AtomicLong的incrementAndGet()

调用顺序为：incrementAndGet--->compareAndSet--->compareAndSwapLong

最后这个函数是一个native函数，这个函数中，第一个形参传入的实参为this（调用者），第三和第四个参数中传入实参是incrementAndGet()中读取的两个变量current和next=current+1。因此cas底层的做法是：如果this的值为current，那就把this的值改为current+1。

源代码如下：

/**
     * 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 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 update Java variable to x if it is currently
     * holding expected.
     * @return true if successful
     */
    public final native boolean compareAndSwapLong(Object o, long offset,
                                                   long expected,
                                                   long x);