5000字 | 詳解 Java 中的 21 種鎖

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序號	鎖名稱	應用
1	樂觀鎖	CAS
2	悲觀鎖	synchronized、vector、hashtable
3	自旋鎖	CAS
4	可重入鎖	synchronized、Reentrantlock、Lock
5	讀寫鎖	ReentrantReadWriteLock，CopyOnWriteArrayList、CopyOnWriteArraySet
6	公平鎖	Reentrantlock(true)
7	非公平鎖	synchronized、reentrantlock(false)
8	共享鎖	ReentrantReadWriteLock中讀鎖
9	獨佔鎖	synchronized、vector、hashtable、ReentrantReadWriteLock中寫鎖
10	重量級鎖	synchronized
11	輕量級鎖	鎖優化技術
12	偏向鎖	鎖優化技術
13	分段鎖	concurrentHashMap
14	互斥鎖	synchronized
15	同步鎖	synchronized
16	死鎖	相互請求對方的資源
17	鎖粗化	鎖優化技術
18	鎖消除	鎖優化技術

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與期望值是否相同，如果相同則進行更新（更新期間加鎖，保證是原子性的）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Java中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"樂觀鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"：","attrs":{}},{"type":"text","text":" ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"CAS","attrs":{}}],"attrs":{}},{"type":"text","text":"，比較並替換，比較當前值（主內存中的值），與預期值（當前線程中的值，主內存中值的一份拷貝）是否一樣，一樣則更新，否則繼續進行CAS操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如上圖所示，可以同時進行讀操作，讀的時候其他線程不能進行寫操作。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2、悲觀鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/12/12bd1e5b9e18de9d324acd86d0518572.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"悲觀鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"是一種悲觀思想","attrs":{}},{"type":"text","text":"，即認爲寫多讀少，遇到併發寫的可能性高，每次去拿數據的時候都認爲其他線程會修改，所以每次讀寫數據都會認爲其他線程會修改，所以每次讀寫數據時都會上鎖。其他線程想要讀寫這個數據時，會被這個線程block，直到這個線程釋放鎖然後其他線程獲取到鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Java中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"悲觀鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"：","attrs":{}},{"type":"text","text":" 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爲了讓線程等待，我們只須讓線程執行一個忙循環（自旋）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現在絕大多數的個人電腦和服務器都是多路（核）處理器系統，如果物理機器有一個以上的處理器或者處理器核心，能讓兩個或以上的線程同時並行執行，就可以讓後面請求鎖的那個線程“稍等一會”，但不放棄處理器的執行時間，看看持有鎖的線程是否很快就會釋放鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"自旋鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"的優點：","attrs":{}},{"type":"text","text":" 避免了線程切換的開銷。掛起線程和恢復線程的操作都需要轉入內核態中完成，這些操作給Java虛擬機的併發性能帶來了很大的壓力。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"自旋鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"的缺點：","attrs":{}},{"type":"text","text":" 佔用處理器的時間，如果佔用的時間很長，會白白消耗處理器資源，而不會做任何有價值的工作，帶來性能的浪費。因此自旋等待的時間必須有一定的限度，如果自旋超過了限定的次數仍然沒有成功獲得鎖，就應當使用傳統的方式去掛起線程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"自旋","attrs":{}}],"attrs":{}},{"type":"text","text":"次數默認值：10次，可以使用參數-XX:PreBlockSpin來自行更改。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"自適應","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"自旋","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"：","attrs":{}},{"type":"text","text":" 自適應意味着自旋的時間不再是固定的，而是由前一次在同一個鎖上的自旋時間及鎖的擁有者的狀態來決定的。有了自適應自旋，隨着程序運行時間的增長及性能監控信息的不斷完善，虛擬機對程序鎖的狀態預測就會越來越精準。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Java中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"自旋鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"：","attrs":{}},{"type":"text","text":" CAS操作中的比較操作失敗後的自旋等待。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"4、可重入鎖（遞歸鎖）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5b/5b1b0c42d80a79dd92e0a37dcc8502af.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"可重入鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"是一種技術：","attrs":{}},{"type":"text","text":" 任意線程在獲取到鎖之後能夠再次獲取該鎖而不會被鎖所阻塞。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"可重入鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"的原理：","attrs":{}},{"type":"text","text":" 通過組合自定義同步器來實現鎖的獲取與釋放。","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"再次獲取鎖：識別獲取鎖的線程是否爲當前佔據鎖的線程，如果是，則再次成功獲取。獲取鎖後，進行計數自增，","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"釋放鎖：釋放鎖時，進行計數自減。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"Java中的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"可重入鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"：","attrs":{}},{"type":"text","text":" ReentrantLock、synchronized修飾的方法或代碼段。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"可重入鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"的作用：","attrs":{}},{"type":"text","text":" 避免死鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"面試題1：","attrs":{}},{"type":"text","text":" 可重入鎖如果加了兩把，但是隻釋放了一把會出現什麼問題？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答：程序卡死，線程不能出來，也就是說我們申請了幾把鎖，就需要釋放幾把鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"面試題2：","attrs":{}},{"type":"text","text":" 如果只加了一把鎖，釋放兩次會出現什麼問題？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答：會報錯，java.lang.IllegalMonitorStateException。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"5、讀寫鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"讀寫鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"是一種技術：","attrs":{}},{"type":"text","text":" 通過","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ReentrantReadWriteLock","attrs":{}}],"attrs":{}},{"type":"text","text":"類來實現。爲了提高性能， Java 提供了讀寫鎖，在讀的地方使用讀鎖，在寫的地方使用寫鎖，靈活控制，如果沒有寫鎖的情況下，讀是無阻塞的，在一定程度上提高了程序的執行效率。讀寫鎖分爲讀鎖和寫鎖，多個讀鎖不互斥，讀鎖與寫鎖互斥，這是由 jvm 自己控制的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"讀鎖：","attrs":{}},{"type":"text","text":" 允許多個線程獲取讀鎖，同時訪問同一個資源。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/19/19617bc4e319894d4daa7193295f0bd9.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"寫鎖：","attrs":{}},{"type":"text","text":" 只允許一個線程獲取寫鎖，不允許同時訪問同一個資源。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f1/f1e3315125f776433cf8c7b16b74704d.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如何使用：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"/**\n* 創建一個讀寫鎖\n* 它是一個讀寫融爲一體的鎖，在使用的時候，需要轉換\n*/\nprivate ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"獲取讀鎖和釋放讀鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 獲取讀鎖\nrwLock.readLock().lock();\n\n// 釋放讀鎖\nrwLock.readLock().unlock();\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"獲取寫鎖和釋放寫鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"// 創建一個寫鎖\nrwLock.writeLock().lock();\n\n// 寫鎖 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多個線程按照申請鎖的順序來獲取鎖。在併發環境中，每個線程會先查看此鎖維護的等待隊列，如果當前等待隊列爲空，則佔有鎖，如果等待隊列不爲空，則加入到等待隊列的末尾，按照FIFO的原則從隊列中拿到線程，然後佔有鎖。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"7、非公平鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/61/611ef24873bb46a7d1fac65e9473f7ce.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"非公平鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"非公平鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"是一種思想：","attrs":{}},{"type":"text","text":" 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有可能造成線程飢餓（某個線程很長一段時間獲取不到鎖）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Java中的非公平鎖：synchronized是非公平鎖，ReentrantLock通過構造函數指定該鎖是公平的還是非公平的，默認是非公平的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"8、共享鎖","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b7/b7ddef8b05a1e4e2371f94d611091b85.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"共享鎖","attrs":{}}],"marks":[{"type":"strong"}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"是一種思想：","attrs":{}},{"type":"text","text":" 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輕量級鎖是在無競爭的情況下使用CAS操作去消除同步使用的互斥量。輕量級是相對於使用操作系統互斥量來實現的重量級鎖而言的。輕量級鎖在沒有多線程競爭的前提下，減少傳統的重量級鎖使用操作系統互斥量產生的性能消耗。如果出現兩條以上的線程爭用同一個鎖的情況，那輕量級鎖將不會有效，必須膨脹爲重量級鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"優點：","attrs":{}},{"type":"text","text":" 如果沒有競爭，通過CAS操作成功避免了使用互斥量的開銷。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"缺點：","attrs":{}},{"type":"text","text":" 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在無競爭的情況下把整個同步都消除掉，連CAS操作都不去做了。偏是指偏心，它的意思是這個鎖會偏向於第一個獲得它的線程，如果在接下來的執行過程中，該鎖一直沒有被其他的線程獲取，則持有偏向鎖的線程將永遠不需要再進行同步。持有偏向鎖的線程以後每次進入這個鎖相關的同步塊時，虛擬機都可以不再進行任何同步操作（例如加鎖、解鎖及對Mark Word的更新操作等）。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"優點：","attrs":{}},{"type":"text","text":" 把整個同步都消除掉，連CAS操作都不去做了，優於輕量級鎖。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"缺點：","attrs":{}},{"type":"text","text":" 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是Java中的接口，可重入鎖、悲觀鎖、獨佔鎖、互斥鎖、同步鎖。","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1.Lock需要手動獲取鎖和釋放鎖。就好比自動擋和手動擋的區別","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2.Lock 是一個接口，而 synchronized 是 Java 中的關鍵字， synchronized 是內置的語言實現。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"3.synchronized 在發生異常時，會自動釋放線程佔有的鎖，因此不會導致死鎖現象發生；而 Lock 在發生異常時，如果沒有主動通過 unLock()去釋放鎖，則很可能造成死鎖現象，因此使用 Lock 時需要在 finally 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