AtomicReference源码解析

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AtomicReference源码解析

AtomicReference类介绍

AtomicReference类提供了一种读和写都是原子性的对象引用变量。原子意味着多个线程试图改变同一个AtomicReference(例如比较和交换操作)将不会使得AtomicReference处于不一致的状态。AtomicReferenc的compareAndSet()方法可以使得它与期望的一个值进行比较，如果他们是相等的，AtomicReference里的对象会被设置成一个新的引用。

类图

AtomicReference类图

主要属性

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private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

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

    private volatile V value;

unsafe：Unsafe类。JDK提供的实现CAS操作的类。

valueOffset：当前值value在当前对象中的偏移量

value：当前值

主要方法

AtomicReference(V)

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

根据给定的值，创建一个新的AtomicReference对象

AtomicReference()

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/**
     * Creates a new AtomicReference with null initial value.
     */
    public AtomicReference() {
    }

创建一个新的AtomicReference对性爱那个

get()

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

获取当前值

set(V)

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

设置当前值为newValue

lazySet(V)

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/**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(V newValue) {
        unsafe.putOrderedObject(this, valueOffset, newValue);
    }

将当前值“慢慢”设置为newValue。这意味着在多线程的极端环境下，可能有一些线程还是能够读取到旧值的。

compareAndSet(V,V)

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/**
     * 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(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

如果当前值等于expect，则将当前值更新为update，然后返回true。否则返回false。

weakCompareAndSet(V,V)

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/**
     * 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(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

如果当前值等于expect，则将当前值更新为update，然后返回true。否则返回false。

getAndSet()

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/**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    @SuppressWarnings("unchecked")
    public final V getAndSet(V newValue) {
        return (V)unsafe.getAndSetObject(this, valueOffset, newValue);
    }

返回当前值，并将当前值设置为newValue

getAndUpdate(UnaryOperator)

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/**
     * 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 V getAndUpdate(UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get();
            next = updateFunction.apply(prev);
        } while (!compareAndSet(prev, next));
        return prev;
    }

获取当前值，并将当前值应用于单元函数，将得到的结果更新至当前值。需要注意的是，这里采用的是一个死循环，极端的多线程情况下，这个循环可能跑很久。

updateAndGet(UnaryOperator)

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/**
     * 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 V updateAndGet(UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get();
            next = updateFunction.apply(prev);
        } while (!compareAndSet(prev, next));
        return next;
    }

将当前值应用于单元函数，并将结果更新值当前值，返回。需要注意的是，这里采用的是一个死循环，极端的多线程情况下，这个循环可能跑很久。

getAndAccumulater(V,BinaryOperator)

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/**
     * 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 V getAndAccumulate(V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get();
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(prev, next));
        return prev;
    }

返回当前值，并将当前值和x以用于双元函数，再将得到的结果更新至当前值。需要注意的是，这里采用的是一个死循环，极端的多线程情况下，这个循环可能跑很久。

accumulateAndGet(V,BianryOperator)

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/**
     * 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 V accumulateAndGet(V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get();
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(prev, next));
        return next;
    }

将当前值和x应用于双元函数，并将结果更新值当前值，返回。需要注意的是，这里采用的是一个死循环，极端的多线程情况下，这个循环可能跑很久。

toString()

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

返回当前引用的字符串形式。

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