了解JAVA并發工具常用設計套路

前言

在學習JAVA并發工具時，分析JUC下的源碼，發現有三個利器：狀態、隊列、CAS。

狀態

一般是state屬性，如AQS源碼中的狀態，是整個工具的核心，一般操作的執行都要看當前狀態是什么，

由于狀態是多線程共享的，所以都是volatile修飾，保證線程直接內存可見。

/**

* AbstractQueuedSynchronizer中的狀態

*/

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;

隊列

隊列一般由鏈表實現(單向鏈表，雙向鏈表)，在線程獲取不到想要的資源或者狀態時，將線程封裝成特定節點，扔到等待隊列中，等待時機成熟，再從隊列中取出，是悲觀鎖思想。

如AQS中的Node節點，代碼太長就不貼了。

CAS

CAS操作是樂觀鎖思想，是輕量級的并發處理。一般用于對上述狀態的修改，而且能保證有且只有一個線程能修改這個狀態。

一般由Unsafe類中的compareAndSwap之類的方法實現。使用CAS,往往伴隨自旋，如果修改狀態失敗，則不斷地重試，直到修改狀態成功。

以上就是java并發編程中的套路，抓住這個思路，想必能在學習中有所幫助。

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