1、非公平鎖不能保證鎖的獲取是按照請(qǐng)求鎖的順序進(jìn)行的。這可能會(huì)導(dǎo)致某個(gè)或某些線程永遠(yuǎn)得不到鎖。
2、CPU喚醒線程的費(fèi)用可以降低，整體吞吐效率會(huì)很高。但是可能會(huì)有線程長時(shí)間甚至永遠(yuǎn)得不到鎖，導(dǎo)致餓死。
實(shí)例
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/** * Sync object for non-fair locks */ static final class NonfairSync extends Sync { private static final long serialVersionUID = 7316153563782823691L; /** * Performs lock. Try immediate barge, backing up to normal * acquire on failure. */ final void lock() { if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread()); else acquire(1); } protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); } } /** * Sync object for fair locks */ static final class FairSync extends Sync { private static final long serialVersionUID = -3000897897090466540L; final void lock() { acquire(1); } /** * Fair version of tryAcquire. Don't grant access unless * recursive call or no waiters or is first. */ protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; } } |
知識(shí)點(diǎn)擴(kuò)展：
非公平鎖，顧名思義，各個(gè)線程獲取到鎖的順序，不一定和它們申請(qǐng)的先后順序一致，有可能后來的線程，反而先獲取到了鎖。
在實(shí)現(xiàn)上，公平鎖在進(jìn)行l(wèi)ock時(shí)，首先會(huì)進(jìn)行tryAcquire()操作。在tryAcquire中，會(huì)判斷等待隊(duì)列中是否已經(jīng)有別的線程在等待了。如果隊(duì)列中已經(jīng)有別的線程了，則tryAcquire失敗，則將自己加入隊(duì)列。如果隊(duì)列中沒有別的線程，則進(jìn)行獲取鎖的操作。
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/** * Fair version of tryAcquire. Don't grant access unless * recursive call or no waiters or is first. **/ protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; } |
非公平鎖，在進(jìn)行l(wèi)ock時(shí)，會(huì)直接嘗試進(jìn)行加鎖，如果成功，則獲取到鎖，如果失敗，則進(jìn)行和公平鎖相同的動(dòng)作。
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原文鏈接：https://www.py.cn/java/jichu/34414.html
