AtomicInteger的CAS算法浅析

　　之前浅析过自旋锁（自旋锁浅析），我们知道它的实现原理就是CAS算法。CAS（Compare and Swap）即比较并交换，作为著名的无锁算法，它也是乐观锁的实现方式之一。JDK并发包里也有许多代码中有CAS的身影闪烁其中，鉴于CAS算法在并发领域的重要性和普适性，还是再结合AtomicInteger这个原子类来浅析一下吧。浅析之前，先借用之前自旋锁测试代码直接看AtomicInteger的自增测试结果，可以拿它跟自旋锁做个比较：

    @Test
    public void testAtomicInteger()
    {
        // 10个线程使用AtomicInteger自增
        AtomicInteger ai = new AtomicInteger();
        for (int i = 0; i < 10; i++)
        {
            new Thread(new Runnable()
            {
                @Override
                public void run()
                {
                    // 自增1万次
                    for (int j = 0; j < 10000; j++)
                    {
                        count = ai.incrementAndGet();
                    }

                    // 一个线程执行完了就减1,10个线程执行完了就变成0，执行主线程
                    latch.countDown();
                }
            }).start();
        }

        // 主线程等待
        try
        {
            latch.await();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        TestCase.assertEquals(count, 100000);
    }

　　运行结果：

count值：100000, 耗时：10毫秒.

　　是不是比自旋锁要简单？必须的，因为AtomicInteger本身已经实现了CAS算法，人家天然就用于并发自增的。之前也说到过，CAS的原理很简单，它包含三个值：当前内存值(V)、预期原来的值(A)以及期待更新的值(B)。如果内存位置V的值与预期原值A相匹配，那么处理器会自动将该位置值更新为新值B,返回true。否则处理器不做任何操作，返回false。举上面的例子，我们有10个线程分别去做自增操作，很明显count是共享变量，它将被这10个线程追杀加1。假如线程1将count追加到100时，正准备更新到101这一刻，线程2插一脚抢先一步把count追加到101，那么线程1该怎么办呢？它将获取最新的count值再去自增。具体怎么实现的，我们接下来看。为了直观点，我们可以换种方式实现上面的例子：

package com.wulinfeng.test.testpilling;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

public class AtomicIntegerTest {

    // 共享变量
    private static int count;

    // 10个线程就先初始化10
    private static CountDownLatch latch = new CountDownLatch(10);

    public static void main(String[] args) {
        // 10个线程使用AtomicInteger自增
        AtomicInteger ai = new AtomicInteger();
        for (int i = 0; i < 10; i++) {
            final int threadNum = i;
            new Thread(new Runnable() {
                @Override
                public void run() {
                    // 自增1万次
                    for (int j = 0; j < 10000; j++) {
                        count = ai.incrementAndGet();
                        System.out.println("线程" + threadNum + ": " + count);
                    }

                    // 一个线程执行完了就减1,10个线程执行完了就变成0，执行主线程
                    latch.countDown();
                }
            }).start();
        }

        // 主线程等待
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

　　截取运行结果后面几行：

线程1: 99993
线程1: 99994
线程1: 99995
线程1: 99996
线程1: 99997
线程1: 99998
线程1: 99999
线程1: 100000

　　这后面的日志打印的都是线程1在追加，把控制台日志往上拉就可以看到其他线程也一直在追加的。那么这10个线程如何在CAS的咒法护身下没有互相冲突的呢？看AtomicInteger的源码便知：

package java.util.concurrent.atomic;
import java.util.function.IntUnaryOperator;
import java.util.function.IntBinaryOperator;
import sun.misc.Unsafe;

/**
 * An {@code int} 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 AtomicInteger} is used in applications such as atomically
 * incremented counters, and cannot be used as a replacement for an
 * {@link java.lang.Integer}. 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 AtomicInteger extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 6214790243416807050L;

    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

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

    private volatile int value;

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

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

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

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

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(int newValue) {
        unsafe.putOrderedInt(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 int getAndSet(int newValue) {
        return unsafe.getAndSetInt(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(int expect, int update) {
        return unsafe.compareAndSwapInt(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(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }

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

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

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

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

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

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the updated value
     */
    public final int addAndGet(int delta) {
        return unsafe.getAndAddInt(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 int getAndUpdate(IntUnaryOperator updateFunction) {
        int prev, next;
        do {
            prev = get();
            next = updateFunction.applyAsInt(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 int updateAndGet(IntUnaryOperator updateFunction) {
        int prev, next;
        do {
            prev = get();
            next = updateFunction.applyAsInt(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 int getAndAccumulate(int x,
                                      IntBinaryOperator accumulatorFunction) {
        int prev, next;
        do {
            prev = get();
            next = accumulatorFunction.applyAsInt(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 int accumulateAndGet(int x,
                                      IntBinaryOperator accumulatorFunction) {
        int prev, next;
        do {
            prev = get();
            next = accumulatorFunction.applyAsInt(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 Integer.toString(get());
    }

    /**
     * Returns the value of this {@code AtomicInteger} as an {@code int}.
     */
    public int intValue() {
        return get();
    }

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

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

}

　　标黄了3个成员变量和两个方法。方法比较简单，一个是返回自增前的值，一个是返回自增后的值。

　　第一个成员变量是unsafe，这个必不可少，没了它CAS就是浮云。CAS算法用到了Unsafe对象的compareAndSwapInt方法，而它是一个本地方法，所以实现源码到此为止，没法再跟进去了。其实CAS算法的精华也就在于此，所以很遗憾。但至少我们知道Unsafe总共有3个CAS方法：

    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);

    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);

　　第二个成员变量是valueOffset，它是共享变量value在AtomicInteger对象上的内存偏移量。它作为compareAndSwapInt的第二个参数，用于修改共享变量value的值。

　　最后就是value了，上面已经介绍了，它就是例子中的count，是共享变量，也就是多线程并发中被追杀的共享资源。它使用volatile修饰，解决了可见性和有序性问题，再由unsafe的CAS保证了原子性，3大问题都解决了，多线程并发问题也就解决了。

　　回过头再看那标黄的两个方法，实现都是unsafe的getAndAddInt，点进去瞧瞧，发现它不是本地方法：

    public final int getAndAddInt(Object var1, long var2, int var4) {
        int var5;
        do {
            var5 = this.getIntVolatile(var1, var2);
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

        return var5;
    }

　　代码看起来是不是有点眼熟？没错，还是自旋锁的套路，只不过这里用到的是Unsafe的CAS算法，而我们的自旋锁用到的是多套了一层马甲的AtmoicXXX的CAS算法，所以说到底，我们用的还是Unsafe的CAS。通过循环，先获取当前值var5（怎么获取当前值的？getIntVolatile就不用看了，还是本地方法），再计算更新值var5+var4，然后通过compareAndSwapInt方法设置value变量。如果compareAndSwapInt方法返回失败，表示value变量的值被别的线程更改了，所以需要循环获取value变量的最新值，再次通过compareAndSwapInt方法设置value变量，直至设置成功，跳出循环，返回更新前的值。

　　从上面看到，CAS底层实现依赖于Unsafe包，我们只要明白CAS的原理即可：预期值与当前值一致，那么执行更新，否则死循环尝试更新，直到成功。