了解JAVA并发工具常用设计套路

前言

在学习JAVA并发工具时，分析JUC下的源码，发现有三个利器：状态、队列、CAS。

状态

一般是state属性，如AQS源码中的状态，是整个工具的核心，一般操作的执行都要看当前状态是什么，

由于状态是多线程共享的，所以都是volatile修饰，保证线程直接内存可见。

/**

* AbstractQueuedSynchronizer中的状态

*/

private volatile int state;

/**

* Status field, taking on only the values:

* SIGNAL: The successor of this node is (or will soon be)

* blocked (via park), so the current node must

* unpark its successor when it releases or

* cancels. To avoid races, acquire methods must

* first indicate they need a signal,

* then retry the atomic acquire, and then,

* on failure, block.

* CANCELLED: This node is cancelled due to timeout or interrupt.

* Nodes never leave this state. In particular,

* a thread with cancelled node never again blocks.

* CONDITION: This node is currently on a condition queue.

* It will not be used as a sync queue node

* until transferred, at which time the status

* will be set to 0. (Use of this value here has

* nothing to do with the other uses of the

* field, but simplifies mechanics.)

* PROPAGATE: A releaseShared should be propagated to other

* nodes. This is set (for head node only) in

* doReleaseShared to ensure propagation

* continues, even if other operations have

* since intervened.

* 0: None of the above

*

* The values are arranged numerically to simplify use.

* Non-negative values mean that a node doesn't need to

* signal. So, most code doesn't need to check for particular

* values, just for sign.

*

* The field is initialized to 0 for normal sync nodes, and

* CONDITION for condition nodes. It is modified using CAS

* (or when possible, unconditional volatile writes).

*/

volatile int waitStatus;

队列

队列一般由链表实现(单向链表，双向链表)，在线程获取不到想要的资源或者状态时，将线程封装成特定节点，扔到等待队列中，等待时机成熟，再从队列中取出，是悲观锁思想。

如AQS中的Node节点，代码太长就不贴了。

CAS

CAS操作是乐观锁思想，是轻量级的并发处理。一般用于对上述状态的修改，而且能保证有且只有一个线程能修改这个状态。

一般由Unsafe类中的compareAndSwap之类的方法实现。使用CAS,往往伴随自旋，如果修改状态失败，则不断地重试，直到修改状态成功。

以上就是java并发编程中的套路，抓住这个思路，想必能在学习中有所帮助。

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