AtomicLong源码浅析(基于jdk1.8.0_231)

AtomicLong 简介

• 在32位操作系统中，64位的long 和 double 变量由于会被JVM当作两个分离的32位来进行操作，所以不具有原子性。而AtomicLong能让long的加1,减1操作,设置新值等操作在多线程中保持原子性；
• AtomicLong 虽然继承了Number 但不是 Long的替代品，即不要滥用；
• AtomicLong的原子性操作并不由加锁支持的，而是有CompareAndSwap(简称 CAS )支持的原子性;

AtomicLong UML

AtomicLong 关键技术分析

CAS关键技术(CompareAndSwap 又称作比较交换)

我们对一个数字A做修改前（注意是前，修改的动作还没发生，我们修改前需要做些准备工作），首先会得到 存储 A 的地址，即为addrA， A的值，即为addA(A)（即为当前addA 中记录的A的值）。得到addA addA（A）就是修改前的准备工作，此期间别的线程也可以读写A哦。
当对A开始修改了，流程如下:
第一步：去addA中去看一下此刻此地址的A值的，即为(V_{A})；
第二步：比较 (V_{A}) 和 addA(A) 是否相等，若相等，则执行更新，如将 A+B的值写入addA，结束。(这步操作是原子性的，是由操作系统的指令支持的，任何一条单独的指令都是原子的，要不执行，要不不执行，设想若指令不是原子性的，那计算机组成中的多级流水线等一系列优化的大厦会轰然倒塌，若是一系列指令完成的，必然要在这些指令的执行“加锁”，如锁住总线，独占CPU,或者让现在线程栈(java虚拟机栈）中修改好缓存，修改缓存期间，其他线程不能修改该缓存对应的主存位置，缓存写好后立即强制写回主存，放开主存锁定，让其他所有线程可见，这一块都是计算机组成的一些知识，仅做抛砖引玉）
第三步： 若 (V_{A}) 和 addA(A) 不相等（说明修改前的准备阶段到准备修改期间，有别的线程在操作A，改了A的值)， 然后记住此刻此地址A 记作addA(A)，再从第一步开始，直到该线程成功更新A.（一直循环判断直到成功修改，我们称为自旋）

AtomicLong 源码解读

package java.util.concurrent.atomic;
import java.util.function.LongUnaryOperator;
import java.util.function.LongBinaryOperator;
import sun.misc.Unsafe;

/**
 * @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();
    }

}

AtomicLong 示例

//单线程下线程不安全和AtomicLong的线程安全

面试session

• CAS 操作一定是线程安全的吗？
  是线程相对安全的，没有绝对的线程安全，但是CAS一般情况下比传统的加锁并发度会更好，性能更加。但是可能会引起一些问题，如ABA问题，自旋引起的性能问题
  ABA问题：线程得到 A 时，是其他线程先将 A 改为 B ,再将 B 改回 为 A, 线程也认为此刻没有其他线程在修改A值 也会成功更新 A.这种情况，大部分情况都是没问题的，有些具体的业务对ABA问题敏感，就要注意。反正和相关的都想想一想，不然干就完了。996谁顶住鸭......
  ABA问题可以在每个操作数加一个版本号，如 A1 B1 A2,这样线程得到的是A1,和A2比较时，就会发现有其他线程在操作A,Java并发包为了解决这个问题，提供了一个带有标记的原子引用类“AtomicStampedReference”，它可以通过控制变量值的版本来保证CAS的正确性。当然改成传统的同步加锁也可以哦。
  自旋： 就是指一直循环判断直到成功修改为止。长时间的自旋操作，特别是多个线程都在自旋，很影响性能的。可考虑换成锁来操作。