大白話聊聊Java併發面試問題之談談你對AQS的理解？【石杉的架構筆記】

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個人新課**《C2C 電商系統微服務架構120天實戰訓練營》在公衆號儒猿技術窩**上線了，感興趣的同窗，能夠點擊下方連接瞭解詳情：

《C2C 電商系統微服務架構120天實戰訓練營》

1、寫在前面

上一篇文章聊了一下java併發中經常使用的原子類的原理和Java 8的優化，具體請參見文章：大白話聊聊Java併發面試問題之Java 8如何優化CAS性能？。

這篇文章，咱們來聊聊面試的時候比較有殺傷力的一個問題：聊聊你對AQS的理解？

以前有同窗反饋，去互聯網公司面試，面試官聊到併發時就問到了這個問題。當時那位同窗心裏估計受到了一萬點傷害。。。

由於首先，不少人還真的連AQS是什麼都不知道，可能聽都沒據說過。或者有的人據說過AQS這個名詞，可是可能連具體全稱怎麼拼寫都不知道。

更有甚者，可能會說：AQS？是否是一種思想？咱們平時開發怎麼來用AQS？

整體來講，不少同窗估計都對AQS有一種雲裏霧裏的感受，若是用搜索引擎查一下AQS是什麼？看幾篇文章，估計就直接放棄了，由於密密麻麻的文字，實在是看不懂！

因此，基於上述痛點，我們這篇文章，就用最簡單的大白話配合N多張手繪圖，給你們講清楚AQS究竟是什麼？讓各位同窗面試被問到這個問題時，不至於不知所措。

2、ReentrantLock和AQS的關係

首先咱們來看看，若是用java併發包下的ReentrantLock來加鎖和釋放鎖，是個什麼樣的感受？

這個基本學過java的同窗應該都會吧，畢竟這個是java併發基本API的使用，應該每一個人都是學過的，因此咱們直接看一下代碼就行了：

上面那段代碼應該不難理解吧，無非就是搞一個Lock對象，而後加鎖和釋放鎖。

你這時可能會問，這個跟AQS有啥關係？關係大了去了！由於java併發包下不少API都是基於AQS來實現的加鎖和釋放鎖等功能的，AQS是java併發包的基礎類。

舉個例子，好比說ReentrantLock、ReentrantReadWriteLock底層都是基於AQS來實現的。

那麼AQS的全稱是什麼呢？AbstractQueuedSynchronizer，抽象隊列同步器。給你們畫一個圖先，看一下ReentrantLock和AQS之間的關係。

咱們來看上面的圖。說白了，ReentrantLock內部包含了一個AQS對象，也就是AbstractQueuedSynchronizer類型的對象。這個AQS對象就是ReentrantLock能夠實現加鎖和釋放鎖的關鍵性的核心組件。

3、ReentrantLock加鎖和釋放鎖的底層原理

好了，那麼如今若是有一個線程過來嘗試用ReentrantLock的lock()方法進行加鎖，會發生什麼事情呢？

很簡單，這個AQS對象內部有一個核心的變量叫作state，是int類型的，表明了加鎖的狀態。初始狀態下，這個state的值是0。

另外，這個AQS內部還有一個關鍵變量，用來記錄當前加鎖的是哪一個線程，初始化狀態下，這個變量是null。

接着線程1跑過來調用ReentrantLock的lock()方法嘗試進行加鎖，這個加鎖的過程，直接就是用CAS操做將state值從0變爲1。

若是不知道CAS是啥的，請看上篇文章，大白話聊聊Java併發面試問題之Java 8如何優化CAS性能？

若是以前沒人加過鎖，那麼state的值確定是0，此時線程1就能夠加鎖成功。

一旦線程1加鎖成功了以後，就能夠設置當前加鎖線程是本身。因此你們看下面的圖，就是線程1跑過來加鎖的一個過程。

其實看到這兒，你們應該對所謂的AQS有感受了。說白了，就是併發包裏的一個核心組件，裏面有state變量、加鎖線程變量等核心的東西，維護了加鎖狀態。

你會發現，ReentrantLock這種東西只是一個外層的API，內核中的鎖機制實現都是依賴AQS組件的。

這個ReentrantLock之因此用Reentrant打頭，意思就是他是一個可重入鎖。

可重入鎖的意思，就是你能夠對一個ReentrantLock對象屢次執行lock()加鎖和unlock()釋放鎖，也就是能夠對一個鎖加屢次，叫作可重入加鎖。

你們看明白了那個state變量以後，就知道了如何進行可重入加鎖！

其實每次線程1可重入加鎖一次，會判斷一下當前加鎖線程就是本身，那麼他本身就能夠可重入屢次加鎖，每次加鎖就是把state的值給累加1，別的沒啥變化。

接着，若是線程1加鎖了以後，線程2跑過來加鎖會怎麼樣呢？

**咱們來看看鎖的互斥是如何實現的？**線程2跑過來一下看到，哎呀！state的值不是0啊？因此CAS操做將state從0變爲1的過程會失敗，由於state的值當前爲1，說明已經有人加鎖了！

接着線程2會看一下，是否是本身以前加的鎖啊？固然不是了，**「加鎖線程」**這個變量明確記錄了是線程1佔用了這個鎖，因此線程2此時就是加鎖失敗。

給你們來一張圖，一塊兒來感覺一下這個過程：

接着，線程2會將本身放入AQS中的一個等待隊列，由於本身嘗試加鎖失敗了，此時就要將本身放入隊列中來等待，等待線程1釋放鎖以後，本身就能夠從新嘗試加鎖了

因此你們能夠看到，AQS是如此的核心！AQS內部還有一個等待隊列，專門放那些加鎖失敗的線程！

一樣，給你們來一張圖，一塊兒感覺一下：

接着，線程1在執行完本身的業務邏輯代碼以後，就會釋放鎖！他釋放鎖的過程很是的簡單，就是將AQS內的state變量的值遞減1，若是state值爲0，則完全釋放鎖，會將「加鎖線程」變量也設置爲null！

整個過程，參見下圖：

接下來，會從等待隊列的隊頭喚醒線程2從新嘗試加鎖。

好！線程2如今就從新嘗試加鎖，這時仍是用CAS操做將state從0變爲1，此時就會成功，成功以後表明加鎖成功，就會將state設置爲1。

此外，還要把**「加鎖線程」**設置爲線程2本身，同時線程2本身就從等待隊列中出隊了。

最後再來一張圖，你們來看看這個過程。

4、總結

OK，本文到這裏爲止，基本藉着ReentrantLock的加鎖和釋放鎖的過程，給你們講清楚了其底層依賴的AQS的核心原理。

基本上你們把這篇文章看懂，之後不再會擔憂面試的時候被問到：談談你對AQS的理解這種問題了。

其實一句話總結AQS就是一個併發包的基礎組件，用來實現各類鎖，各類同步組件的。它包含了state變量、加鎖線程、等待隊列等併發中的核心組件。

併發系列文章，正在更新中，歡迎關注：
大白話聊聊Java併發面試問題之volatile究竟是什麼？ 大白話聊聊Java併發面試問題之Java 8如何優化CAS性能？ 大白話聊聊Java併發面試問題之談談你對AQS的理解？
大白話聊聊Java併發面試問題之公平鎖與非公平鎖是啥？敬請期待
大白話聊聊Java併發面試問題之微服務註冊中心的讀寫鎖優化？敬請期待

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