AtomicReferenceArray源码解析

AtomicReferenceArray类介绍

java.util.concurrent.atomic.AtomicReferenceArray类提供了可以原子读取和写入的底层引用数组的操作，并且还包含高级原子操作。 AtomicReferenceArray支持对底层引用数组变量的原子操作。 它具有获取和设置方法，如在变量上的读取和写入。 也就是说，一个集合与同一变量上的任何后续获取相关联。 原子compareAndSet方法也具有这些内存一致性功能。

类图

主要属性

private static final Unsafe unsafe;
private static final int base;
private static final int shift;
private static final long arrayFieldOffset;
private final Object[] array; // must have exact type Object[]

unsafe：Unsafe，JDK提供的实现CAS算法的类

base：数组中第一个元素的内存地址

shift：左移偏移量。基于对象的数组，shift是根据不同对象来决定不同值的用来计算，数组中的元素在整个AtomicReferenceArray对象中的偏移量。数组中第i个元素的地址就是 i << shift + base.

arrayFieldOffset： array 相对于对象的偏移量

上述三个字段会在静态代码块中完成初始化

static {
        try {
            unsafe = Unsafe.getUnsafe();
            arrayFieldOffset = unsafe.objectFieldOffset
                (AtomicReferenceArray.class.getDeclaredField("array"));
            base = unsafe.arrayBaseOffset(Object[].class);
            int scale = unsafe.arrayIndexScale(Object[].class);
            if ((scale & (scale - 1)) != 0)
                throw new Error("data type scale not a power of two");
            shift = 31 - Integer.numberOfLeadingZeros(scale);
        } catch (Exception e) {
            throw new Error(e);
        }
    }

toString()：存放元素的数组

主要方法

checkedByteOffset(int)

private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }

检验索引是否越界，并返回对应位置的偏移量

byteOffset(int)

private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }

返回对应索引的偏移量

AtomicReferenceArray(int)

/**
     * Creates a new AtomicReferenceArray of the given length, with all
     * elements initially null.
     *
     * @param length the length of the array
     */
    public AtomicReferenceArray(int length) {
        array = new Object[length];
    }

创建指定长度的原子引用数组

AtomicReferenceArray(E[])

/**
     * Creates a new AtomicReferenceArray with the same length as, and
     * all elements copied from, the given array.
     *
     * @param array the array to copy elements from
     * @throws NullPointerException if array is null
     */
    public AtomicReferenceArray(E[] array) {
        // Visibility guaranteed by final field guarantees
        this.array = Arrays.copyOf(array, array.length, Object[].class);
    }

根据给定的数组，创建相同长度的原子引用数组。

length()

/**
     * Returns the length of the array.
     *
     * @return the length of the array
     */
    public final int length() {
        return array.length;
    }

返回当前数组的长度

get(int)

/**
     * Gets the current value at position {@code i}.
     *
     * @param i the index
     * @return the current value
     */
    public final E get(int i) {
        return getRaw(checkedByteOffset(i));
    }

返回指定索引上的值

getRaw(long)

@SuppressWarnings("unchecked")
    private E getRaw(long offset) {
        return (E) unsafe.getObjectVolatile(array, offset);
    }

调用unsafe，获取指定位移上的值

set(int, E)

/**
     * Sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     */
    public final void set(int i, E newValue) {
        unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);
    }

将指定位置上的值设置为newValue

lazySet(int,E)

/**
     * Eventually sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(int i, E newValue) {
        unsafe.putOrderedObject(array, checkedByteOffset(i), newValue);
    }

调用unsafe的方法直接将位置i上的值设置为给定的新值。但是这个方法有别于上面的set方法，它在一段时间内是有可能让其他线程查到原来的旧值，而不是立马查到新值。因此，它可以算是一个“最终一致性”的方法

getAndSet(int,E)

/**
     * Atomically sets the element at position {@code i} to the given
     * value and returns the old value.
     *
     * @param i the index
     * @param newValue the new value
     * @return the previous value
     */
    @SuppressWarnings("unchecked")
    public final E getAndSet(int i, E newValue) {
        return (E)unsafe.getAndSetObject(array, checkedByteOffset(i), newValue);
    }

获取当前值，返回，并将当前值设置为newValue

compareAndSet(int,E,E)

/**
     * Atomically sets the element at position {@code i} to the given
     * updated value if the current value {@code ==} the expected value.
     *
     * @param i the index
     * @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 i, E expect, E update) {
        return compareAndSetRaw(checkedByteOffset(i), expect, update);
    }

如果位置i上的值等同于except，那就将该值更新为update，并返回true，否则返回false。

compareAndSetRaw(long,E,E)

private boolean compareAndSetRaw(long offset, E expect, E update) {
        return unsafe.compareAndSwapObject(array, offset, expect, update);
    }

如果指定偏移量处的值等同于except，那就将该值更新为update，并返回true，否则返回false。

weakCompareAndSet(int, E,E)

/**
     * Atomically sets the element at position {@code i} 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 i the index
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public final boolean weakCompareAndSet(int i, E expect, E update) {
        return compareAndSet(i, expect, update);
    }

这个方法的本意是，调用weakCompareAndSet方法时不能保证指令重排的发生，因此，这个方法有时候会毫无理由地失败。

但是从实现上来看，这个方法还是和compareAndSet一模一样的。不过建议在使用的时候，谨慎对待，保不定什么时候，方法实现就修改了。

getAndUpdate(int,UnaryOperator)

/**
     * Atomically updates the element at index {@code i} 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 i the index
     * @param updateFunction a side-effect-free function
     * @return the previous value
     * @since 1.8
     */
    public final E getAndUpdate(int i, UnaryOperator<E> updateFunction) {
        long offset = checkedByteOffset(i);
        E prev, next;
        do {
            prev = getRaw(offset);
            next = updateFunction.apply(prev);
        } while (!compareAndSetRaw(offset, prev, next));
        return prev;
    }

获取位置i上的值，并将该值应用于单元函数updateFunction，并将其结果更新于当前值。需要注意的是，此处使用的是一个循环，如果一直更新不成功的话，线程会一直自旋在此处。

updateAndGet(int, UnaryOperator)

/**
     * Atomically updates the element at index {@code i} 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 i the index
     * @param updateFunction a side-effect-free function
     * @return the updated value
     * @since 1.8
     */
    public final E updateAndGet(int i, UnaryOperator<E> updateFunction) {
        long offset = checkedByteOffset(i);
        E prev, next;
        do {
            prev = getRaw(offset);
            next = updateFunction.apply(prev);
        } while (!compareAndSetRaw(offset, prev, next));
        return next;
    }

将当前值应用单元函数，并将获得的值更新当前值，返回。需要注意的是，此处使用的是一个循环，如果一直更新不成功的话，线程会一直自旋在此处。

getAndAccumulate(int,E,BinaryOperator)

/**
     * Atomically updates the element at index {@code i} 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 at index {@code i} as its first
     * argument, and the given update as the second argument.
     *
     * @param i the index
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the previous value
     * @since 1.8
     */
    public final E getAndAccumulate(int i, E x,
                                    BinaryOperator<E> accumulatorFunction) {
        long offset = checkedByteOffset(i);
        E prev, next;
        do {
            prev = getRaw(offset);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSetRaw(offset, prev, next));
        return prev;
    }

返回当前值。并将x和当前值应用于双元函数，将其结果更新值当前值上。需要注意的是，此处使用的是一个循环，如果一直更新不成功的话，线程会一直自旋在此处。

accumulateAndGet（int,E,BinaryOperator)

/**
     * Atomically updates the element at index {@code i} 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 at index {@code i} as its first
     * argument, and the given update as the second argument.
     *
     * @param i the index
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the updated value
     * @since 1.8
     */
    public final E accumulateAndGet(int i, E x,
                                    BinaryOperator<E> accumulatorFunction) {
        long offset = checkedByteOffset(i);
        E prev, next;
        do {
            prev = getRaw(offset);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSetRaw(offset, prev, next));
        return next;
    }

将x和当前值应用于双元函数，并将其结果更新至当前值，返回。需要注意的是，此处使用的是一个循环，如果一直更新不成功的话，线程会一直自旋在此处。

toString()

/**
     * Returns the String representation of the current values of array.
     * @return the String representation of the current values of array
     */
    public String toString() {
        int iMax = array.length - 1;
        if (iMax == -1)
            return "[]";

        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(getRaw(byteOffset(i)));
            if (i == iMax)
                return b.append(']').toString();
            b.append(',').append(' ');
        }
    }

返回当前数组的字符串形式

readObject(ObjectInputStream)

/**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException,
        java.io.InvalidObjectException {
        // Note: This must be changed if any additional fields are defined
        Object a = s.readFields().get("array", null);
        if (a == null || !a.getClass().isArray())
            throw new java.io.InvalidObjectException("Not array type");
        if (a.getClass() != Object[].class)
            a = Arrays.copyOf((Object[])a, Array.getLength(a), Object[].class);
        unsafe.putObjectVolatile(this, arrayFieldOffset, a);
    }

从输入流中读取数据并构造数组。