CAS如何保证安全性？（最后更新时间：20170328）

CAS是compareAndSet的缩写，是Unsafe类中的方法，我们来看源码：

   /**
     * 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(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }

CAS方法中的四个变量分别为：当前可见值，偏移量，之前预期值，需要更新的值；