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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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/licenses/publicdomain
 */

package java.util.concurrent.atomic;
import sun.misc.Unsafe;


A long value that may be updated atomically. See the atomic package specification for description of the properties of atomic variables. An AtomicLong is used in applications such as atomically incremented sequence numbers, and cannot be used as a replacement for a Long. However, this class does extend Number to allow uniform access by tools and utilities that deal with numerically-based classes. 
Author:Doug Lea
Since:1.5
/**
 * 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.

/**
     * 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.

/**
     * 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.

Params:
  • initialValue – the initial 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 0. 
/**
     * Creates a new AtomicLong with initial value {@code 0}.
     */
    public AtomicLong() {
    }

    
Gets the current value.

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

    
Sets to the given value.

Params:
  • newValue – the new 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.

Params:
  • newValue – the new value
Since:1.6
/**
     * 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.

Params:
  • newValue – the new value
Returns:the previous value
/**
     * 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) {
        while (true) {
            long current = get();
            if (compareAndSet(current, newValue))
                return current;
        }
    }

    
Atomically sets the value to the given updated value if the current value == the expected value. 
Params:
  • expect – the expected value
  • update – the new value
Returns:true if successful. False return indicates that
the actual value was not equal to the expected value.
/**
     * 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 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 == the expected value. 
May fail spuriously and does not provide ordering guarantees, so is only rarely an appropriate alternative to compareAndSet. 
Params:
  • expect – the expected value
  • update – the new value
Returns:true if successful.
/**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return 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.

Returns:the previous value
/**
     * Atomically increments by one the current value.
     *
     * @return the previous value
     */
    public final long getAndIncrement() {
        while (true) {
            long current = get();
            long next = current + 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    
Atomically decrements by one the current value.

Returns:the previous value
/**
     * Atomically decrements by one the current value.
     *
     * @return the previous value
     */
    public final long getAndDecrement() {
        while (true) {
            long current = get();
            long next = current - 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    
Atomically adds the given value to the current value.

Params:
  • delta – the value to add
Returns:the previous value
/**
     * 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) {
        while (true) {
            long current = get();
            long next = current + delta;
            if (compareAndSet(current, next))
                return current;
        }
    }

    
Atomically increments by one the current value.

Returns:the updated value
/**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final long incrementAndGet() {
        for (;;) {
            long current = get();
            long next = current + 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    
Atomically decrements by one the current value.

Returns:the updated value
/**
     * Atomically decrements by one the current value.
     *
     * @return the updated value
     */
    public final long decrementAndGet() {
        for (;;) {
            long current = get();
            long next = current - 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    
Atomically adds the given value to the current value.

Params:
  • delta – the value to add
Returns:the updated value
/**
     * 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) {
        for (;;) {
            long current = get();
            long next = current + delta;
            if (compareAndSet(current, next))
                return next;
        }
    }

    
Returns the String representation of the current value.

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


    public int intValue() {
        return (int)get();
    }

    public long longValue() {
        return get();
    }

    public float floatValue() {
        return (float)get();
    }

    public double doubleValue() {
        return (double)get();
    }

}