1、非公平鎖不能保證鎖的獲取是按照請求鎖的順序進行的。這可能會導致某個或某些線程永遠得不到鎖。
2、CPU喚醒線程的費用可以降低，整體吞吐效率會很高。但是可能會有線程長時間甚至永遠得不到鎖，導致餓死。
實例
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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; } } |
知識點擴展：
非公平鎖，顧名思義，各個線程獲取到鎖的順序，不一定和它們申請的先后順序一致，有可能后來的線程，反而先獲取到了鎖。
在實現上，公平鎖在進行lock時，首先會進行tryAcquire()操作。在tryAcquire中，會判斷等待隊列中是否已經有別的線程在等待了。如果隊列中已經有別的線程了，則tryAcquire失敗，則將自己加入隊列。如果隊列中沒有別的線程，則進行獲取鎖的操作。
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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，在進行lock時，會直接嘗試進行加鎖，如果成功，則獲取到鎖，如果失敗，則進行和公平鎖相同的動作。
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