前言学完阿里P8面试官推荐的Java高并发核心编程文档后终于拿到了蚂蚁p6的offer，这份文档包含的内容有点多。

JUC 提供的同步类主要有如下几种:Semaphore is a classic concurrency tool.CountDownLatch is a very simple yet very common utility for blocking until a given number of signals， events， or conditions hold.A CyclicBarrier is a resettable multiway synchronization point useful in some styles of parallel programming.A Phaser provides a more flexible form of barrier that may be used to control phased computation among multiple threads.An Exchanger allows two threads to exchange objects at a rendezvous point， and is useful in several pipeline designs.也许基于多线程的并发编程不是最好的，但却是最悠久的。