- * Returns whether underlying JVM supports lockless CompareAndSet
- * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
- */
- private static boolean VMSupportsCS8() {
- final Class klazz = java.util.concurrent.atomic.AtomicLong.class;
- return java.security.AccessController.doPrivileged
- (new java.security.PrivilegedAction