AtomicLong.java

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

/*
 * 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;
import java.util.function.LongUnaryOperator;
import java.util.function.LongBinaryOperator;
import sun.misc.Unsafe;

/**
 * A {@code long} value that may be updated atomically.  See the
 * {@link java.util.concurrent.atomic} package specification for
 * description of the properties of atomic variables. An
 * {@code AtomicLong} is used in applications such as atomically
 * incremented sequence numbers, and cannot be used as a replacement
 * for a {@link java.lang.Long}. However, this class does extend
 * {@code Number} to allow uniform access by tools and utilities that
 * deal with numerically-based classes.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class AtomicLong extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 1927816293512124184L;

    // setup to use Unsafe.compareAndSwapLong for updates
    private static final Unsafe 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 native boolean VMSupportsCS8();

    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicLong.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile long value;

    /**
     * Creates a new AtomicLong with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicLong(long initialValue) {
        value = initialValue;
    }

    /**
     * Creates a new AtomicLong with initial value {@code 0}.
     */
    public AtomicLong() {
    }

    /**
     * Gets the current value.
     *
     * @return the current value
     */
    public final long get() {
        return value;
    }

    /**
     * Sets to the given value.
     *
     * @param newValue the new value
     */
    public final void set(long newValue) {
        value = newValue;
    }

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(long newValue) {
        unsafe.putOrderedLong(this, valueOffset, newValue);
    }

    /**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    public final long getAndSet(long newValue) {
        return unsafe.getAndSetLong(this, valueOffset, newValue);
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(long expect, long update) {
        return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, so is
     * only rarely an appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public final boolean weakCompareAndSet(long expect, long update) {
        return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
    }

    /**
     * Atomically increments by one the current value.
     *
     * @return the previous value
     */
    public final long getAndIncrement() {
        return unsafe.getAndAddLong(this, valueOffset, 1L);
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the previous value
     */
    public final long getAndDecrement() {
        return unsafe.getAndAddLong(this, valueOffset, -1L);
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the previous value
     */
    public final long getAndAdd(long delta) {
        return unsafe.getAndAddLong(this, valueOffset, delta);
    }

    /**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final long incrementAndGet() {
        return unsafe.getAndAddLong(this, valueOffset, 1L) + 1L;
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the updated value
     */
    public final long decrementAndGet() {
        return unsafe.getAndAddLong(this, valueOffset, -1L) - 1L;
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the updated value
     */
    public final long addAndGet(long delta) {
        return unsafe.getAndAddLong(this, valueOffset, delta) + delta;
    }

    /**
     * Atomically updates the current value with the results of
     * applying the given function, returning the previous value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.
     *
     * @param updateFunction a side-effect-free function
     * @return the previous value
     * @since 1.8
     */
    public final long getAndUpdate(LongUnaryOperator updateFunction) {
        long prev, next;
        do {
            prev = get();
            next = updateFunction.applyAsLong(prev);
        } while (!compareAndSet(prev, next));
        return prev;
    }

    /**
     * Atomically updates the current value with the results of
     * applying the given function, returning the updated value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.
     *
     * @param updateFunction a side-effect-free function
     * @return the updated value
     * @since 1.8
     */
    public final long updateAndGet(LongUnaryOperator updateFunction) {
        long prev, next;
        do {
            prev = get();
            next = updateFunction.applyAsLong(prev);
        } while (!compareAndSet(prev, next));
        return next;
    }

    /**
     * Atomically updates the current value with the results of
     * applying the given function to the current and given values,
     * returning the previous value. The function should be
     * side-effect-free, since it may be re-applied when attempted
     * updates fail due to contention among threads.  The function
     * is applied with the current value as its first argument,
     * and the given update as the second argument.
     *
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the previous value
     * @since 1.8
     */
    public final long getAndAccumulate(long x,
                                       LongBinaryOperator accumulatorFunction) {
        long prev, next;
        do {
            prev = get();
            next = accumulatorFunction.applyAsLong(prev, x);
        } while (!compareAndSet(prev, next));
        return prev;
    }

    /**
     * Atomically updates the current value with the results of
     * applying the given function to the current and given values,
     * returning the updated value. The function should be
     * side-effect-free, since it may be re-applied when attempted
     * updates fail due to contention among threads.  The function
     * is applied with the current value as its first argument,
     * and the given update as the second argument.
     *
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the updated value
     * @since 1.8
     */
    public final long accumulateAndGet(long x,
                                       LongBinaryOperator accumulatorFunction) {
        long prev, next;
        do {
            prev = get();
            next = accumulatorFunction.applyAsLong(prev, x);
        } while (!compareAndSet(prev, next));
        return next;
    }

    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value
     */
    public String toString() {
        return Long.toString(get());
    }

    /**
     * Returns the value of this {@code AtomicLong} as an {@code int}
     * after a narrowing primitive conversion.
     * @jls 5.1.3 Narrowing Primitive Conversions
     */
    public int intValue() {
        return (int)get();
    }

    /**
     * Returns the value of this {@code AtomicLong} as a {@code long}.
     */
    public long longValue() {
        return get();
    }

    /**
     * Returns the value of this {@code AtomicLong} as a {@code float}
     * after a widening primitive conversion.
     * @jls 5.1.2 Widening Primitive Conversions
     */
    public float floatValue() {
        return (float)get();
    }

    /**
     * Returns the value of this {@code AtomicLong} as a {@code double}
     * after a widening primitive conversion.
     * @jls 5.1.2 Widening Primitive Conversions
     */
    public double doubleValue() {
        return (double)get();
    }

}