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/* |
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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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* Written by Doug Lea and Martin Buchholz with assistance from |
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* members of JCP JSR-166 Expert Group and released to the public |
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* domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166e.extra; |
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import static java.lang.Double.doubleToRawLongBits; |
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import static java.lang.Double.longBitsToDouble; |
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import sun.misc.Unsafe; |
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/** |
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* A {@code double} value that may be updated atomically. See the |
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* {@link java.util.concurrent.atomic} package specification for |
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* description of the properties of atomic variables. An |
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* {@code AtomicDouble} is used in applications such as atomically |
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* incremented sequence numbers, and cannot be used as a replacement |
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* for a {@link java.lang.Double}. However, this class does extend |
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* {@code Number} to allow uniform access by tools and utilities that |
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* deal with numerically-based classes. |
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* description of the properties of atomic variables. An {@code |
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* AtomicDouble} is used in applications such as atomic accumulation, |
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* and cannot be used as a replacement for a {@link Double}. However, |
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* this class does extend {@code Number} to allow uniform access by |
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* tools and utilities that deal with numerically-based classes. |
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* |
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* <p>This class differs from the primitive double {@code ==} operator |
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* and from {@link Double#equals} in that it uses purely bitwise |
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* equality in methods such as {@link #compareAndSet}, as if |
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* implemented by: |
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* <pre> {@code |
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* boolean bitEquals(double x, double y) { |
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* long xBits = Double.doubleToRawLongBits(x); |
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* long yBits = Double.doubleToRawLongBits(y); |
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* return xBits == yBits; |
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* }}</pre> |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @author Martin Buchholz |
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*/ |
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public class AtomicDouble extends Number implements java.io.Serializable { |
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private static final long serialVersionUID = 1927816293512124184L; |
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static final long serialVersionUID = -8405198993435143622L; |
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// setup to use Unsafe.compareAndSwapLong for updates |
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private static final Unsafe unsafe = Unsafe.getUnsafe(); |
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private static final sun.misc.Unsafe unsafe = getUnsafe(); |
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private static final long valueOffset; |
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/** |
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* Records whether the underlying JVM supports lockless |
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* compareAndSwap for longs. While the Unsafe.compareAndSwapLong |
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* method works in either case, some constructions should be |
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* handled at Java level to avoid locking user-visible locks. |
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*/ |
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static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8(); |
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/** |
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* Returns whether underlying JVM supports lockless CompareAndSet |
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* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS. |
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*/ |
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private static native boolean VMSupportsCS8(); |
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static { |
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try { |
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valueOffset = unsafe.objectFieldOffset |
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* Eventually sets to the given value. |
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* |
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* @param newValue the new value |
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* @since 1.6 |
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*/ |
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public final void lazySet(double newValue) { |
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unsafe.putOrderedLong(this, valueOffset, doubleToRawLongBits(newValue)); |
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long newBits = doubleToRawLongBits(newValue); |
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while (true) { |
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long currentBits = value; |
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if (compareAndSet(currentBits, newBits)) |
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if (unsafe.compareAndSwapLong(this, valueOffset, |
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currentBits, newBits)) |
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return longBitsToDouble(currentBits); |
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} |
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} |
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while (true) { |
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long currentBits = value; |
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long nextBits = doubleToRawLongBits(longBitsToDouble(currentBits) + delta); |
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if (compareAndSet(currentBits, nextBits)) |
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if (unsafe.compareAndSwapLong(this, valueOffset, |
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currentBits, nextBits)) |
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return longBitsToDouble(currentBits); |
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} |
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} |
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return get(); |
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} |
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/** |
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* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
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* Replace with a simple call to Unsafe.getUnsafe when integrating |
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* into a jdk. |
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* |
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* @return a sun.misc.Unsafe |
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*/ |
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private static sun.misc.Unsafe getUnsafe() { |
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try { |
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return sun.misc.Unsafe.getUnsafe(); |
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} catch (SecurityException se) { |
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try { |
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return java.security.AccessController.doPrivileged |
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(new java.security |
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.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
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public sun.misc.Unsafe run() throws Exception { |
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java.lang.reflect.Field f = sun.misc |
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.Unsafe.class.getDeclaredField("theUnsafe"); |
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f.setAccessible(true); |
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return (sun.misc.Unsafe) f.get(null); |
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}}); |
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} catch (java.security.PrivilegedActionException e) { |
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throw new RuntimeException("Could not initialize intrinsics", |
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e.getCause()); |
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} |
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} |
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} |
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} |
