LongAdderImpl.java

/*
 * 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/
 *
 * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/LongAdder.java?revision=1.14&view=markup
 */

package io.dropwizard.metrics;

import java.io.Serializable;
import java.util.concurrent.atomic.AtomicLong;

// CHECKSTYLE:OFF
/**
 * One or more variables that together maintain an initially zero {@code long} sum.  When updates
 * (method {@link #add}) are contended across threads, the set of variables may grow dynamically to
 * reduce contention. Method {@link #sum} (or, equivalently, {@link #longValue}) returns the current
 * total combined across the variables maintaining the sum.
 * <p/>
 * <p>This class is usually preferable to {@link AtomicLong} when multiple threads update a common
 * sum that is used for purposes such as collecting statistics, not for fine-grained synchronization
 * control.  Under low update contention, the two classes have similar characteristics. But under
 * high contention, expected throughput of this class is significantly higher, at the expense of
 * higher space consumption.
 * <p/>
 * <p>This class extends {@link Number}, but does <em>not</em> define methods such as {@code
 * equals}, {@code hashCode} and {@code compareTo} because instances are expected to be mutated, and
 * so are not useful as collection keys.
 * <p/>
 * <p><em>jsr166e note: This class is targeted to be placed in java.util.concurrent.atomic.</em>
 *
 * @author Doug Lea
 * @since 1.8
 */
@SuppressWarnings("all")
class LongAdderImpl extends Striped64 implements LongAdder, Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * Version of plus for use in retryUpdate
     */
    @Override
    final long fn(long v, long x) {
        return v + x;
    }

    /**
     * Creates a new adder with initial sum of zero.
     */
    LongAdderImpl() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(long x) {
        Cell[] as;
        long b, v;
        HashCode hc;
        Cell a;
        int n;
        if ((as = cells) != null || !casBase(b = base, b + x)) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1 ||
                    (a = as[(n - 1) & h]) == null ||
                    !(uncontended = a.cas(v = a.value, v + x)))
                retryUpdate(x, hc, uncontended);
        }
    }

    /**
     * Equivalent to {@code add(1)}.
     */
    public void increment() {
        add(1L);
    }

    /**
     * Equivalent to {@code add(-1)}.
     */
    public void decrement() {
        add(-1L);
    }

    /**
     * Returns the current sum.  The returned value is <em>NOT</em> an atomic snapshot; invocation
     * in the absence of concurrent updates returns an accurate result, but concurrent updates that
     * occur while the sum is being calculated might not be incorporated.
     *
     * @return the sum
     */
    public long sum() {
        long sum = base;
        Cell[] as = cells;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    sum += a.value;
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero.  This method may be a useful alternative to
     * creating a new adder, but is only effective if there are no concurrent updates.  Because this
     * method is intrinsically racy, it should only be used when it is known that no threads are
     * concurrently updating.
     */
    public void reset() {
        internalReset(0L);
    }

    /**
     * Equivalent in effect to {@link #sum} followed by {@link #reset}. This method may apply for
     * example during quiescent points between multithreaded computations.  If there are updates
     * concurrent with this method, the returned value is <em>not</em> guaranteed to be the final
     * value occurring before the reset.
     *
     * @return the sum
     */
    public long sumThenReset() {
        long sum = base;
        Cell[] as = cells;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    sum += a.value;
                    a.value = 0L;
                }
            }
        }
        return sum;
    }

    /**
     * Returns the String representation of the {@link #sum}.
     *
     * @return the String representation of the {@link #sum}
     */
    @Override
    public String toString() {
        return Long.toString(sum());
    }

    /**
     * Equivalent to {@link #sum}.
     *
     * @return the sum
     */
    @Override
    public long longValue() {
        return sum();
    }

    /**
     * Returns the {@link #sum} as an {@code int} after a narrowing primitive conversion.
     */
    @Override
    public int intValue() {
        return (int) sum();
    }

    /**
     * Returns the {@link #sum} as a {@code float} after a widening primitive conversion.
     */
    @Override
    public float floatValue() {
        return sum();
    }

    /**
     * Returns the {@link #sum} as a {@code double} after a widening primitive conversion.
     */
    @Override
    public double doubleValue() {
        return sum();
    }

    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        s.defaultWriteObject();
        s.writeLong(sum());
    }

    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = s.readLong();
    }
}
// CHECKSTYLE:ON