Atomicity（原子性）

Operations A and B are atomic with respect to each other if, from the perspective of a thread executing A, when another thread executes B, either all of B has executed or none of it has. An atomic operation is one that is atomic with respect to all operations, including itself, that operate on the same state.

什么是原子性？简单来讲就是两个线程同时对一个状态做操作。当线程A对这个状态进行操作时，线程B对于A的操作要么尚未开始，要么已经结束。这样就保证了这个状态的原子性，
同时也可以说线程A与线程B对这个状态的操作是原子性的。

摘自<Java Concurrency In Practise> 2.2. Atomicity